SOIL POTASSIUM and FERTILITY Loi High-Potassium Minerals in the Silt and Sand Fractions
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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.