PO T ASSIUM - IPNI

1998 Number 3

P O T A S S I U M

A G R I C U L T U R E

F O R

Editor: Donald L. ArmstrongAssistant Editor: Katherine P. Griffin Editorial Assistant: Melody DannerCirculation Manager: Carol MeesDesign: Debbie Nguyen, S.O.H.O.

Potash & Phosphate Institute (PPI) J.M. Van Brunt, Chairman of the Board

Agrium Inc.J.H. Sultenfuss, Vice Chairman of the Board

CF Industries, Inc.

HEADQUARTERS: NORCROSS, GEORGIA, U.S.A.D.W. Dibb, President B.C. Darst, Executive Vice President R.T. Roberts, Vice President C.V. Holcomb, Assistant Treasurer S.O. Fox, Executive Assistant W.R. Agerton, Communications SpecialistS.J. Couch, Information Management SpecialistS.K. Rogers, Statistics/Accounting

NORTH AMERICAN PROGRAMSBrookings, South DakotaP.E. Fixen, Senior Vice PresidentP. Pates, Secretary

REGIONAL DIRECTORS-North America T.W. Bruulsema, Guelph, OntarioA.E. Ludwick, Bodega Bay, California T.S. Murrell, Minneapolis, MinnesotaH.F. Reetz, Jr., Monticello, Illinois T.L. Roberts, Saskatoon, Saskatchewan C.S. Snyder, Conway, Arkansas W.M. Stewart, Lubbock, TexasN.R. Usherwood, Norcross, Georgia

INTERNATIONAL PROGRAMSSaskatoon, Saskatchewan, Canada

M.D. Stauffer, Senior Vice President, International Programs (PPI), and President, Potash & Phosphate Institute of Canada (PPIC)

INTERNATIONAL PROGRAM LOCATIONS Brazil T. Yamada, POTAFOS, PiracicabaChina S.S. Portch, Hong Kong

J. Wang, Hong Kong Jin Ji-yun, Beijing Chen Fang, WuhanTu Shihua, ChengduWu Rongguí, BeijingLiu Rongle, Beijing

India G. Dev, Dundahera, GurgaonT.N. Rao, Secunderabad, Andhra PradeshK.N. Tiwari, Uttar Pradesh

N. Latin America J. Espinosa, Quito, EcuadorLatin America-Southern Cone F.O. Garcia, Buenos AiresMexico I. Lazcano-Ferrat, QuerétaroSoutheast Asia E. Mutert, Singapore

T.H. Fairhurst, Singapore

BETTER CROPS WITH PLANT FOOD(ISSN:0006-0089) is published quarterly by thePotash & Phosphate Institute (PPI). Periodicalspostage paid at Norcross, GA, and at additional mail-ing offices (USPS 012-713). Subscription free onrequest to qualified individuals; others $8.00 per yearor $2.00 per issue. POSTMASTER: Send addresschanges to Better Crops with Plant Food, 655 EngineeringDrive, Suite 110, Norcross, GA 30092-2837. Phone(770) 447-0335; fax (770) 448-0439. www.ppi-far.orgCopyright 1998 by Potash & Phosphate Institute.

Members: Agrium Inc. • Cargill, Incorporated • CF Industries, Inc. • Farmland Hydro, Inc.IMC Global Inc. • Mississippi Chemical Corporation • Potash Corporation of Saskatchewan Inc.

Potassium: 3An Essential Plant Food Nutrient

Functions of Potassium in Plants 4

Production and Use of Potassium 6

Effects of Potassium on 9Crop Maturity

Potassium Interactions 12with Other Nutrients

Potassium Availability and Uptake 14

Yield and Economic Responses 16to Potassium

Potassium Deficiency Symptoms 20in Some Crops

Potassium Application Methods 24

The Influence of Potassium in 28Crop Quality

Influence of Potassium on 30Nitrogen Fixation

Potassium in Animal Nutrition 32

Potassium Reduces Stress from 34Drought, Cool Soils, and Compaction

Effects of Potassium on 37Plant Diseases

Powerful Potassium 40

C O N T E N T S

Vol. LXXXII (82) 1998, No. 3

Note: Reprints of this issue are available under the title of Potassium for Agriculture. Contact PPI at the address shown at left.

This publication presents current infor-mation on the importance of potassium(K) for agriculture. A forerunner booklet

on this subject was published by the Institutein the 1950s. During the more than 40 yearssince then, several editions have been re-leased. Our knowledge of plant nutrition andcrop production has greatly increased. Yieldshave doubled and tripled. Fertilization prac-tices have improved. There are newer and bet-ter hybrids and varieties. However, we stillhave much to learn about nutrient manage-ment as it relates to high yield, high profit cropproduction that is environmentally friendly.

There are still widespread areas of nutri-ent depletion. Africa is a well known example,but nutrient mining exists everywhere, even insome of the U.S. Corn Belt states. That is,more phosphorus (P) and K are being removedin harvested crops than are being applied infertilizers and animal manures. Improving themanagement of P, K and other nutrients ismore important now than it has ever been.

Potassium is basic to plant and animal

life. Except for nitrogen (N), plantsrequire more Kthan any othernutrient. Potassium is the third most abundantmineral in the human body, and it plays manyvital roles in plant nutrition.

Unlike N, P and most of the other essen-tial nutrients, K does not become a part of thechemical structure of the plant. Thus, itsmobility in the plant allows it to influencealmost all aspects of plant growth. Potassiumencourages root growth, strengthens stalks,activates enzymes, controls plant turgidity,transports sugar and starch, helps in proteinformation, controls diseases...and is involvedin many other plant functions.

Potash & Phosphate Institute (PPI) scien-tists have updated and summarized materialfor use in this booklet. We hope this scientificsummary helps contribute to a better under-standing of the importance of K in food andfiber production.

Better Crops/Vol. 82 (1998, No. 3) 3

Potassium: An Essential Plant Food Nutrient

David W. Dibb, President

Potassium is vital to many plant process-es. A review of its role involves under-standing the basic biochemical and

physiological systems of plants. While K doesnot become a part of the chemical structure ofplants, it plays many important regulatoryroles in development.

Enzyme ActivationEnzymes serve as cata-

lysts for chemical reactions,being utilized but not con-sumed in the process. Theybring together other molecules in such a waythat the chemical reaction can take place.Potassium “activates” at least 60 differentenzymes involved in plant growth. The Kchanges the physical shape of the enzymemolecule, exposing the appropriate chemical-ly active sites for reaction. Potassium alsoneutralizes various organic anions and othercompounds within the plant, helping to stabi-lize pH between 7 and 8...optimum for mostenzyme reactions.

The amount of K present in the cell deter-mines how many of the enzymes can be acti-vated and the rates at which chemical reac-tions can proceed. Thus, the rate of a givenreaction is controlled by the rate at which Kenters the cell.

Stomatal Activity (Water Use)Plants depend upon K to regulate the

opening and closing of stomates...the poresthrough which leaves exchange carbon diox-ide (CO2), water vapor, and oxygen (O2) withthe atmosphere. Proper functioning of stom-ates is essential for photosynthesis, water andnutrient transport, and plant cooling.

When K moves into the guard cellsaround the stomates, the cells accumulatewater and swell, causing the pores to open andallowing gases to move freely in and out.

When water supply is short, K is pumped outof the guard cells. The pores close tightly toprevent loss of water and minimize droughtstress to the plant.

If K supply is inadequate, the stomatesbecome sluggish – slow to respond – and

water vapor is lost. Closuremay take hours rather thanminutes and is incomplete.As a result, plants with aninsufficient supply of K aremuch more susceptible towater stress.

Accumulation of K in plant roots pro-duces a gradient of osmotic pressure thatdraws water into the roots. Plants deficient inK are thus less able to absorb water and aremore subject to stress when water is in shortsupply.

PhotosynthesisThe role of K in photosynthesis is com-

plex. The activation of enzymes by K and itsinvolvement in adenosine triphosphate (ATP)production is probably more important in reg-ulating the rate of photosynthesis than is therole of K in stomatal activity.

When the sun’s energy is used to com-bine CO2 and water to form sugars, the initialhigh-energy product is ATP. The ATP is thenused as the energy source for many otherchemical reactions. The electrical charge bal-ance at the site of ATP production is main-tained with K ions. When plants are K defi-cient, the rate of photosynthesis and the rate ofATP production are reduced, and all of theprocesses dependent on ATP are sloweddown. Conversely, plant respiration increaseswhich also contributes to slower growth anddevelopment.

In some plants, leaf blades re-orienttoward light sources to increase light intercep-tion or away to avoid damage by excess light,

4 Better Crops/Vol. 82 (1998, No. 3)

Functions of Potassium in Plants

Potassium (K) increasescrop yield and improvesquality. It is required fornumerous plant growthprocesses.

in effect assisting to regulate therate of photosynthesis. Thesemovements of leaves are broughtabout by reversible changes inturgor pressure through move-ment of K into and out of special-ized tissues similar to thatdescribed above for stomata.

Transport of SugarsSugars produced in photo-

synthesis must be transportedthrough the phloem to other partsof the plant for utilization and storage. Theplant’s transport system uses energy in theform of ATP. If K is inadequate, less ATP isavailable, and the transport system breaksdown. This causes photosynthates to build upin the leaves, and the rate of photosynthesis isreduced. Normal development of energy stor-age organs, such as grain, is retarded as aresult. An adequate supply of K helps to keepall of these processes and transportation sys-tems functioning normally.

Water and Nutrient TransportPotassium also plays a major role in the

transport of water and nutrients throughout theplant in the xylem. When K supply is reduced,translocation of nitrates, phosphates, calcium(Ca), magnesium (Mg), and amino acids is de-pressed. As with phloem transport systems,the role of K in xylem transport is often in con-junction with specific enzymes and plantgrowth hormones. An ample supply of K isessential to efficient operation of these systems.

Protein SynthesisPotassium is required for every major

step of protein synthesis. The “reading” of thegenetic code in plant cells to produce proteinsand enzymes that regulate all growth process-es would be impossible without adequate K.When plants are deficient in K, proteins arenot synthesized despite an abundance of avail-able nitrogen (N). Instead, protein “raw mate-rials” (precursors) such as amino acids,amides and nitrate accumulate. The enzymenitrate reductase catalyzes the formation of


How potassium works to increase crop yields:

• Increases root growth and improves drought resistance• Activates many enzyme systems• Maintains turgor; reduces water loss and wilting• Aids in photosynthesis and food formation• Reduces respiration, preventing energy losses• Enhances translocation of sugars and starch• Produces grain rich in starch• Increases protein content of plants• Builds cellulose and reduces lodging• Helps retard crop diseases

proteins, and K is likely responsible for itsactivation and synthesis.

Starch SynthesisThe enzyme responsible for synthesis of

starch (starch synthetase) is activated by K.Thus, with inadequate K, the level of starchdeclines while soluble carbohydrates and Ncompounds accumulate. Photosynthetic activ-ity also affects the rate of sugar formation forultimate starch production. Under high K lev-els, starch is efficiently moved from sites ofproduction to storage organs.

Crop QualityPotassium plays significant roles in

enhancing crop quality. High levels of avail-able K improve the physical quality, diseaseresistance, and shelf life of fruits and vegeta-bles used for human consumption and thefeeding value of grain and forage crops. Fiberquality of cotton is improved. Quality can alsobe affected in the field before harvesting suchas when K reduces lodging of grains orenhances winterhardiness of many crops.

The effects of K deficiency can causereduced yield potential and quality longbefore visible symptoms appear. This “hiddenhunger” robs profits from the farmer who failsto keep soil K levels in the range high enoughto supply adequate K at all times during thegrowing season. Even short periods of defi-ciency, especially during critical developmen-tal stages, can cause serious losses.

Quality factors are addressed in moredetail on pages 28 and 29.

Potassium (K) is essential to all forms ofplant and animal life. It is a chemicallyactive metal. Because of its highly

active characteristics, K is never found in itspure elemental state in nature. It is alwayscombined with one or more other elements.

Potassium is foundwidely distributed throughoutthe Earth’s crust. It is the sev-enth most abundant elementin the world. It is not onlyfound in every living plantand animal, but is also foundin rocks, minerals, soils,lakes, rivers and oceans.

Agricultural soils maycontain from 2 to 30 tons/A oftotal K in the upper six inch-es. However, most of this ischemically bound in insoluble mineral formsand is unavailable, or only slowly available toplants.

