SOIL POTASSIUM AND FERTILITY lOI high-potassium minerals in the silt and sand fractions. These minerals include two micas (muscovite and biotite) and Soil Potassium two feldspars (orthoclase and micro- cline). The minerals in soils of arid regions are relatively unweathered and and Fertility therefore are effective suppliers of potassium to plants. Biotite seems to R, F. Reitemeier decompose most readily. It can be an important source of potassium for crops in tropical soils when tempera- Plants need large amounts of ture and moisture are high. potassium, one of the three Many soils also contain potassium- bearing, clay-size minerals, the hy- major fertilizer elements. It drous micas or illites, which have less is supplied to roots by natural potassium and more water than the primary micas. Because of their smaller sources in the soil and by fer- size, which gives them a greater ex- tiHzers, manures, and mulches. posed surface area, some forms of illite effectively supply potassium. The high Our soils, except acid sandy soils and potassium-supplying power of some organic soils, such as peat and muck, soils in the Corn Belt is attributed to usually have high contents of potas- this source. sium—often 2 to 3 percent in the sur- Such clay minerals as illite, mont- face foot. The total potassium content morillonite, vermiculite, and kaolinite, of our soils generally increases from and soil organic matter have cation- east to west; that is, in the direction of exchange capacity—that is, the ability less severe soil weathering. The con- to retain on their surfaces cations that tent tends to increase from south to can be replaced rapidly by other cat- north in the eastern half of the country. ions. Potassium ions often constitute Potassium (K) is a silvery-white, soft, from I to 3 percent of the exchangeable highly reactive metal, much like so- cations in the soil. dium. In nature it does not occur in a Exchangeable potassium is the im- free metallic state but is combined in portant reservoir of readily available many compounds and minerals. It is potassium. It may be derived from found in all living matter, and its salts potassium-bearing minerals or from are used as fertilizers. The term "pot- fertilizers, other soil additives, or crop ash" originally was used for K2CO3 residues. It generally represents only (potassium carbonate), produced in a small part of the total potassium. For the burning of plants, but commonly is example, in a soil containing 40 thou- applied also to KOH (caustic potash) sand pounds of potassium (to convert and to potassium salts such as KCl to potash, K2O, multiply by 1.2) an (muriate of potash) and K2SO4 (sul- acre at plow depth, the exchangeable fate of potash). In fertilizer and soil potassium might be 400 pounds; that analyses, however, potash signifies the is, I percent of the total. hypothetical potassium oxide, K2O. In turn, the soluble potassium that is Soil potassium exists in a number of free to move with the soil water forms. One form is soluble in water. amounts to a small fraction of the ex- Some forms are insoluble even in changeable quantity, about i to 5 per- strong acids. Others are of intermedi- cent. The soluble and exchangeable ate solubility. Most soil potassium is forms are in equilibrium with each not available to plants even after years other. A reduction of the soluble form of cropping. by crop removal or leaching is followed A large proportion of it is in primary instantly by a transfer from the ex- 102 YEARBOOK OF AGRICULTURE 1957 changeable form so as to maintain the able forms. Its availability generally is equilibrium relationship. intermediate between that of exchange- When a soluble potassium salt is able and natural nonexchangeable added to the soil, a transfer occurs in forms. In the case of illites, fixation the opposite direction, from solution to may be regarded as the restoration exchange surfaces, and the equilibrium of potassium previously lost from the is reestablished rapidly at a higher crystal lattice by weathering, leaching, available potassium level. Because of and cropping. this relationship, no distinction is m.ade Another equilibrium exists between between soluble and exchangeable exchangeable and clay lattice forms, forms in the usual determination of but this equilibrium is more sluggish exchangeable potassium by salt or acid than that between soluble and ex- extractions or other methods. changeable forms. The overall equilib- When a soluble potassium fertilizer, rium may be represented as follows: such as muriate of potash, is applied to Soluble ;=± Exchangeable ;=± Lattice some soils that contain expandable lat- (fixed; illitic). tice clay minerals (like montmorillon- The time required for equilibrium ite and vermiculite and some forms of to be established between each pair of illite) a substantial part of it may be forms increases from left to right. An converted into a form which is not increase of soluble potassium, as from readily available. Drying appears nec- a fertilizer application, results in a essary for this fixation by montmoril- movement of some potassium to the lonite, but not by illite. G. Brown, of right, and a decrease, as from crop- the Rothamsted Experimental Station, ping, in a movement to the left. in England, has named a potassium- The release of the nonexchangeable fixing form of illite in Irish soils "de- forms to the more readily available grading illite." exchangeable and soluble forms has Fixation and release in these minerals been increased by cropping, freezing are viewed as the comparatively slow and thawing, liming, and drying. entry and exit of potassium ions within Drying fixes potassium in some soils the layers of cations located between if the readily