Many soils have been depleted of avail-able and slowly available K by decades oreven centuries of continuous farming withcrop removal exceeding inputs. For example,in any given year, the major field crops grownin the U.S. and Canada, including hay andforage, often remove many times more K thanis applied.

Potash refers to a variety of K-bearingminerals that are used for fertilizer. It includespotassium chloride [KCl, or muriate of potash(MOP)], potassium sulfate [K2SO4, or sulfateof potash (SOP)], potassium-magnesium sul-fate (K2SO4•MgSO4, or sulfate of potash mag-nesia), potassium nitrate (KNO3, or saltpeter),and mixed sodium-potassium nitrate (NaNO3+ KNO3, or Chilean saltpeter).

The term “potash” comes from “potashes” which, in Colonial days in NorthAmerica, were the primary source of K for fer-tilizers and for soap, glass, gunpowder, and

other industrial uses. Potash is sold on the basis of its oxide or

K2O equivalent content. For example, chemi-cally pure KCl contains 52.44 percent K or63.17 percent K2O equivalent. Convertingfrom K to K2O or vice versa is a simple calcu-

lation: percent K2O = percentK x 1.2; percent K = percentK2O x 0.83.

Potash FertilizerMaterials

In North America, MOPaccounts for approximately95 percent of all potash fertil-izers. Potassium sulfate andK2SO4•MgSO4 are also wide-ly used sources, but accountfor less than 5 percent of the

total. Other forms of potash are used only to a very limited extent. The K content of themost common potash fertilizers is shown inTable 1.

Other Potash MaterialsSome industrial wastes offer a source of

potash. Tobacco stems, wool waste, sugar beetfactory waste, flue-dust, and similar wastesfrom many industries can have limited use.

Mixed FertilizersWhile nearly all the potash fertilizer used

comes in one of the forms described above, aconsiderable proportion of it is applied to the


Production and Use of Potassium

Potash fertilizers are minedand refined from under-ground ore deposits, saltlakes and brines. NorthAmerica, the Former SovietUnion, and Europe accountfor almost 90 percent ofworld output. Known potashreserves are estimated to beequivalent to 9 billion tonsK2O.

TABLE 1. Composition of important potash fertilizers.

Form of %potash K2O K

KCl 60-62.5 49.8-51.9K2SO4 50-52 41.5-43.2K2SO4•MgSO4 22 18.3KNO3 44 36.5

soil as part of a fertilizer which contains one ormore other nutrients in addition to potash.

Thus in a 5-20-20 fertilizer, there is 5percent nitrogen (N), 20 percent P2O5, and 20percent K2O by weight.

The analyses are always stated in thesame order. Blended fertilizers, a physicalmixture of such materials as KCl and diammo-nium phosphate, are widely used. Fluid fertil-izers contain relatively small amounts of K inclear solutions but relatively high amounts insuspensions.

Mining of PotashMost potash fertilizers are mined from

underground bedded deposits.In Saskatchewan, Canada, most deposits

are more than 3,000 feet below the surfacewhile in New Brunswick the deposits areapproximately 2,500 feet below the surface.The Canadian deposits are, however, of highgrade, thick, and relatively easy to mine oncea shaft is sunk to the level of the bed. In thearea of Carlsbad, New Mexico, deposits arelocated between 700 and 1,800 feet under-ground.

Solution mining is another method ofextracting potash from underground deposits.In this method, water is injected into thedeposit through a well to dissolve the salts,and a solution containing the K and sodium(Na) chlorides is withdrawn through another

nearby well. This method has advantageswhen the deposit is irregular in shape or whenthe depth is too great for conventional mining.

A third method for mining potash is theevaporation of water from salt lakes andsubsurface brines. This technique is used inthe U.S. at the Great Salt Lake and BonnevilleSalt Flats in Utah, in Searles Lake inCalifornia, and in Israel and Jordan at theDead Sea. The water that is already near thesaturation point in K, Na and Mg salts is fur-ther evaporated in shallow ponds, and the dif-ferent salts are separated.


TABLE 2. World potash production (1997) and reserves by country.

Mine ReserveCountry production base

million tons K2O

Canada 9.3 10,700Russia 3.1 2,400Belarus 2.9 1,100Germany 3.5 960Brazil 0.3 660Israel 1.4 640Jordan 1.3 640China 0.1 350U.S.A. 1.6 260Others 2.4 340World total 25.9 18,050

Numbers may be rounded. Source: U.S. Dept. of Interior, Bureau of Mines.

Giant mining machines cut into the potash ore in underground bedded deposits.

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Potash Production and ConsumptionTotal world production increased from

15.9 million short tons K2O in 1966 to 27.2 in1976 and 31.7 in 1986. Production declinedslightly to 25.3 million tons in 1996 and 25.9million tons in 1997.

Seven countries produced 90 percent ofthe world’s potash in 1997 (Table 2). Canadawas the largest producer at 36 percent, fol-lowed by Germany (14 percent), Russia (12percent), and Belarus (11 percent). U.S. pro-duction represented 6 percent of world output.Other producing countries include Brazil,Chile, China, France, Spain, Ukraine, and theUnited Kingdom, while very little or no potashis produced on the continents of Africa orOceania.

Since the initiation of potash mining inCanada in 1962, production in North Americahas shifted from the U.S. to Canada. The high-grade ore reserves in Saskatchewan and NewBrunswick are more economical to mine thanthe lower grade ore reserves remaining in theCarlsbad, New Mexico, basin.

Europe and the Former Soviet Unionaccounted for 30 percent of world potash con-sumption in 1997, followed by Asia at 25 per-cent, the U.S. at 25 percent, and LatinAmerica at 15 percent.

In the fertilizer year ending June 30,1997, 86 percent of the 10.8 million tons K2Oproduced in North America came fromCanada. Saskatchewan produced about 92percent of Canada’s potash. More than half ofthe Canadian production (60 percent) wasshipped to the U.S.

The use of potash in the U.S. and Canadashowed phenomenal growth in the threedecades preceding 1980. In 1950, 1.5 milliontons K2O were used. By 1960, the figure was2.2 million, and in 1970, 3.5 million. Con-sumption in the two countries peaked at morethan 6 million tons in 1980 with more than 90percent used in the U.S.

Since 1980, potash use in North Americahas fluctuated between 5 and 6 million tonsK2O. Much of this fluctuation is attributable toreduced prices for most crops in recent years,coupled with acreage control programs in theU.S.

Reserves and ResourcesPotash reserves are those that can be

mined economically under short-term market-ing conditions. The reserve base includesdemonstrated resources that are currently eco-nomic (reserves), marginally economic, andsome that are currently subeconomic. Re-sources include proven, probable and inferredreserves.

In 1997, global reserves were estimatedto be 9 billion tons K2O, while the reservebase was estimated at about 18 billion tons(Table 2). Canada and Russia have over 70percent of the total reserve base.

In addition to those listed in Table 2,other producing countries have small reserves(Chile, France, Spain, Ukraine, and theUnited Kingdom) and undeveloped depositsexist in a number of other countries.

Estimated world resources of potash totalabout 250 billion tons. Canada’s potashresources are estimated at about 48 billiontons, and total U.S. resources are estimated atabout 6 billion tons. Most of U.S. resourcesoccur as bedded deposits between 6,000 and10,000 feet below the surface in Montana andNorth Dakota. Some 2 billion tons occur 4,000feet below surface in Utah and at least 25 mil-lion tons about 7,000 feet deep in Michigan.At these depths, solution mining is the onlyrecovery method that is feasible. At presentproduction levels, North America has suffi-cient potash resources for several thousandyears.


Canada is the world’s largest producer of potash.Large reserves remain there and in other countries.

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Crop stress caused by too little K canexhibit itself in many ways...increasedincidence of diseases...lodging in corn

and other crops...less winterhardiness in for-age legumes and grasses...and premature“cutout” in cotton. Potassium stress on corn,for example, will result in theaccumulation of high sugarcontent in the stalks by mid-season. Apparently, the low Klevels in the sap prevent thenormal translocation of sugarto ears, disrupting the growthcycle and preventing normalgrain development. Potas-sium, then, has a direct influ-ence on crop maturity becausethe grain never develops.

Research has shown thatapplication of starter fertiliz-ers containing nitrogen (N),phosphorus (P) and K has-tens the maturity of corn froma few days to as much as aweek. In addition, adding thenutrient that is in shortestsupply advances maturity,whether it is applied throughthe planter or broadcast. The greater the yieldand growth response to the nutrient, the morematurity is hastened.

Potassium hastened silking in corn in aKentucky study. It reduced days from emer-gence to silk, but delayed maturity by asmany as five days. The net effect was anincrease of seven days in time of grain devel-opment...and higher yields. Results are shownin Table 1.

Other studies have shown that K hastenssilking, but most show that it does not shortenthe total production cycle. In other words,there is a longer grain filling period.

The percent of ears silked on a given date

can give an exaggerated impression of maturi-ty differences from fertilizer applications.Data in Table 2 illustrate the point. Resultswere taken from the northern Corn Belt.

The plot that was only 25 percent silkedon August 5 was probably 90 percent silked

about three or four days later,approximately the differencein maturity indicated by themoisture contents of 40.9percent and 39.2 percent.

Illinois research showedthat corn silk emerged soonerwhen K was added to a medi-um K soil. The conclusionswere that the K influence onsilking could boost corn yieldby lengthening the grain fill-ing time. Adequate K helpsprevent pollen shed and silk-ing times from getting mis-matched during hot, dryweather when silking is oftendelayed. Results are shownin Table 3.

Even with earlier silking,K can delay maturity if itdoes influence the length of

time for grain fill. In an Ohio study, optimum


Effects of Potassium on Crop Maturity

Potassium (K) influencescrop maturity in differentways. It speeds silking incorn, but lengthens time ofgrain fill, thus increasingyield potential. Higher mois-ture at harvest in an ade-quate K environment indi-cates K stretches the grow-ing season in corn.Potassium deficiency delaysmaturity in soybeans. Withadequate K, cotton yieldsare increased by extendingthe boll-filling period andpreventing prematuresenescence or “cutout”.The effects of K on maturityof fruits and vegetables arevariable.

Adequate K nutrition offers benefits in developmentand maturity of corn, soybeans and other crops.

N and K rates increased yields, improved Nutilization efficiency, and tended to increasegrain moisture at harvest...an indication of anextended period of grain fill and a definiteeffect on maturity, Figure 1.

The tendency for added K to increasemoisture content may have an indirect effecton physiological maturity as well. Studieshave shown that low K causes early death of


TABLE 2. Relationship between silking date and corn grain moisture.

Percent silked Percent moistureon August 5 at harvest

25 40.980 40.490 39.2

TABLE 3. Potassium hastens corn silking.

K2O rate, lb/A % of plants silked

0 1450 34

100 38200 67

225

200

175

150

125

100

75

50

28

27

26

25

24160 200 232 269 278

Soil test K, lb/A

Yiel

d, b

u/A

Grain m

oisture, %

0 80 160 240 320

N, lb/A

Figure 1. Effect of K and N on corn grain yieldsand moisture content (4-year average).

Yield

55

50

45

40

14.5

14

13.5

13

12.5157 215 297

Soil test K, lb/A

Soyb

ean

yiel

d, b

u/A

Moisture, %

grain moisture

Figure 2. Effect of K levels on soybean grainyields and moisture (4-year average).

corn plant tissue, allowing stalk rot tokill the plant prematurely.

Soybeans can suffer from delayedmaturity because of K starvation.When K is low, plants are usuallystunted. Leaves show yellowish mar-gins, and both leaves and stemsremain green after normal plants havematured and dropped their leaves.

Other Ohio studies comparing combina-tions of low, medium, and high soil tests of Pand K showed no significant interactionsbetween these two nutrients on corn or soy-bean grain moisture (maturity). High soil testK levels reduced soybean grain moisturefrom 14.3 to 13.1 percent at harvest, andyields were increased by almost 10 bu/A(Figure 2).

The overall influence of K on cottonmaturity ranges from speeding it up to noeffect to a delay. Many of these differences canbe related to the level of N nutrition andimbalances with K. Similar to disease and Knutrition effects in corn, low K can result in anincrease in foliar leaf spot diseases in cotton,premature defoliation, and reduced yields. InLouisiana on a low K, acid soil, K had noeffect on first harvest yields without lime. Butwhen dolomitic lime was applied, K increasedboth first harvest and total yields (Table 4).

Proper K fertilization in Mississippiincreased lint yield, with proportionately moreof the yield developing later in the season. Theeffect was similar among four varieties whichrepresented a range in earliness (Table 5).

Recent soil and foliar K fertilizationresearch with no-till cotton in Tennesseeshowed that adequate soil-applied K reduced

TABLE 1. Potassium increases number of grain development days for corn.

K2O Days from Days from Days ofrate, emergence emergence grain Yield,lb/A to silk to maturity development bu/A

0 83 138 55 14260 81 142 61 155

240 80 142 62 170


the percent first harvest from 80 percent downto 65 percent (Table 6), yet increased thefirst harvest yield by 68 percent and doubledtotal lint yield on a low K soil. Some cottonproducers mistakenly believe that K delaysmaturity. Instead, K increases the first harvestyields, the second harvest yields, and total lintyields by allowing plants to take full advan-tage of soil moisture, sunlight and other plantnutrients. Extension of the boll filling periodas a result of improved K nutrition would have

TABLE 7. Effect of K on the number of broccoliterminals mature by midseason.

K2O rate, Terminals mature by midseasonlb/A Location 1 Location 2 Location 3

0 147 139 14885 124 127 130

170 142 116 142225 102 134 119

TABLE 5. Cotton lint yield and percent harvests at two K fertilization rates, averaged among fourvarieties (2-year average).

K2O rate, Lint % at first % at 2 weeks % at 4 weekslb/A yield, lb/A harvest after first harvest after first harvest

0 876 34 34 32100 960 29 34 36

TABLE 6. Potassium effects on earliness of no-till cotton.

Proportion inK treatment, lb K2O/A Lint yield, lb/A 1st harvest,

Soil-applied Foliar-applied 1st harvest 2nd harvest Total %

0 0 411 103 514 800 17.6 623 192 815 76

120 0 694 359 1,053 65120 17.6 648 362 1,010 64

TABLE 4. Potassium and lime increase first harvest and total yields of seed-cotton.

K2O rate, First harvest Total yield,lb/A yield, lb/A lb/A

No lime0 684 1,008

64 685 1,194Dolomitic lime

0 1,057 1,28364 1,400 1,920

little potential for negative economic conse-quence except in the most northern reaches ofthe cotton production belt in North America.

In the more northerly regions with shortgrowing periods, appropriate cultivar selec-tion and timely planting can enhance the abil-ity to capture potential yield benefits fromadequate K nutrition, before a killing frostarrests boll development.

The effects of K on maturity of fruit andvegetable crops vary. In one study, K increasedearly harvest and total yields of tomatoes, buthad no effect in another study. Adequate Khas been shown to reduce premature drop oftomatoes and citrus fruits. There may be aslight early season advantage from K on cauli-

flower. But high K rates have delayed maturi-ty of broccoli, Table 7.

In a North Carolina study, K increasedyields and hastened the maturity of blueber-ries. Early maturity of fruits and vegetablesalmost always offers the advantage of higherprices in the marketplace.

Potassium affects nitrate (NO3) absorptionand reduction. Rapid NO3 uptakedepends on adequate K in the soil solu-

tion. Activity of the enzyme glutamine synthe-tase in wheat is lower when K is deficient.

Potassium stimulates leaf protein synthe-sis. Up to 65 percent of thevariability in grain qualitytraits, such as amino acidmakeup, is due to non-geneticfactors, including K nutrition.

In a nutrient solutionstudy, higher rates of Kallowed for the efficient use ofmore nitrogen (N), whichresulted in better early vegeta-tive growth and higher grainand straw yields as K and Nrates increased.

In the field, better N uptake and utilizationwith adequate K mean improved N use andhigher yields. Crops respond to higher K levelswhen N is sufficient, and greater yield responseto N fertilizer occurs when K is sufficient. Cornstudies in Illinois and Ohio provide examples ofthis economically and environmentally impor-tant interaction (Figure 1).

Potassium UptakeNitrogen form can affect K absorption. For

example, tomatoes grown in nutrient solutionwith NO3-N have shown a higher relative growthrate than plants supplied with ammonium-N(NH4-N). After 4 days, the total K content

decreased in NH4-growntomato plants and remainedconstant in those suppliedwith NO3.

Similarly, when corn wasgrown with either NH4 or NO3as the N source, both yield andtotal N uptake were lower withNH4-N as the source.

However, when the high-est K rate was used, vegetativegrowth (yield) and N and Kuptake were improved with the

NH4-fed plants.It is clear that K interacts with N and is

important in its utilization throughout the cropgrowth and yield production cycle.

Potassium-Phosphorus InteractionsResearch has shown that K interacts with

phosphorus (P) and that together they may inter-act with othernutrients. Agood exampleis the observ-ed reductionof P-inducedzinc (Zn) defi-ciency of cornwhen avail-able K levelsare increased.Manganese(Mn) contentof the cornplants alsoi n c r e a s e s ,


Potassium Interactions with Other Nutrients

Potassium (K) is uniqueamong the essential nutri-ents in the diversity andnumber of roles it plays inplant chemical processes.To perform these varied andmultiple roles, K uptake andutilization often interact withthe availability and uptake ofother nutrients.

50

100

150

200

250

00 80 160 240 320

50

100

150

200

250

00 60 120 180 240

Illinois Ohio

Fertilizer K2O, lb/A 144 96 48 0

Corn

gra

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ield

, bu/

A

Fertilizer N rate, lb/A

Soil test K, lb/A 278 269 232 200 160

Figure 1. Potassium improves yield response to N fertilizer and N efficiency.

indicating there is some relationship of K, P,Mn, and Zn in this complex effect...resulting inless severe Zn deficiency.

A more simple P-K interaction, but per-haps of more widespread importance, is theirsynergistic effect on yield (Tables 1 and 2).

In these cases, besides their individualeffects on yield, P and K together produced anextra 15 percent positive yield interaction forsoybeans and 50 percent for Coastal bermuda-grass.

Potassium, Calcium and MagnesiumInteractions

Low magnesium (Mg) or calcium (Ca) inforages can affect animals by producing lowblood serum Mg or Ca (grass tetany).

Incidence of tetany tends to be lower if for-age Mg exceeds 0.2 percent and Ca exceeds 0.4percent. High plant K can have an antagonisticeffect on Mg concentrations, particularly whenMg is low in soils.

Seasonal changes in forage compositionmay be associated with factors such as levelsand forms of N absorbed by plants. Absorptionof NH4-N may result in greatly reduced uptakeof Ca and Mg while having lesser effects on K.

Large amounts of NH4-N in the soil wouldhave the same effect on a forage as that of K,causing depressed uptake of Ca and Mg.

Sudden rises in temperature tend to beassociated with wider K/(Ca+Mg) ratios, whichcorrespond with a higher grass tetany potential.Higher temperatures tend to increase the Kuptake faster than that of Ca and Mg.

Generally, additions of K, Ca or Mg resultin a lower concentration of the remaining twocations, regardless of the crop grown.

Research has shown that P fertilization offescue pastures can significantly increase Mgand Ca contents of leaves early in the springwhen the potential for tetany is highest.

Potassium/Sulfur InteractionsSulfur (S) nutrition of barley plants has an

influence on the effect of K on Zn uptake fromnutrient solutions. Apparently, good S levelsalong with adequate K improve Zn uptake.

Potassium/Micronutrient InteractionsMany interactions have been reported

between K and micronutrients. Some of thosereported with Zn (as they have involved P and S)have already been noted. Interactions with someof the micronutrients...boron (B), iron (Fe) andmolybdenum (Mo)...have resulted in decreaseduptake when K was added. For others...copper(Cu), Mn and Zn...use of K has increasedmicronutrient utilization.

An interesting observed interaction is thatbetween K and sodium (Na) on alfalfa. When Kis deficient, the classical K deficiency symptomis quite apparent. However, for alfalfa grown onsoils high in Na, the K deficiency symptom hasa somewhat different appearance. See photos onpage 21.

The interactions between K and micronu-trients have not yet been well characterized.Further study, especially under field conditions,is necessary.

SummaryPotassium is known to interact with almost

all of the essential macronutrients, secondarynutrients, and micronutrients. Future improve-ments in yield and quality will require a betterunderstanding and management of these inter-actions.

As livestock feeding operations, industrialuses, and food processors move to special vari-eties and identity-preserved marketing, nutrienteffects on grain quality traits will become evenmore important.


TABLE 1. Phosphorus and K interact for higher soybean yields.

P2O5 K2O Yield,lb/A bu/A

0 0 25.830 0 30.80 120 46.2

30 120 54.9

TABLE 2. Positive P-K interaction produces higher Coastal bermudagrass yields.

P2O5 K2O Yield,lb/A tons/A

0 0 2.680 360 2.63

230 0 3.26230 360 4.55

Plants differ in their ability to take up Kdepending on several factors. The fac-tors that affect availability of K in the

soil and resulting plant uptake are soil factors,plant factors, and fertilizer and managementpractices.

Soil FactorsThe soil itself. This

includes the material fromwhich the soil was formed,the amounts and types of clayminerals in it, the vegetationand climate under which itwas formed, topography anddrainage, and the length oftime it has been forming.

The cation exchangecapacity (CEC) of the soil.This reflects the soil’s abilityto hold K and other cationsand store them in the soil for crop uptake.Clay minerals and soil organic matter are thetwo parts of soil that contribute to CEC. Ingeneral, the higher the CEC of the soil, thegreater the storage capacity and supplyingpower for K.

The quantity of available K in thesoil. This is the value the K soil test reflects.It is the sum of exchangeable K and water sol-uble K. As the level of soil test K decreases,the crop response to applied K increases.

The nonexchangeable or slowlyavailable K. This is the K that is in equilib-rium with available K and renews the soil’ssupply of exchangeable K. For most soils, themore crops depend on nonexchangeable K,the lower the yields.

The K fixation capacity of the soil.Some soils have clay types that can fix largeamounts of K from fertilizers or other sources.This reduces the availability of K to the crop.

The amount of K in the subsoil and

the density or consistency of subsoil lay-ers. Some subsoils are high in K available toroots. Others, such as those formed undergrass in the central Corn Belt, have low Kavailability.

If dense layers (fragipans, etc.) develop inthe subsoil, root penetrationand rooting volumes aredecreased, reducing theavailability of K and othernutrients that are there. Rootsystems are frequently shal-low, with roots concentratedin the upper layers where Ksupply may be adequate, butwhere shortage of water canmake it unavailable to plants.

Soil temperature. Lowsoil temperatures reduce Kavailability and uptake rateby crops. The optimum soil

temperature for K uptake for a crop such ascorn is about 85ºF.

Effects of low temperature can be some-what offset by increasing soil K levels. Row Kcan be important with lower soil temperatures,especially for early planted and minimum tillcrops.

Soil moisture. Moisture is needed for Kto move to plant roots for uptake.

Moisture is needed for root growththrough the soil to “new” supplies of K. It isneeded for mass-flow movement of K to theplant roots with water and for the diffusion ofK to the roots to resupply that taken up by theroots. Drought stress or excess moisturereduces K availability and uptake by crops.Increasing soil K levels can help overcome theadverse effects.

Soil tilth. Tilth is related to the friabilityand ability to get air into the soil. Air is need-ed for root respiration for K uptake. Tillagewhen soils are too wet leads to compaction.


Potassium Availability and Uptake

Availability and uptake ofpotassium (K) is often com-plicated by many interactingcomponents. Two factorsthat have a predominatingeffect are the soil and plantcharacteristics involved. Athird factor, improved fertiliz-er and management prac-tices, can be used to modifythe inherent characteristicsof soils and plants involvingK uptake.

Plant FactorsThe crop. Crops differ in their ability to

take up K from a given soil.This is associated with the type of root

system and surface area of the roots. Grasses,for example, have a much greater capacity totake up K from the plow layer than alfalfadoes. Grasses have many more fibrous,branching roots, increasing the K absorbingsurface.

The variety or hybrid. Crop geneticscome into play with the differences amongvarieties or hybrids of a given crop.Differences are developed through plantbreeding. They usually relate back to the typeof root system, root density, and metabolicactivity that affect K uptake and, hence, avail-ability of K for a given K test. Plant K content,in turn, has major influence on plant waterrelations and metabolic processes, often serv-ing as a regulator of physiological processes.Potassium as a nutrient has a very positiveeffect on root branching and density.

Another factor is that new varieties oftenhave higher yield potentials which increasethe demands placed on soil to supply enoughK. Additional K will be needed under higheryields.