available level is high silica layers of the lattice. When a large but releases it if the level is low. fraction of these cation sites is occu- The exchangeable form is never de- pied by potassium ions, the lattice is pleted completely by cropping but it contracted. The degree of subsequent often reaches a minimum level char- availability of the fixed potassium de- acteristic of the soil and the cropping pends on such factors as the kind of situation. In soils containing all the crop, type of mineral, moisture con- various forms, the exchangeable po- tent, and the levels of exchangeable tassium value is usually lowest at har- potassium, calcium, and hydrogen. vest time and highest in the spring The illite of some river sediment soils before planting. in the Netherlands, named "ammer- Other cations can afí'ect some of the sooite" by J. Temme and H. W. van forms of potassium. der Marel, fixes fertilizer potassium so When water is added to a soil, a firmly that potatoes and clover cannot small fraction of the exchangeable po- be grown successfully without ex- tassium changes to the soluble form tremely high potash applications, al- because of replacement by calcium though sugar beets, oats, barley, and and magnesium ions originally in solu- wheat yield satisfactorily. tion. During a drying period, some Fixed potassium should not be con- soluble potassium replaces an equiva- sidered as a total loss but as an addi- lent amount of calcium and magnesium tion to the reserve supply forms, which in the exchangeable forms. helps to reduce leaching and luxury Both the fixation and the release of consumption of soluble and exchange- potassium in illite are favored by an SOIL POTASSIUM AND FERTILITY 103 increase in the exchangeable calcium Crops remove large amounts of relative to exchangeable hydrogen, potassium from soil, as compared with which is the basic process in liming. other nutrients except nitrogen and The increased calcium ions evidently calcium. The actual amounts are expand some of the clay mineral lat- afí'ected by the species, variety, and tice interlayers sufficiently to facilitate size of plant and by such factors as the entry or exit of potassium ions. level of available potassium, supplies Ammonium ions and hydrogen ions of other elements, soil moisture, soil (actually hydronium ions, HgO''') are aeration, and temperature. The potas- of about the same size as potassium sium contents needed for average to ions and therefore interfere and com- good acre yields therefore should be pete with potassium in fixation and re- regarded only as approximate needs. lease reactions involving the inter- The grain portion of barley, oats, layers of expandable lattice minerals. and wheat crops contains about 10 Ammonium added to a soil containing pounds. The straw contains about vermiculite, montmorillonite, or de- 30 pounds. Corn grain contains 15 grading illite thus may become fixed pounds and the stover about 50 and thereby decrease the fixation of pounds. The aboveground part of a subsequently added potassium. The cotton crop may have a content of 40 converse would occur where the po- pounds, of which about one-third is in tassium was applied first. the lint and seed. Various grasses con- tain 25 to 50 pounds. Alfalfa and LOSSES OF SOIL POTASSIUM occur sweetclover contain 100 to 150 pounds, in cropping, leaching, and erosion. and other legumes 50 to 75 pounds. Soils that cannot supply significant Potatoes contain 150 pounds, 100 amounts of natural and fixed non- pounds in the tubers and 50 pounds in exchangeable potassium (such as or- the vines. A 15-ton crop of celery may ganic soils; acid, coarse-textured soils; contain 200 pounds of potassium. and acid soils that do not contain When a high supply of readily avail- illites) have no reservoir of reserve po- able potassium is present and other tassium to maintain the exchangeable growth conditions are favorable, the form at a moderate or high level. Po- uptake by crops may far exceed aver- tassium removed from such soils by age requirements. That may result cropping must be replaced frequently from an increase in the size of the by potassium in fertilizers. plants and from the luxury consump- Exchangeable potassium is subject tion of potassium. to leaching with water by exchange Annual crops do not take up potas- with hydrogen and other cations, and sium at a constant rate but approxi- leaching losses in permeable soils in mately according to the size of the humid regions must be replaced. If plant at each stage. clay is abundant in the subsoil, potas- In experiments with potatoes at the sium leached from the surface soil may Virginia Truck Experiment Station, become concentrated there in ex- R. L. Carolus learned that during the changeable and fixed forms. Erosion 8th week of growth the crop absorbed of surface soil in extreme cases may about I pound of potassium an acre a cause an appreciable loss of available day but absorbed 4 pounds a day potassium by the removal of fertilizer between the loth and 12th weeks. particles and of soil particles and or- The potassium supply during the ganic matter that have high exchange- period of greatest growth should be able potassium content. Because the enough to meet a high rate of demand. total content of potassium usually does Plant roots readily absorb soluble not vary abruptly with depth, erosion and exchangeable potassium, as must does not alter appreciably the total be obvious from the high potassium potassium of the surface soil. requirements of some annual crops. I04 YEARBOOK OF AGRICULTURE 1957 Potassium apparently enters the root are growing in a similar place and are cells in combination with the organic getting enough potassium. Further- compounds produced in metabolic