Plant populations. As plant popula-tions increase, yields of some crops aregreater, and demands on soil K are increased.Yields often will not increase with higher pop-ulations unless adequate levels of K are in thesoil, from native or fertilizer sources.


135 145 155 225

5057

6370

K soil test, ppm

Yiel

d, b

u/A

Figure 1. Higher K soil tests can increase soybean yields.

Crop yield. As crop yields increase,total K uptake increases. The uptake per unitof crop yield, such as pounds of K per bushelor ton, may be nearly constant at optimumyield levels.

Fertilizer and Management PracticesIncreased use of nitrogen (N) and

other limiting nutrients. When adequate Kis available, addition of N and/or phosphorus(P) greatly increases K uptake, as yields areincreased. Usually the uptake of K by cropsclosely parallels N uptake and may be greater.So, as limiting nutrients are added, thedemands on soil K increase.

Applications of K in fertilizers,manures or crop residues. The major wayto increase K availability is to apply adequateamounts. Potassium is readily available fromall these sources, provided they are locatedwhere roots can absorb the K. Figure 1 illus-trates how higher soil test levels can increaseyields.

Placement of K. Broadcast plow-downapplications of K are more available than sur-face applied disked-in K. Row K at moderaterates and soil test levels is usually twice asavailable to corn as similar amounts broad-cast. Deep placement or drip irrigation helpsmove K down. Gypsum applied with K alsohelps move K down in very fine textured soils.

Conservation tillage limits availabili-ty of surface applied K. Soil K levelsshould be built to high levels before shifting tominimum or conservation tillage. Thisimproves K distribution within the plow layer.In many fine textured soils, surface applied Kmoves very little in the soil and has low avail-ability, particularly under dryland conditions.

Drainage increases K availability.Draining soils of excess moisture helps manysoils warm up earlier and improves the aera-tion of the soil. This improves the availabilityof soil K.

Weed and insect control. Controllingweeds and insects reduces competition formoisture and nutrients, so that the crop beingproduced has relatively more K available.

Successful farmers know that they mustspend money to make money. They lookfor income-earning inputs that make their

farming operations profitable. However, pes-simism and negative talk cause some to try tosave money by skimping on or omitting income-earning practices. Cuttingcosts in the wrong place mayreduce profits by loweringyields, which makes the unitcosts of production higher.

Potassium (K) fertiliza-tion is one of the vitalincome-earning inputs thatenables farmers to producecrops at lower unit cost. Notonly can K boost yields, but itis also one of the least expensive nutrients tobuy. Many experiments clearly show the eco-nomic benefits of K fertilization for a widerange of soils and cropping conditions in theU.S. and Canada. Several examples have beenselected from such studies to illustrate the sub-stantial impact of K on improving earnings.

In this article, the costs considered for Kfertilization were soil sample analyses, fertiliz-er, application, and harvest costs. Soil sam-ples, taken every 2 years, representing 5

acres, and analyzed for phosphorus (P), K andpH, were assumed to cost $0.75/A/year forchemical analysis. Potassium fertilizer pricewas set at $0.14/lb K2O. Application costs of$3.00/A, associated with dry bulk applica-tions, were assumed unless otherwise noted.

Harvest costs for grain were$0.17/bu, which includedhandling (auger, tractor andlabor), hauling from field tofarmstead, and hauling fromfarmstead to market. Forcorn, an additional harvestcost of $0.18/bu was incurredfor drying, assuming harvest-ed corn was dried to 15.5 per-cent at $0.022 per percent-

age point. This brings the fixed costs of Kapplications (including sampling) to $3.75/Aand the variable harvest costs to $0.17/bu(grain crops other than corn) or $0.35/bu(corn). Base costs for crop production footnot-ed in each table included direct and overheadexpenses.

Corn: Table 1 shows how K fertilizationin Ohio increased corn yields and profits on aCrosby soil testing low to medium in K. Thesedata demonstrate that applying needed K can


Yield and Economic Responses toPotassium

TABLE 1. Potassium fertilization increases corn yields and return per acre by lowering the unit costof production (Ohio).

Additional AdditionalCorn Total harvest costs input costs

K2O grain yield from yield from K Total cost Netrate, yield, income, response to K, fertilization, per bushel, profit,lb/A bu/A $/A $/A $/A $/bu $/A

0 146 328.50 – – 2.05 28.5050 167 375.75 7.35 9.25 1.90 59.15

100 174 391.50 9.80 16.25 1.87 65.45200 187 420.75 14.35 30.25 1.84 76.15400 188 423.00 14.70 58.25 1.98 50.05

Base cost without K = $300/A; soil test K = 126 to 209 lb/A.Corn = $2.25/bu; K applied every other year, so amortized application costs were $1.50/A/yr.

Farmers must reduce theircosts per unit of harvestedcrop in order to optimizeprofits during times ofdepressed crop prices. Highyields distribute productioncosts over more bushels ortons, resulting in lower costsper unit of crop production.

result in higher yields, which lowers produc-tion costs per unit of crop yield.

Tillage and soil management systems canchange the need for fertilizer K. In Minnesota,yield responses to K banded below the seed inridge-till systems have been observed, evenon soils testing high in K (Table 2). Thisresponse is probably related to lower soil Klevels within the ridge. Reduced tillage sys-tems can often have stratified levels of soil Kthat can reduce the availability of K to thecrop under adverse conditions. Other factorswhich can lead to K responses on high K soils

include cool, wet conditions, low soil mois-ture, compaction, low pH, high amounts ofcalcium (Ca), magnesium (Mg), and/or sodium(Na), and the presence of K-fixing minerals.Also, some soils may simply lack the capacityto supply K fast enough to satisfy crop needsduring critical periods of rapid uptake.

Soybeans: Soybeans respond well to K,giving consistent, profitable increases at manylocations. The Ohio data in Table 3 demon-strate the reduction in costs per bushel result-ing from K fertilization. Ohio data have alsoshown the importance of K fertilization in


TABLE 3. Potassium fertilization increases soybean yields and reduces cost per bushel (Ohio).

Additional AdditionalTotal harvest costs input costs

K2O Soybean yield from yield from K Total cost Netrate, yield, income, response to K, fertilization, per bushel, profit,lb/A bu/A $/A $/A $/A $/bu $/A

0 40 250.00 – – 5.13 45.0040 43 268.75 0.51 9.35 4.99 53.8980 45 281.25 0.85 14.95 4.91 60.45

120 48 300.00 1.36 20.55 4.73 73.09

Base cost without K = $205/A; soybean sale price = $6.25/bu.

TABLE 2. Deep banded K boosts yields and profitability on a soil testing 314 lb/A K in a ridge-till system (Minnesota).

Additional AdditionalDeep Corn Total harvest costs input costs

banded grain yield from yield from K Total cost NetK2O rate, yield, income, response to K, fertilization, per bushel, profit,

lb/A bu/A $/A $/A $/A $/bu $/A

0 153 344.25 – – 1.96 44.2520 162 364.50 3.15 7.55 1.92 53.8040 162 364.50 3.15 10.35 1.94 51.0060 159 357.75 2.10 13.15 1.98 42.5080 165 371.25 4.20 15.95 1.94 51.10

Base cost without K = $300/A; corn sale price = $2.25/bu; band application = $4.00/A.

TABLE 4. Potassium fertilization increases soybean yields and quality on a low K soil in Virginia.

AdditionalDockage harvest costs Additional

Total for poor from yield input costs TotalK2O Soybean yield Moldy soybean response from K cost per Netrate, yield, income, beans, quality, to K, fertilization, bushel, profit,lb/A bu/A $/A % $/A $/A $/A $/bu $/A

0 38 237.50 31 132.24 – – 8.87 -99.74120 47 293.75 12 56.40 1.53 20.55 6.03 10.27

Base cost without K = $205/A; soybean sale price = $6.25/bu; dockage for poor soybean quality = $0.12/pt./buover 2%.

adverse conditions, with the greatest soybeanyield and profit increases coming from K indry years. Yield losses incurred from goodyears to dry years were also cushioned by theuse of K.

Substantial improvements in the econom-ics of soybean production occur from thefavorable influence of K on quality. Thisimportant aspect of K fertilization is readilyapparent in Table 4.

Small Grains: Although many of thesoils in the heart of the Wheat Belt are high inavailable K, there are some requiring K fertil-ization for profitable wheat production. Forexample, yield increases of at least 4 to 10bu/A resulted from K additions in six statesand provinces. Returns on investments from Kapplications, usually less than 60 lb/A, were200 percent or higher approximately 60 per-cent of the time.

On a low K soil in northeastern Saskat-chewan, potassium chloride (KCl) fertilizationraised spring wheat yields from 34 to 63 bu/Aand greatly increased profit (Table 5).

No credit was given for higher grain pro-tein in these calculations. When marketdemands exist for higher protein content, highprotein wheat can sell at a premium, providingadditional profits.

Applications of KCl on high K testingsoils have frequently increased yields of bothhard red winter and hard red spring wheat inthe Plains states and Prairie Provinces ofCanada. These responses are apparently dueto K in some situations and to Cl in others.

Responses of barley and wheat to chlo-

ride (Cl) fertilization have been extensivelystudied, and research continues. Chloride isgenerally beneficial in high disease environ-ments where soil Cl levels are low. A numberof diseases in wheat such as the take-all, com-mon and dryland root rots, leaf and striperusts, tan spot, and septoria are suppressed byCl. Response data from more than 200 respon-sive and non-responsive trials in Kansas,Manitoba, Minnesota, Montana, North Dakota,South Dakota, Saskatchewan, and Texas havebeen summarized. These data show that for allsites studied, the average yield increase to Clfertilization was 2.4 bu/A...the average being5.2 bu/A on responsive sites. Assuming anapplication of 30 lb Cl/A, this translates to netreturns of $2.34/A and $11.67/A for all sitesand responsive sites only, respectively.

Cotton: Potassium is essential for maxi-mizing the profitability of cotton production.Table 6 shows a cotton yield response tobroadcast applications of K. Costs consideredfor K fertilization were those assumed earlierin this article ($0.14/lb K2O and fixed inputcosts of $3.75/A). Using K also improved cot-ton lint quality properties including micron-aire and fiber length and elongation (see page28).

Foliar applications of K on cotton can beprofitable for fast-fruiting cultivars grown onsoils low in K. A study in Tennessee showedthat foliar applications of K were profitable forat least two years, even when relatively highrates of K were soil-applied each year. In thisstudy, K was soil-applied at rates from zero to120 lb K2O/A. In addition, 40 lb/A of potassi-


TABLE 5. Adequate K improves spring wheat yield and grain protein content on a low K soil in northeastern Saskatchewan.

Additional AdditionalSpring Total harvest costs input costs

K2O wheat yield Grain from yield from K Total cost Netrate, yield, income, protein, response to K, fertilization, per bushel, profit,lb/A bu/A $/A % $/A $/A $/bu $/A

0 34 119.00 13.3 – – 4.26 -26.0030 45 157.50 13.8 1.87 7.95 3.44 2.6860 43 150.50 13.9 1.53 12.15 3.69 -8.17

120 54 189.00 13.9 3.40 20.55 3.13 20.05240 60 210.00 14.8 4.42 37.35 3.11 23.23480 63 220.50 15.0 4.93 70.95 3.51 -0.38

Base cost without K = $145/A; spring wheat sale price = $3.50/bu.

um nitrate (KNO3) was foliar applied in 10lb/A increments on a 9 to 14 day interval start-ing at or shortly after bloom. The KNO3 was$0.26/lb, and the foliar application costs wereabout $9/A. A recent 10-year averageTennessee cotton price of $0.584/lb wasused. Net revenue gain from foliar fertiliza-tion (Table 7) was calculated by subtractingthe product and application costs from

the additional rev-enue gained from cot-ton yield responses.

Alfalfa: High-yielding alfalfa re-moves large amountsof K from the soil,usually from 50 to 75lb of K20/ton of drymatter. Most farmersdon’t apply enough K for their alfalfa, losingyields and profits while draining soil K sup-plies. The profitability of K fertilization, asin other crops, depends in part on soil test Klevels.

Normally, less K is required on soilswith higher K levels. Some examples ofusing K at the most economic rate are shownin Table 8. Data for Table 8 came fromWisconsin, New York and Pennsylvania. Atlower soil K levels, more K is needed tomaximize profitability.


TABLE 6. Fertilizer K boosts cotton yields (Mississippi).

K2O rate, Lint yield, K input costs, Return to K,lb/A lb/A $/A $/A

0 1,061 – –120 1,169 20.55 42.52

Cotton price = $0.584/lb.

TABLE 7. Economic responses of cotton to foliar applications of K (Tennessee).

Net revenue gained from foliar application of KNO3, $/A,Initial soil test at various rates of soil applied K, lb K2O/AK level, ppm1 Tillage Year 0 30 60 90 120

45 Conventional 1 -3 27 41 39 212 21 18 14 8 23 15 13 3 -14 -394 40 76 82 58 6

40 No-till 1 16 35 46 49 432 46 27 18 20 333 66 31 7 -6 -94 123 66 20 -14 -36

1ppm = parts per million.

TABLE 8. Potassium recommendations and net returns change with soil test levels.

Optimum K rate, K input costs, Yield response, Net return to K,Soil test K lb K2O/A $/A ton/A $/A

Very low 335 50.65 1.2 57.35Low 260 40.15 1.0 49.85High 90 16.35 0.2 1.65

Hay price = $90/ton.

SummaryThe most profitable farm operations are

not the ones that have the lowest operationalcosts. Instead, wise investments are the key tofinancial success. Applications of K can bevery profitable when K is in short supply. Insuch cases, increased returns are achievedonly through increased investments. Identify-ing situations where K is limiting and invest-ing in K fertilizer inputs is a practice that cansubstantially boost profits.

In modern commercial agriculture, symp-toms of potassium (K) deficiency are not ascommonly seen as in earlier years. How-

ever, visual symptoms and “hidden hunger”do still occur in some circumstances. Ofcourse, other conditions can cause poor

growth or yield limitation. It’s also important toconsider possible interactions with othernutrients or treatments.

Following are photos and descriptions ofK deficiency for several major crops.


Potassium Deficiency Symptoms in Some Crops

Soybeans: Firing or scorching begins on outer edge ofleaf. When leaf tissue dies, leaf edges become brokenand ragged...delayed maturity and slow defoliation...shriveled and less uniform beans, many worthless.

Corn: Firing or scorching appearson outer edge of leaf, while midribremains green. May be some yellowstriping on lower leaves. (Sorghumand most grasses also react thisway.) Poor root development, defec-tive nodal tissues, unfilled, chaffyears, and stalk lodging are othersymptoms in corn.

Wheat: Frequently, out-standing hunger signs onleaf itself (no discol-oration, scorching, ormottling), but sharp dif-ference in plant size andnumber, length, and con-dition of roots. Lodgingtendency. Smaller ker-nels. In advanced stages,withering or burn of leaftips and margins, begin-ning with older leaves.

Cotton: Cotton “rust”...first a yel-lowish or bronze mottling in theleaf. Leaf turns yellowish green,brown specks at tip around marginand between veins. As breakdownprogresses, whole leaf becomesreddish brown, dies, sheds prema-turely. Short plants with fewer,smaller bolls of short, weak fibers.In the past, K deficiency symptomshave been described as occurringon older, mature leaves at the bot-tom of the plant. In recent years,symptoms have been observed atthe top onyoung leaves ofsome heavilyfruited cottonvarieties.


Alfalfa: With classical symptoms (shown at top right), first signs of K deficiency are small whiteor yellowish dots around outer edges of leaves...then edges turn yellow and tissue dies andbecomes brown and dry. However, for alfalfa grown on soils high in sodium (Na), the K deficiencysymptom has a different appearance, as indicated in the photo at left above.

Potatoes: Upper leaves usuallysmaller, crinkled and darker greenthan normal with small necroticpatches...middle to lower leavesshow marginal scorch and yellowing.Early indicator: dark green, crinkledleaves, though varieties differ in nor-mal leaf color and texture.


Apples: Yellowish green leaves curlupward along entire leaf...scorchedareas develop along edges thatbecome ragged. Undersized andpoorly colored fruit may drop pre-maturely. Poor storage, shippingand canning qualities in fruit.

Rice: Rice deficient in K mayshow symptoms as stunted plants,a slight reduction in tillering, andshort, droopy, dark green upperleaves. Yellowing may appear ininterveinal areas of lower leaves,starting from the top and eventu-ally drying to a light brown. Long,thin panicles and black, deterio-rated roots may be related to Kdeficiency.

Sugarbeets: The first sign of K deficiency appears as tan-ning and leathering of the edges of recently matured leaves.When the soil solution is very low in Na, a severe inter-veinal leaf scorch and crinkling proceeds to the midrib.Under high Na conditions, tanning and leaf scorch lead toa smooth leaf surface.


Canola: Potassium deficiency reducesgrowth, resulting in smaller leaves and thin-ner stems. Plants are more easily lodged andmay wilt. Under severe deficiency, the edgesof older leaves become yellow, or scorchedand may die completely, but remain attachedto the stem.

Coastal Bermudagrass:Potassium plays an importantrole in heat, drought and coldtolerance of forage grasses.Leafspot diseases may be thefirst symptom of K deficiencyrecognized in Coastal and otherhybrid bermudagrasses. Yellow-ing of older leaves, followed byleaf tip and leaf margin chloro-sis, can occur with severe defi-ciency. Reddish-brown to purplespots, caused by fungal infec-tion, may also be scattered overyounger leaf blades. Thinningstands and reduced growth, fol-lowed by death of older leaves,are frequent symptoms.

Peanuts: Because K is easily redis-tributed from mature to youngerorgans, deficiency symptoms arefirst observable in the older, lowerleaves. Deficiency is expressed bychlorosis of the leaves, beginning atthe leaf margin. Potassium deficien-cy occurs frequently in acidic soils,and symptoms usually appear withinfive weeks of planting.

Grapes: Potassium deficiency symptoms typically appearin early summer on leaves on the middle portion on theshoots. The leaves fade, becoming chlorotic beginning atthe leaf margin, while the center portion of the leaf andveins remain green. The leaves tend to cup downward. Inwhite wine varieties (such as Chardonnay, shown in photo)the leaves become mostly yellow or yellow bronze.

Potassium is a monovalent cation and isreadily adsorbed by the soil’s cationexchange sites. It is not generally con-

sidered to be mobile in soil. Thus, the K avail-able to plants is that in close proximity to theroots. Placement of supplemental K close tothe plant is important forimproving uptake and useefficiency under certain con-ditions. Research is currentlyfocusing on identifying whensuch placement providesbenefits beyond those attainedfrom maintaining higher soiltest K levels.

MethodsResearchers have explored

a number of K applicationmethods including: (1) surface broadcast withand without incorporation; (2) direct seedplacement; (3) row placement (banded) –including all combinations of distances belowand to the side of the seed; (4) plow sole place-ment; (5) deep or knife placement; (6) surface

strip applications; (7) fertigation; (8) highpressure injection; (9) point injection; and(10) combinations of the various methods.

All of these application methods can beconsidered variations or combinations of twobasic placements: (1) banding in high concen-

trations with a minimum ofsoil contact and (2) broad-casting.

ResultsResponses to K place-

ment vary among crop types,soil and environmental con-ditions, and tillage practices.

Corn. Soil characteris-tics can have significanteffects on how corn respondsto K application methods.

Corn on three Illinois soils, low to medium insoil test K, responded differently to broadcastand banded K. Banded K was more effectiveon all three soils tested. Even at high rates ofapplication, broadcast K was not as effectiveas that banded near the seed on two of thethree soils.


Potassium Application Methods

Crop responses to place-ment of potassium (K) fertil-izer are receiving moreattention. Current researchefforts focus upon identify-ing soil characteristics orgrowing conditions that maywarrant placement of highconcentrations of K in thevicinity of developing plantroots.

150

140

130

120

110

100

90

80

0 30 60 90K2O, lb/A

Yiel

d, b

u/A

Row

Soil test K, 224 ppm

Broadcast

Row

Soil test K, 112 pm

Broadcast

Figure 1. Differences between K application responses are smaller as K test rises (Tennessee); ppm=parts per million.

175

147

162

142143

117

105

152152

154152158

158 137

129

127

167158

227

331

Soil K, lb/A 020

4060

Gra

in y

ield

s, b

u/A

Row K 2O, lb

/A

Figure 2. Starter K can be effective even at highsoil test levels (Wisconsin).

Differences among themethods of K applicationusually diminish as K soiltest values rise and as ratesof application increase(Figure 1). The relationshipof starter K response to soiltest K can vary with soil typeand with year. Starter K con-tinued to increase cornyields even at the highestsoil test level in a Wisconsinexperiment (Figure 2).

Tillage is an importantfactor for determining theresponsiveness of corn toapplied K. Potassium defi-ciencies are being observedin the upper Midwest on soilstesting very high in K.Although all of the causes forthis phenomenon have yet tobe revealed, stratification ofK in soils under reducedtillage is an important factor.Potassium supplies near thesurface may be inaccessibleby roots under drier condi-tions, when roots proliferate deeper in soil.Data from Minnesota show that K bandedbelow the seed boosts yields in ridge-till sys-tems even when soil test K levels are high(Table 1). Iowa research has recently shownthat deep-banded K consistently producesmodest yield increases in no-till systems


under optimum to very high soil test K levels(Figure 3).

Compacted, cold or extremely dry soilconditions may favor K starter responses dueto slowed diffusion of soil K to plant roots,even on high K soils. Large amounts of sur-face residue leading to lower soil tempera-tures and higher soil bulk density underreduced till conditions may require starter K

TABLE 1. Corn responds to banded K in ridge-till systems at a high soil test level of 156 ppm (Minnesota).

K2O K2O rate, Corn grain yield, bu/Aplacement lb/A Hybrid A Hybrid B

— — 54 59Fall band 40 92 102Fall broadcast 40 57 68

TABLE 2. Seed-placed K increased barley yield over band or broadcast on K deficient soils (Alberta).

K2O, Application Yield increase, bu/Alb/A method 6 tests 13 tests

15 Broadcast 8.615 Banded 12.8 6.215 With seed 18.8 10.730 Broadcast 17.030 Banded 18.8 8.030 With seed 21.0 12.2

TABLE 3. Chloride contained in muriate of potash can produceyield and kernel weight increases in small grains (Montana).

Rate of chloride WB881 durum WB881 durum (from muriate of potash), wheat yield, wheat kernel weight,

lb/A bu/A g/1,000 kernels

0 49.2 38.240 60.2 40.4

Responsive All trials

180

160

140

120

100

80 Chec

k

Bro

adca

st

Dee

p ba

nd

Chec

k

Bro

adca

st

Dee

p ba

nd

Corn

gra

in y

ield

, bu/

A

Figure 3. Corn responds to deep band place-ment of K on soils testing high to very high in K (Iowa).

38

36

34

32

0 40 80 160

K2O, lb/A sidedressed

Yiel

d, b

u/A

Figure 4. Soybeans respond to K sidedressed at early flower (Alabama).

TABLE 4. Fertilizer K boosts cotton yields (Mississippi).

K2O rate, Lint yield, Boll mass, Lint, Seed mass,lb/A lb/A g/boll % mg/seed

0 1,061 4.1 38.6 90120 1,169 4.4 39.3 94

Increase from K, % 9 7 2 4

TABLE 5. Fertilizer K improves cotton fiber quality (Mississippi).

K2O Span, Length,rate, Strength, Elongation, 2.5% 50% Uniformity Maturity,lb/A g/tex % cm ratio MIC %

0 21.1 7.97 2.82 1.35 48.0 3.7 74.1120 20.7 8.25 2.82 1.37 48.7 4.1 78.3

Increasefrom K, % 0 3 0 1 1 10 5

for most profitable yields. Starter K signifi-cantly increased corn yields on compactedsoils in a Wisconsin study and continued toimprove yields as K soil tests increased.

Soybeans. Soybeans require high Kavailability for best yields and profitability.Soybeans generally do not respond differentlyto broadcast or banded K applications.Responses to applied K have been good ondeficient soils, whether broadcast or banded.Banded applications may be appropriate incool soils or soils with low K fertility. Soybeansare easily injured by contact with salts, so noK fertilizer should be placed in direct contactwith the seed. Foliar applications of K have


produced modest yield increases in somecases, but predictable increases have not yetbeen proven.

Potassium should be applied early forsoybeans, but research from the Midsouth hasshown that K can be applied as late as earlypod development and still produce significantyield increases when moisture is adequate(Figure 4).

Alfalfa. High yield alfalfa has one of thehighest K needs of any crop, frequentlyexceeding 60 lb K2O per ton of hay. Broad-casting and incorporating K to build nutrientlevels before seeding and then topdressing formaintenance is the best approach. This makesK applications more efficient, allowing consis-tently higher yields to be attained at lowerannual application rates (Figure 5).