more, this is a general deficiency symp- processes within the plant. Once inside tom for many nutrients. Analyses and the roots, the potassium evidently tests of the soil and plant may reveal a reverts to an ionic form and can move potassium deficiency. rapidly through the plant. Although it The onset of characteristic visual is retained moderately tightly by symptoms, which signifies a more se- living cells, it does not become perma- vere deficiency, means that production nently combined in organic molecules has already been seriously impaired. or structural components and is easily The application then of fertilizer po- removed at the death of the cell. tassium cannot overcome the damage During the time that seeds, fruits, already incurred, especially in annual and nuts develop, potassium moves to quick-growing crops. them from the leaves. Cereal plants If legumes, such as clovers and alfal- normally appear even to lose a portion fa, and grasses are growing together, a of their potassium to the soil as they shortage of potassium may lead to the approach maturity. Appreciable frac- reduction or disappearance of the tions of the potassium content of plants legume without the occurrence of any are lost sometimes by the leaching of severe deficiency symptoms. Grasses potassium from the leaves by rain. and weeds can thrive at levels of avail- This high mobility has hindered the able soil potassium that are inadequate determination of the essential functions for forage legumes. of potassium in the growth of plants. The general leaf pattern when po- tassium is low begins with a yellowing POTASSIUM IS NECESSARY for several of the tips and edges. The yellow area basic physiological functions—the for- then gets broader. The tissues at the mation of sugars and starch and their edges and later the entire leaf die as movement between different parts of the deficiency becomes more severe. the plant, the synthesis of proteins, These symptoms appear first in the normal cell division and growth, and older leaves and later in the younger the neutralization of organic acids. leaves, because in line with the general Potassium also assists different plants tendency of potassium to concentrate in a number of more specialized ways. in the rapidly growing tissues it moves It enhances the size, flavor, and color from the older leaves (at their expense) of some fruits and vegetables. It in- to the younger leaves. creases the resistance of some plants At first, in clover and alfalfa you to particular diseases. Potassium im- usually see small, white spots around proves the rigidity of straw and stalks, the edges of the leaf. As the deficiency so there is less lodging. It increases the worsens, the spots become more nu- oil content of oil-bearing seeds. It helps merous, the edges and entire leaf turn overcome influences of adverse weath- yellow, the edges are scorched, and er, such as low soil moisture and low the older leaves drop. temperature, and of poor physical soil In corn, the tips of the older leaves first conditions, such as compaction and become yellow. Streaks of yellow run inadequate aeration. lengthwise through the leaves. The You may not be able to notice any edges become scorched. The stalks are deficiency effects in plants that have weak and short. The ears are small, moderately inadequate levels of potas- poorly filled, and chaffy at the tip. sium because the usual symptom of A deficiency symptom in cotton, this degree of deficiency is a general known as cotton rust, first appears as reduction in growth. That is not easy yellowish, mottled margins of the leaf to detect unless you compare the size and yellow spots between the veins. of the plants with that of others that They finally merge to make a dry, SOIL POTASSIUM AND FERTILITY 105 curled, reddish-brown leaf, which drops slum level and low nitrogen or phos- prematurely. Unopened and partly phorus supply, luxury consumption of opened bolls containing cotton of poor potassium is to be expected. quality result from this extreme defi- ciency of potassium. SODIUM IS NOT CONSIDERED to be an essential plant nutrient, but some THE SUPPLY of potassium to plants plants (for example, beets, celery, often affects and is affected by the turnips, and cabbage) require it for level of other nutrients. maximum production even in the In soils containing expandable lat- presence of ample potassium. tice clay minerals, an increase in ex- Another group of plants, including changeable calcium sometimes causes barley, oats, wheat, cotton, tomatoes, the fixation or release of nonexchange- asparagus, and alfalfa, respond to able potassium, depending on whether sodium when the potassium supply is the exchangeable potassium level is inadequate. relatively high or low. Under the con- A third large group of crops respond dition of very high soluble or ex- to sodium slightly or not at all at any changeable calcium and very low potassium level. Among them are corn, exchangeable potassium, the calcium rye, potatoes, lettuce, and soybeans. may depress the immediate supply of Sodium or potassium generally will potassium to plants. That appears depress the uptake of the other cation, typical also of some calcareous soils. but the results of this mutual relation- But normally within a wide range of ship in any particular situation will saturation of the cation-exchange ca- depend on the levels of available so- pacity with calcium, the calcium has dium and potassium and the relative only a minor effect on the uptake of ease of absorption of the two cations exchangeable potassium. Actually an by the plant. increase in the exchangeable potassium level may reduce the uptake of calcium THE EFFICIENT MANAGEMENT of soil or magnesium and cause the lux- with