Deep and extensive root systems allowalfalfa to make good use of soil K. As the standages, large removals of K from past cuttingsmake alfalfa more dependent upon annual Kapplications. Figure 6 shows that alfalfaestablished for several years can have greateryield responses to applied K than newly estab-lished stands.

Small Grains. Limited root systems,shorter growing seasons, and cooler tempera-tures enhance yield advantages of seed-placed over broadcast K for small grains.Barley data from Alberta showed a consider-able advantage for K placed in direct seedcontact at fairly low rates of application(Table 2). High rates of K in direct seed

>150

120-150

90-120

70-90

<700 70 140 210 280 350

K2O applied, lb/A/year

Soil test K, ppm

3.5-3.753.25-3.53-3.252.75-3

Alfalfa yield, tons/A

Figure 5. Applications of K to alfalfa become more efficient as soil test K increases (Wisconsin).

contact may cause germination damage whenhoe or disk-opener drills are used.

Much of the wheat in the U.S. is grown inthe Great Plains. Soils in this arid environ-ment usually contain high levels of K. Conse-quently, K-containing fertilizers have not tra-ditionally been applied. In some areas, lack offertilization has resulted in a depletion of soilchloride (Cl) levels. Chloride is a mobilenutrient and can be leached from soils justlike nitrate. Chloride is an essential nutrientand provides small grains with improved dis-ease resistance or an improved ability to with-stand disease. Muriate of potash (KCl) broad-cast or applied near the seed has producedincreases in yield and kernelweight in some cultivarswhen Cl nutrition has beeninadequate (Table 3).Caution should be used toavoid salt injury whenapplying KCl near the seed.

Cotton. Low root lengthdensity compared to othermajor field crops makes cot-ton particularly sensitive tolow soil K supplies. In fact,cotton can exhibit deficiency symptoms onsoils not considered K deficient. Potassium isimportant for maximizing cotton lint yield andquality. Broadcast applications have beencommonly used to boost cotton yield and fiberquality (Tables 4 and 5). Banded applica-tions of K may be more efficient than broad-cast applications in soils that fix large quanti-ties of applied K.


Potassium is required by cotton through-out the growing season, but needs are greatestduring boll set and development. Preplant ormid-season sidedress applications of K arecommonly used to meet the K needs of cotton.However, even where soil test K levels areconsidered adequate, late-season K deficien-cies have been observed throughout theCotton Belt. Reasons for this phenomenon arestill being investigated, but foliar applicationsof K have provided yield and quality increas-es in many cases (Table 6). Where appropri-ate, foliar fertilization should be used to sup-plement soil applications of K.

SummaryPotassium placement is receiving in-

creasing attention in modern crop production.Proper placement and timing are essential forgetting the most yield response from an appli-cation of K. While many issues remain unre-solved, research will continue to reveal whichmanagement practices provide the most bene-fits to producers.

2.5

2.0

1.5

1.0

0.5

0.01 2 3 4 5 6 7

Age of stand, years

Yiel

d re

spon

se,

tons

/A

Alfalfa yield response to annually applied K

QuebecPennsylvaniaNew YorkManitobaMissouri

Figure 6. Yield response to K application gets larger as the alfalfa stand ages.

TABLE 6. Cotton responds to foliar application of potassiumnitrate (California).

Foliar K applicationapplied during thefollowing weeks Lint yield, Lint yield response,after first flower: lb/A lb/A

Control 1,291 –1 & 2 1,360 693 & 4 1,411 1205 & 6 1,367 767 & 8 1,313 22

What is the effect of K on crop quali-ty? For some crops, improved qual-ity might be more protein or higher

forage feeding value for livestock. It could beimproved persistence of alfalfa stands orreduced drying cost of corn grain or less dock-age for diseased and shriv-eled soybean seed. For veg-etables, it might be greaterconsumer acceptance. Theeconomic return from theinvestment in K can originatefrom: (1) improvement intotal yield; (2) a greater per-centage of total yield which ismarketable; (3) better cropquality; (4) lower cost perunit of production; (5) diseaseresistance; (6) stress toler-ance; and (7) more effective use of otherinputs such as nitrogen (N).

Potassium requirements for top profitproduction systems is best determined on acrop-by-crop basis. Inadequate K disruptsplant development in different ways. Plantsymptoms or growth irregularities signal ashortage of K. Potassium benefits plant growthin the following ways.

Corn• Earlier silking and longer grain fill• Uniform maturity and grain moisture• Improved stalk quality and reduced

lodging• More kernels per ear and better test

weight• Improved N use effectiveness

Soybeans• Improved seed size• Fewer shriveled and moldy beans• Improved oil and protein content• More and larger nodules for N fixation• Better tolerance to pests and improved

resistance to disease

Wheat• Improved grain protein• Better milling and

baking qualities• More efficient use of N• Improved disease

resistance

Forages• Increased winter hardi-

ness and stand longevity• Increased protein quantity and quality• Better N fixation and nodule activity• Increased legumes in legume-grass

swards• Increased vitamin and mineral content• Higher total digestible nutrients• Improved palatability and digestibility of

feed to animals

Fiber Crops Cotton fiber quality is evaluated in terms

of length, strength and fineness of the fiber aswell as its color and cleanliness. Researchstudies show that K improves cotton boll size,micronaire, and strength of cotton fibers. Cloth


The Influence of Potassium on Crop Quality

Potassium (K) is oftendescribed as the “qualitynutrient” for crop produc-tion. With a shortage of K,photosynthesis, respiration,translocation, and a numberof enzyme systems don’tfunction very well. Theresult can be a reduction ofplant growth and, often, ofcrop quality.

TABLE 1. Potassium and N increase marketable yield of tomatoes.

N, lb/AK2O, 120 180 240lb/A tons/A (% marketable)

0 7.1 (41) 7.5 (56) 9.3 (55)300 17.6 (80) 20.8 (85) 26.7 (85)

woven from K deficient cotton fiber resulted inan inferior grade of cloth due to unsatisfactorycolor dying resulting from the nappiness of thefiber.

Vegetables Irish and sweet potatoes, cabbage, cassa-

va, and other vegetable crops require K forboth yield and quality. Where K is limited,tomatoes, potatoes and cabbage often showdiscoloration of the internal tissue.

Both tomatoes and potatoes respond wellto applied K in terms of total yield and percentof that yield meeting strict market standards.Nitrogen and K interact to help achieve maxi-mum economic yield (MEY) of tomatoes(Table 1).

Citrus Potassium is essential for producing

quality citrus. Research with “pineapple”oranges revealed that K influences size offruit, thickness of the rind, and fruit color.Potassium also improved the acid/sugar ratio,soluble solids, and vitamin C content. Theimproved yield was due, in part, to reducedfruit fall from the tree and larger fruit size.Under conditions of severe K deficiency, stemend deterioration of fresh fruit results ingreater loss during transport and a shortershelf life in the super market.

Turf Requirements for K are quality oriented

and include grass color, turf density, winterhardiness, resistance to disease, and resil-ience to traffic. Potassium for improved rootgrowth is believed to be one of the major ben-efits which allow turf grasses to grow out ofstress conditions brought on by insects, dis-ease, and adverse climatic conditions.Producers of sod are interested in how K canimprove plant tiller count, rhizome length, androot density. Balancing K with N nutrition iskey for maintaining a healthy, vigorous turf.

Tobacco Plants fertilized with K resulted in

increased K content, a reduction in nicotine,and an increase in sugar concentrations.

Grapes Quality is influenced as K improves yield

of marketable grapes and helps prevent clus-ter tip, uneven ripening, and pre-harvest shat-tering of fruit.

Sugarcane Yield and quality are closely tied to K

nutrition. This is due in part to K’s influenceon photosynthesis, total leaf area, droughtstress, and disease resistance. A balanced fer-tilization program with N and K produces highjuice quality and the most economical yieldlevel.

Banana Yield and quality are strongly influenced

by K nutrition. It improves fruit weight andnumber per bunch. In addition, K stimulatesearlier fruit shooting and shortens the numberof days to fruit maturity. The beneficial effectsof K on banana fruit quality continue over andabove the level of K required for top fruityield.

Summary The role of K in crop quality has been

documented throughout the world. The influ-ence exists for crops grown in temperate andhumid regions, for legumes and non legumeplants, for annual and perennial crops, and forother crops needed for food, fiber or ornamen-tal purposes.

The quantity of K required to obtainMEY plus quality varies with crop require-ments for K in the growth environment. Insome cases the amount of K required for topyield is adequate for top quality. In othercases, however, the desired level of crop qual-ity and top profit require levels of K exceedingthose normally needed for yield alone. Thisinfluence has been documented for crops suchas tobacco, turf, ornamentals, and some foodand fiber crops.

A balanced nutrition program allows K tocontribute its best toward highest crop yield,quality and profitability.


Nitrogen fixation by trees, shrubs, grass-es, and legumes varies widely in eco-nomic importance to farmers. The most

important involves plants belonging to thefamily Leguminosae and the bacteria genusRhizobium. Crops included within this groupare the pea, bean, soybean,alfalfa, clover, cowpea, andlentil. Each crop performsbest when provided with itsspecific strain of Rhizobiumbacteria through the processof seed inoculation in the topsoil or residual from previouscrops. The bacteria then enter the seedling byinfecting root hair cells and forming a noduleof enlarged plant cells filled with thousands ofbacteria. These bacteria derive nutrition fromthe plant and generate ammonium (NH4

+)from atmospheric N by a process catalyzed bythe enzyme Nitrogenase. Conditions favoringN fixation by Rhizobium bacteria are similar tothose necessary for good growth, vigor and drymatter production of the host plant.

The amount of N generated by a legumevaries with the crop species, soil and cropgrowth conditions, and crop managementpractices. As the total amount of N fixed bybacteria increases, their need for an energysource to reduce N2 to NH4

+ also increases.This energy source is the sugar produced dur-ing photosynthesis. Any disruption in the pho-tosynthesis process will also disrupt the N fix-

ation process. The essential role of K in pho-tosynthesis makes K a vital contributor toeffective N fixation by legumes.

Table 1 provides an estimate of therange in amount of N fixed by severallegumes. The total amount of N fixed will usu-

ally increase with yield level.Any growth factor, such as K,which is necessary for opti-mum legume production,would also influence theamount of N fixed.

The N fixation process isinfluenced by and dependent

upon K for very distinct reasons. • Potassium is the predominant cation in

the plant like calcium (Ca) is in the soil. Its high chemical activity and the pres-ence of a water film around the K ion give it special properties. It acts to neutralize organic acid formed during carbohydrate metabolism, to maintain hydration of cel-lular structures such as membranes and to serve as a cofactor to help enzymes improve the movement of sugars across membranes within the plant.

• Potassium activates more than 60 enzyme systems even though it is not a part of any enzyme structure. The enzyme Nitrogenase, for example, is vital for N fixation.

• Potassium is essential for photosynthesis. Carbohydrates generated by photosyn-thesis provide the energy needed by bac-teria in nodules to fix atmospheric N. Potassium allows for photosynthesis to operate at peak capacity for a longer peri-od of time. Basically, it controls the open-ing and closing of leaf stomates which control the movement of carbon dioxide into the plant and water out into the air. When K is in short supply, photosynthe-sis and water use efficiency decline. At


Influence of Potassium on Nitrogen Fixation

Adequate potassium (K) fertility is important for thesymbiotic relationship thatenables bacteria to fix nitrogen (N) from the air foruse by legumes.

TABLE 1. Estimated N fixed by selected legumes grown under conditions ofoptimum production.

N fixed, lb/AAverage yield High yield

Alfalfa 120 350Clovers 75 250Soybeans 60 240Peas, Vetch 50 180

the same time, the rate of plant respira-tion remains high, resulting in the exces-sive consumption of carbohydrates that should be available for root growth and N fixation.

• Potassium contributes to good root growth and has been shown to improve the num-ber and size of nodules on roots. As shown in Table 2, the application of K to responsive soils can increase both nodule size and number. This results in improvednodule activity and conversion of atmos-pheric N into organic forms of N.

• Potassium allows carbohydrates producedin leaves to get to the root system for use by nodules. One function of K is to serve as a cofactor that is required for the action of the enzyme needed to transport carbohydrates across cell membranes and into the phloem. Once in the phloem,

these sugars can move quickly into the root system to stimulate growth of new root hairs as well as nodule development and function.A highly fertile soil is essential for opti-

mum N fixation. During the first few days aftergermination, seedlings must rely upon energyfrom the seed for initial growth. Then, nutri-ents absorbed by an immature and developingroot system plus photosynthesis occurring in alimited leaf area are expected to nourish theplant for the next 10 days to two weeks or untilnodulation occurs and the N fixing processbegins.

The legume’s inability to fix much N dur-ing the first two weeks of growth is one reasongrowers follow the practice of applying 25 to30 lb of N at planting. This is especiallyimportant for stimulating rapid growth of soy-beans planted immediately after the harvest ofa small grain crop.

Phosphorus (P) and K speed the processof seedling use of N which allows for the ear-liest possible development of an effectivenodulation system.

In summary, the primary importance of Kto N fixation by legumes is to assure rapidseedling development, earliest date of N gen-eration by nodules, and then the formationand delivery of carbohydrates required foroptimum nodule performance.


TABLE 2. Effect of K on soybean yield, nodulation and protein production.

K2O rate, Yield, Nodules Seed protein,lb/A bu/A number/plant weight, g/cu. ft lb/A

0 26 59 10 662120 55 114 26 1,289

Potassium in balance with other nutrientsencourages N fixation by bacteria in nodules onroots of soybeans and other legumes.

Phot

o so

urce

: Run

k/Sc

hoen

berg

er fr

om G

rant

Hei

lman

.

Potassium has been recognized as anessential nutrient in animal nutritionsince its importance was pointed out by

Sidney Ringer in 1883. Potassium is essentialfor life. Young animals will fail to grow andwill die within a few days when the diet isextremely deficient in K.

Potassium is the thirdmost abundant mineral ele-ment in the animal body, sur-passed only by calcium (Ca)and phosphorus (P). Potas-sium concentrations in cellsexceed the concentration ofsodium (Na) by 20 to 30 times.Outside the cell the reverse istrue. Potassium comprisesabout 5 percent of the totalmineral content of the body.

Muscle contains most ofthe total K in the bodies ofanimals (Table 1).

Potassium is contained almost entirelywithin the cells and is the most plentiful ion ofthe intracellular fluids. Potassium is found inevery cell. It is present in tissues and cellsonly in ionic form (K+).

Functions of PotassiumPotassium functions in the intracellular

fluids the same as Na does in the extracellularfluids. The major functions of K in the humanand animal body are to:

• maintain water balance• maintain osmotic pressure• maintain acid-base balance• activate enzymes • help metabolize carbohydrates and

proteins • regulate neuromuscular activity (along

with Ca)• help regulate heartbeat.

Potassium DeficiencyThere are several causes of K deficiency:

inadequate amounts of K in diet, K losses indigestive secretions caused by vomiting anddiarrhea, high intake of Na, increased urina-tion, and stress conditions.

Potassium deficiency maycommonly be manifested bydepressed growth, muscularweakness, stiffness, decreas-ed feed intake, intracellularacidosis, nervous disorders,reduced heart rate, and ab-normal electrocardiograms.

The first sign of K defi-ciency is reduced feed in-take. Many of the other signsstem from reduced feedintake. Potassium must besupplied in the daily rationbecause it is a mobile nutri-

ent and there are not any appreciable reserves.

Potassium Uptake and ControlPotassium is absorbed in the small intes-

tine. Its availability in digestion is nearly 100percent. Most K is lost or excreted in urine.


Potassium in Animal Nutrition

Potassium (K) is essentialfor human and animal life.Potassium is involved inmany body functions and isrequired for proper muscledevelopment. Adequate K isalso important for goodheart function. The recom-mended daily allowance(RDA) of K varies dependingon species, stage of growth,and level of other dietaryminerals.

TABLE 1. Concentration and distribution of Kin animal body.

Tissue or organ K, meq/kg K, %

Muscle 110.0 56.0Skin 58.6 11.1Digestive tract 96.6 5.6Liver 95.0 5.3Red blood cells 106.0 4.2Blood plasma 4.2 2.2Brain 98.6 1.4Kidney 77.6 0.9Lung 79.3 0.5Spleen 130.0 0.4Heart 77.8 0.4Bones and other — 12.6

There is a small amount lost inperspiration. Kidneys play themost important role in mainte-nance and control of K. Understress conditions the kidneystend to excrete more K andconserve more Na.

Potassium in HumanNutrition

The usual American dietnormally contains adequate K.The RDA is 2,500 milligrams(mg). The usual intake is2,000 to 4,000 mg per day.

Problems with K intakecan occur. Diets low in carbohydrates lowerblood K and can cause an irregular heartbeat.Potassium deficiency can become serious dueto K depletion in cases of cirrhosis of the liver,diarrhea, vomiting, diabetic acidosis, bodyburns, and severe protein-calorie malnutri-tion.

Potassium plays important functions ingood cardiac health. Blood pressure is influ-enced by K. It helps overcome the adverseeffect of Na on blood pressure. Sodium can bebalanced with K to maintain normal bloodpressure.

Potassium in Animal NutritionPotassium is especially important in diets

of chickens and turkeys during the first 8weeks. During heat stress, or if there is anydiarrhea, the needed levels may be higher.Adequate K in the ration of laying hensassures good egg production, egg weight, andshell thickness. In starter chicks and turkeypoults, adequate K increases weight gain,improves feed efficiency, and reduces mortality.

Swine K requirement is higher for youngpigs than for older ones. It ranges from about0.33 percent (dry matter basis) in rations ofsmall pigs weighing up to 8 lb, to 0.19 percentin rations of pigs weighing more than 180 lb(Table 2). The K requirement for gestatingand lactating sows is 0.20 percent. Potassiumrequirement increases in diets with higher Naand chloride (Cl) levels.

Ruminants have a higher K requirementthan nonruminants. Potassium is essential for


TABLE 2. Recommended K level, % in dry ration.

Animal Recommended level1

Beef cattle 0.6-0.7Dairy cattle 0.65-1.0Sheep 0.5Swine 0.19-0.33Horses 0.25-0.45Poultry:

Starting chicks 0.30Laying or breeding hens 0.40Turkeys 0.6-0.8

1National Research Council of National Academyof Science.

Proper level of K in the diet is important for all types of animals.Lactating dairy cattle have a high K requirement.

rumen microorganisms. The single most con-sistent effect of suboptimal K in the ration ofruminants is decreased feed intake.

Lactating dairy cattle, particularly high-producing cows, require the highest levels ofdietary K. Under heat stress, their optimallevel of dietary K can be as high as 1.9 per-cent, but the normal National ResearchCouncil (NRC) recommendation is 1.0 per-cent of dietary dry matter (Table 2).

Less K (0.65 percent) is recommendedfor dry cows, calves and heifers. During thelast three to four weeks before calving, exces-sive K in the dry cow diet can increase theincidence of milk fever and retained placen-tas. This can lead to reduced milk productionduring the subsequent lactation. The maxi-mum amount of K desirable in the dry cowdiet depends on the use of anionic salts and

(continued on page 36)

Why does water stress often causeplants to look like they are sufferingfrom K deficiency?

Potassium deficiency can be the mostharmful effect of dry weather. In a drought,water films surrounding soil particles becomevery thin. Because most Kmoves to plant roots throughthese films, drought makes itmuch more difficult for cropsto take up enough K to satis-fy their needs.

Higher K concentrations in the soil solu-tion help to speed K delivery to the root. Thisis why it is so important to have high levels ofK fertility in dry years. With higher levels of Kin the soil, the crop doesn’t have to work ashard to take up the K it needs.

In dry years, crops often contain loweramounts of K than in years with adequaterainfall. For example, Iowa scientists mea-sured sharply lower amounts of K in cornleaves under drought conditions (Table 1).Applying 160 lb/A of K2O raised leaf K con-centrations in the dry year but did not entire-ly compensate for the harmful effects of waterstress. Most agronomists feel that corn leavesmust contain at least 1.7 percent K in the earleaf at tasseling to allow the crop to producemaximum yields.

Research in several states and provinceshas shown that the largest yield increases fromapplied K often occur in dry years.

One example is a long-term K study in

Ohio (Table 2). A good weather year was fol-lowed by a dry year on two occasions. Cornwas the test crop over the first two-year periodand soybeans over the second period. For bothcrops, yields and profits from adding K weregreatest in the year with water stress.

Another 18-year experi-ment in Indiana showed soy-bean response to K to begreater in dry years. Figure 1shows a higher yield increase

from K in years with the lowest rainfall duringthe critical 12-week period after planting.

Potassium cannot protect against extremedroughts, but it does help maintain yields inyears with moderate water stress, typical ofCorn Belt conditions.

Another way that K helps drought-stressed plants is to lower the amount of waterlost through the leaves. Plants have tiny open-ings in their leaves called stomates throughwhich water transpires to the atmosphere.Closing the stomates is a defense mechanism


Potassium Reduces Stress from Drought, Cool Soils, and Compaction

Higher levels of potassium(K) in the soil help cropswithstand stress conditions.

TABLE 1. Stress caused by dry weather reduces the amount of K taken up by corn (Iowa).

K2O rate, K in corn leaves, %lb/A No stress Stress

0 1.1 0.7160 1.6 1.2

100

80

60

40

20

00 5 10 15 20 25

12-week rainfall, inches

Perc

ent y

ield

incr

ease

from

K

Figure 1. Soybean response to potash was greatest in years with lower rainfall during the critical 12-week period afterplanting (18-year study, Indiana).

to conserve water.Plants with inade-quate K can be slow-er (and incomplete)in closing their sto-mates. For example,a Montana experi-ment showed thatbarley plants ex-posed to hot, windyconditions were ableto slow water losswithin 5 minuteswhen they had ade-quate K. But without adequate K, about 45minutes were required for water loss to bereduced.

Additionally, K in plant cells helps keepphotosynthesis going. Plant cells that lose toomuch water slow down in photosynthesisbecause of distortion of their internal parts.Potassium within plant cells has an osmoticeffect that helps retain water. For this reason,tissue K concentrations that are above opti-mum for normal conditions can be necessaryfor stress conditions.

In cool soils, root growth is less, androots are less able to absorb nutrients. Cropsgrowing in cool soils often have reduced Kuptake. If the K shortage is too severe, yieldscan be reduced. Higher K soil tests may benecessary in cooler climates. For example, itrequired a 50 percent higher level of soil K innorthern Wisconsin than in southernWisconsin to obtain a 2 percent level of K inalfalfa plants.

Early planting situations,where the crop grows in coolersoils, may require higher K fer-tility. Extra K increased cornyields 26 bu/A when plantedApril 26, but only 12 bu/Awhen planted June 2 on a low-K soil in Indiana. Plantsreceiving inadequate K arealso more susceptible to frostdamage. A higher yield poten-tial is another factor that mayincrease K need with earlierplanting.

Soil compaction stress-


TABLE 2. The greatest profits and the biggest yield responses to K occurred in the stress years of this long-term experiment (Ohio).

K2O Good Stress Good Stressrate, year year year yearlb/A Corn yield, bu/A Soybean yield, bu/A

0 163 81 56 3550 163 113 59 44

100 167 121 60 52

Response to K, bu 4 40 4 17Profit from K, $/A -5 76 11 92

Corn, $2.25/bu; soybeans, $6.25/bu; K2O, 14¢/lb; medium K soil.

TABLE 3. Row-applied K partially reduced yield losses due to compaction on a soil testing low to medium in K (2-year average, Wisconsin).

Row CompactionK2O, Low High Losslb/A Corn yield, bu/A

0 151 129 2245 169 164 5

Response to K, bu/A 18 35Profit from K, $/A 39 81

Low compaction: < 5 tons/A. High compaction: 19 tons/A.Soil test K: 204 lb/A. Corn, $2.50/bu; K2O, 14¢/lb.

es plants by restricting root growth and mak-ing it more difficult for the roots to take in ade-quate amounts of K and other nutrients.

On soils that are low to medium in K,some of the yield loss from compaction maybe reduced by added K. In a Wisconsinexperiment, 45 lb/A of row-applied K2Oreduced corn yield lost to compaction to 5bu/A from 22 bu/A without K (Table 3).Yield response to K and profits were greateron the compacted soil.

Compaction is one of the reasons whyresponses to K occur more frequently in no-till and ridge-till cropping systems. Other rea-sons include stratification of K near the soilsurface, cooler soil temperatures, and greaterlikelihood of anaerobic soil conditions. An-aerobic conditions can increase the fixation ofK by soil minerals, making it less available toplants. Roots also depend on oxygen and areless active in taking up K when soils becomeanaerobic.