respect to potassium must be ury consumption of potassium, even based on a number of soil-manage- though the absolute potassium level is ment factors: The kind of crop, the very low relative to the level of the rotation system, the livestock-manage- other two cations, as is customary. ment system, the nature of the soil, the Such an effect has been observed in liming and fertilizer practices, and the alfalfa, clover, tomatoes, apples, and weather. prunes. Satisfying the potassium require- The usual effects of nitrogen and ments of a cropping system should be phosphorus, the other two major fer- based first on the natural potassium- tilizer nutrients, are associated with supplying ability of the soil. In this nutrient balance in the plant. If the regard soils range from organic and supply of nitrogen and phosphorus is acid sandy soils (which cannot be de- high relative to that of potassium, pended on for any natural reserve sup- growth may be rapid at first, but the ply) to clay soils that contain large potassium concentration in the plant amounts of relatively unweathered po- may become reduced to a deficiency tassium minerals, which do not have to level. Thus, even though the total be supplemented by potash fertilizer. potassium uptake by the plant may be Soils having little or no reserve po- increased by the high nitrogen and tassium supply and low cation-ex- phosphorus levels, additions of potas- change capacities require the frequent sium to the soil would be necessary to additions of small or moderate amounts maintain the nutrient balance re- of potassium. Large single applications quired for rapid, continued growth. to such soils may result in higher losses In situations of high available potas- through leaching and unbalanced nu- io6 YEARBOOK OF AGRICULTURE 1957 trient relations in the crop through plant, to animal, to manure, to soluble, luxury consumption. Leaching losses to exchangeable, to mineral (fixed). from rain can be serious in winter- After the forage plants have absorbed fallowed soil of regions in which win- soluble soil potassium which has been ters are warm. A winter cover crop replaced from the exchangeable form reduces this loss. The continued re- and released earlier from mineral lat- moval of hay crops, however, severely tice forms, the pastured animals con- depletes the soil potassium, and it must sume it during their feeding. A large be increased by adding potash. fraction of it is returned to the soil in When rotations include row crops the animal manure, some of which be- that respond markedly to potash (for comes, in turn, soluble, exchangeable, example, cotton, tobacco, and pota- and finally fixed in mineral lattices. toes), a potash application should be The main loss of potassium in this cycle made to these crops at planting time. arises from the removal of the animals Excessive rains after planting may and animal products from the land. make additional applications necessary Byron T. Shaw, of the Department to replace leaching losses. of Agriculture, has estimated that in Legume crops that are removed from such a system, 75 to 90 percent of the the land severely lower the available potassium removed from the soil would potassium level of soils. Liming of acid be returned to it. Other cropping and soils improves various growth condi- livestock systems will conserve a small- tions and thereby increases potassium er fraction. If sheltered animals are fed requirements of legumes, but it also hay from the same farm, losses of po- reduces the leaching of potassium. In tassium may occur if the manure is a study at the North Carolina Agri- improperly conserved before it is ap- cultural Experiment Station, Adolph plied to the soil. Mehlich found that even clay subsoils If the straw of small grain and corn would not retain potassium effectively crops is not removed from the land, until they were limed. only about one-fourth of the potaissium If sodium is applied to the soil, either in the crop is permanently lost from as sodium nitrate or as a treatment for the soil. The harvested portions of po- a sodium-responsive crop, the extent tatoes, celery, other vegetable crops, of the substitution of this cation for and tobacco contain much higher frac- potassium should be considered in tions of the total crop potassium. Much estimating potash applications. of the potassium in leaves of deciduous Loams and clays containing an abun- fruit and nut trees that drop on the dance of illitic clay minerals or un- ground eventually will return to avail- weathered primary potassium minerals able soil forms. In places where the can be expected to supply from a native potassium supply is inadequate moderate fraction to all of the potas- to replace losses, fertilizer potash must sium required for a cropping system. be applied. In many soils, a fraction of Younger soils in this group can be it will be fixed, and the availability of cropped for many years before ferti- the application to the current crop will lizer potash has to be included in the be lowered accordingly. This fixed soil-management program. The exact potassium will be slowly available in length of time, of course, depends on the future, however, and so improve the the amount of potassium in crops re- ability of the soil to supply potassium. moved from the land and on other The determination of currently avail- losses from the soil. able potassium is made by a soil test or by a plant tissue test. Information IN A SOIL-PASTURE-LIVESTOGK man- about the reserve supply is obtained agement system on such soils, the potas- from other laboratory measurements sium cycle would be as follows: Min- and from the history and general eral, to exchangeable, to soluble, to knowledge of the particular soil.