Higher K levels clearly help get cropsthrough periods of stress. Many observationsshow the need to plan a strong K soil fertility

program to make crop yields more certain inan uncertain environment.


Potassium helps crops use water moreeffectively. The positive benefits ofadequate K fertility are:

• Deeper roots. Potash helps plant roots penetrate to access deeper soil water, as illustrated at right.

• Faster closing of the crop canopy. When the crop canopy closes, the ratio of transpiration to evaporation increases, which means more of the water available is used by the crop.

• Greater osmotic gradient. The more K inside the plant cell, the more strongly it can attract water from the soil – and better control its water loss.

• Earlier maturity. Adequate K helps ensure plants will get through the critical pollination period earlier – before drought.

Effective Water Use

other factors, but generally forage K shouldbe less than 2.5 percent. Cool-season for-ages tend to contain more K than warm-sea-son grasses. Thus, problems of excess occurless frequently in southern than in northernregions.

The RDA of beef cattle is about 0.5 to0.7 percent of dry ration (Table 2). Severalstudies have been reported with weightgains of steers on rations containing opti-mum levels of K. In Texas and Tennessee,elevating K levels to 1.4 percent of dietarydry matter helped reduce the stress of ship-ping calves and lambs to feedlots.

Grass tetany and wheat pasture poison-ing are metabolic diseases of lactating cat-tle. These occur most frequently in animals

grazing cool-season forages in which mag-nesium (Mg) concentration or availability islow (less than 0.2 percent). High levels ofK, unbalanced with Mg, can increase risk ofgrass tetany. Milliequivalent ratios ofK/(Ca+Mg) above 2.2 in forage dry matterare considered hazardous. Grass tetany riskis reduced by feeding Mg supplements.Also, fertilizing with phosphorus (P) canenhance plant uptake of Mg.

The assistance of Dr. Steve Leeson, Dr.C.F.M. de Lange, and Dr. Jock Buchanan-Smith of the University of Guelph and Dr.Larry Chase of Cornell is gratefullyacknowledged.

Potassium in Animal Nutrition... (continued from page 33)

Soil Fertility

Disease resistance in crops is genetical-ly controlled. However, natural diseaseresistance mechanisms can be

enhanced by plant nutrients. Potassium defi-ciency symptoms such as thin cell walls,weakened stalks and stems, smaller and short-er roots, sugar accumulationin the leaves, and accumula-tion of unused nitrogen (N)encourage disease infection.Each of these reduces theability of the plant to resistentry and infection by fungal,bacterial and viral diseaseorganisms. A healthy plant,free from stress, is much moreresistant to disease attack.Sound fertility managementand fertilization practices pro-vide assurance that stressinduced by K deficiency is nota factor in crop production.

Several factors influence the effectivenessof K fertilizer in reducing crop stress and dis-ease incidence. These factors include K statusof the soil, K rate and source, nutrient balance,variety/hybrid susceptibility, and disease organ-ism virulence and population. The following areexamples of K fertilization reducing diseasepressure on various crops and the correspond-ing increases in crop yield and quality.

The incidence of leaf spot disease causedby Cercospora, Stemphylium and Alternaria incotton has been related to K fertility. ATennessee study demonstrated the importanceof K fertilization in reducing the severity ofAlternaria leaf spot disease in cotton on a soiltesting low in K. This disease organism cancause significant yield reduction where pre-mature plant defoliation is extensive.Potassium was broadcast and incorporated atthe rate of 0, 30, 60, and 120 lb K2O/A. Therewas also a foliar application component in this

experiment. Partial results from this study arelisted in Table 1.

Disease occurrence may be encouragedby an imbalance between N and K. As pro-duction levels are pushed higher, striving formaximum economic yields, K must be bal-

anced with increased addi-tions of N. The penalties fromthis and other nutrient imbal-ances can be rather severe.An Illinois study showed thebenefits of K fertilization on asoil testing high in availableK with 300 lb/A of N applied.Potassium increased yieldsover four growing seasons byan average of 21 bu/A.Lodging, frequently associat-ed with stalk rot, was reducedin 3 of 4 years (Table 2).

Good K fertility is associ-ated with strong cell walls that enhance dis-ease resistance and the ability of the crop tomaintain firm, healthy stalks. Therefore, a


Effects of Potassium on Plant Diseases

Of all the nutrients essentialfor plant growth and function,potassium (K) is most oftenassociated with reducing dis-ease severity. It should berecognized, however, that Kdoes not work alone. Thehealthiest, most profitablecrops are produced with bal-anced fertility managementpractices that minimize nutri-ent stress throughout thegrowing season.

TABLE 1. Effect of K fertilization on Alternarialeaf spot, defoliation, and cotton lint yields under conventional tillage and no-till conditions.

K2O, Lint yield,lb/A Alternaria1 Defoliation lb/A

Conventional tillage0 7.7 6.9 350

30 5.8 4.5 55660 5.5 2.9 621

120 4.7 1.3 760No-tillage

0 7.5 5.8 36030 6.1 4.2 53160 5.1 1.6 528

120 4.5 0.6 6691Alternaria leaf spot and defoliation ratings, 0=none, 10=highest.

properly fertilized crop such as cornwill have better standability until nat-ural maturity is achieved. The data inTable 3 illustrate the influence of Kfertilization on corn stalk strength andrind thickness.

The incidence of stalk rot andlodging in grain sorghum may also beinfluenced by K fertilization. Twosources of K fertilizer increased grainyield and reduced severity of stalk rot at thehigher levels of fertilization in a Kansas study(Table 4). In addition, a separate Kansasstudy showed that 75 lb/A K2O increasedgrain sorghum yield by 11 bu/A and reducedvisual signs of stalk rot from 6 to 5 nodes affect-ed per plant.

In addition to higher yields andreduced disease susceptibility, good Kfertility management often results inimproved crop quality. A Wisconsinsoybean study illustrated how build-ing soil fertility in K and phosphorus(P) pays in higher crop yield, with lessshriveled and diseased seed thatresults in less dockage at the elevator(Table 5).

The effect of soil K status on soy-bean stem canker infection and plantK concentration was demonstrated in aMississippi study (Table 6). High fertility sta-tus (125 lb P/A and 250 lb K/A) was main-tained on one half of the plots at each of 6sites. The remaining low fertility plots were notfertilized.

Potassium fertilization can increase for-age yield, stand longevity, and disease resis-tance. A Texas study demonstrated the influ-ence of K fertilization on the occurrence ofHelminthosporium cynodontis in Coastalbermudagrass at 2 sites (Table 7). Coastalbermudagrass production in Louisiana bene-fited where leaf-spot disease was evident andforage was infected with the fungus Helmin-thosporium cynodontis (Table 8).

Potassium fertilization reduces diseaseinfection, increases yields, and enhances thequality of many other crops. For example,foliar application of K from several sources tothe first true leaf of cucumber, before inocula-tion with powdery mildew, induced up to 94


TABLE 2. Potassium fertilization effect on corn yields and lodging.

Yield, bu/A Stalk lodging, %Year Control 120 lb/A K2O Control 120 lb/A K2O

1st 148 164 56 602nd 148 164 30 253rd 151 187 30 164th 104 120 52 27

TABLE 3. Effect of K fertilization on corn stalk crushing strength and rind thickness.

K2O Rind Crushingrate, thickness, strength,lb/A mm kg

0 0.91 25460 0.97 349

120 1.00 374

TABLE 4. Effect of K fertilization on sorghum grain yield and stalk rot infection.

K2O, Grain yield, bu/A Plants with stalk rot, %lb/A KCl K2SO4 KCl K2SO4

0 52 52 22 2240 61 75 20 3580 72 74 26 19

160 76 78 16 20320 83 86 4 9

percent systemic protection from this diseaseorganism in a greenhouse study. Research inOregon has shown that a well-managed K pro-gram resulted in increased total yields andreduced incidence of diseases such as stemsoft rot, hollow heart, and brown center inpotatoes. Potassium benefits the market qual-ity of fresh fruits and vegetables and helpsmaintain quality in storage, with less rot anddecay. In turfgrasses, K is associated with the

TABLE 5. Effect of P and K fertilization on soybean yield and quality.

ShriveledFertilization*, lb/A Soybean & diseasedP2O5 K2O yield, bu/A seed, %

0 0 35 20.8400 0 29 12.5

0 400 38 1.8400 400 52 1.3

*Applied only in first year of the five-year study.Soil test P and K levels - medium and low, respectively.

dress application on leaf rust severity andyield of winter wheat. Several Cl sources areavailable, and research has generally foundthat the various sources are equally effective.

SummaryA well-planned fertility program is

essential to the production of healthy cropsthat produce optimum yields and maximizeprofits. Potassium is a critical component ofany fertility program. Increasing the crop’sability to resist disease infection is only one ofmany important practical functions that Kperforms. The number of crops and associateddiseases affected by K is extensive and notpractical to list here.

suppression of several disease organisms andalso increases tolerance to stress and traffic.

Chloride (Cl), like K, is important in reduc-ing the incidence of plant diseases in manycrops (Table 9). Chloride interactions with cropdiseases are well documented. However, themechanisms involved are not well defined.Proposed mechanisms involve suppression ofthe pathogen or increased host tolerance.

Researchers in Texas found that Cl fertil-ization increases wheat yields in years withhigh leaf rust and Septoria pressure. The datain Table 10 demonstrate the effect of Cl top-


TABLE 7. Influence of P and K fertilization on disease severity on Coastal bermudagrass.

P2O5 rate, lb/A Disease scale1

Site 1 Site 2 K2O rate Site 1 Site 2

0 0 0 2.7 3.8282 241 0 3.1 3.9

0 0 120 1.1 1.40 0 240 1.0 1.0

141 120 120 1.3 1.5282 241 240 1.0 1.1

1Disease scale: 1=trace of leaf spot, 5=2 or more leaves dead from disease.

TABLE 8. Potassium fertilization improves yield and stand longevity of Coastal bermudagrass.

Annual Total Two yearK2O rate, Spring stand density estimates, % change, average yield,

lb/A 1st year 2nd year 3rd year % tons/A

0 57 46 39 -18 5.9100 24 55 50 26 6.6200 35 60 72 37 7.1400 42 67 83 41 7.3600 36 60 81 45 7.6

Soil test K level - low.

TABLE 9. Diseases suppressed by chloride.

Crop Disease

Wheat Take-all and common root rot, tan spot, Septoria, leaf rust, stripe rust

Barley Common root rot, spot blotch,Fusarium root rot

Corn Stalk rotRice Stem rot, sheath blightPotatoes Hollow heart, brown centerCelery Fusarium yellowsPearl millet Downy mildewCoconut palm Gray leaf spot

TABLE 10. Chloride decreases leaf rust and increases wheat yield.

Cl rate, Rust rating, Yield,Source lb/A % bu/A

Check 0 67.5 35.4NH4Cl 40 26.3 41.7KCl 40 27.5 42.0MgCl2 40 28.8 40.9Rust rating-flag leaf coverage.

TABLE 6. The influence of reduced levels of soil P and K on soybean plant K concentration and stem cankerinfection level.

Infection Plant Kincrease due decrease due

to low to lowSite fertility, % fertility, %

1 16.6 23.02 36.5 33.03 34.1 33.14 20.7 15.65 41.1 37.86 0 11.8

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PeriodicalsPostage

POWERFUL POTASSIUM

WHAT IS MORE PRECIOUS than silver or gold? Yes, even more necessary than oil or computers? POTASSIUM!

It is essential for all plant, animal and human life.

The importance of potassium in plant life has been recognized formany years. Good yields are impossible without it. High quality cropsdemand it.

Adequate potassium must be present to produce desired sweetness infruits and vegetables, to assure high protein in grains, to use water efficient-ly, to encourage resistance to diseases and insects. No wonder potassiumand quality are synonymous.

This publication tells the story of potassium or “potash” as it is oftenreferred to in agricultural lingo. It has a fascinating history – and will havea key role in the world’s future.

The producers of potassium have long recognized their responsibilityin developing information on proper use of potassium. Over 60 years ago,they established the American Potash Institute (now the Potash &Phosphate Institute). Later, the Foundation for Agronomic Research wasformed. These organizations support research and education through manyprograms, including fellowships, grants and publications. They have anenviable reputation for integrity.

“Integrity is doing what’s right when nobody’s looking.”

PO T ASSIUM - IPNI

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