Soil Potassium and Fertility

R. F. Reitemeier.

Plants need large amounts of potassium, one of the three major fertilizer elements. It is supplied to roots by natural sources in the soil and by fertilizers, manures, and mulches.

Our soils, except acid sandy soils and organic soils, such as peat and muck, usually have high contents of potassium often 2 to 3 percent in the surface foot. The total potassium content of our soils generally increases from east to west; that is, in the direction of less severe soil weathering. The content tends to increase from south to north in the eastern half of the country.

Potassium (K) is a silvery-white, soft, highly reactive metal, much like sodium. In nature it does not occur in a free metallic state but is combined in many compounds and minerals. It is found in all living matter, and its salts are used as fertilizers. The term "potash" originally was used for K₂CO₃ (potassium carbonate), produced in the burning of plants, but commonly is applied also to KOH (caustic potash) and to potassium salts such as KCI (muriate of potash) and K₂SO₄ (sulfate of potash). In fertilizer and soil analyses, however, potash signifies the hypothetical potassium oxide, K₂O.

Soil potassium exists in a number of forms. One form is soluble in water. Some forms are insoluble even in strong acids. Others are of intermediate solubility. Most soil potassium is not available to plants even after years of cropping.

A large proportion of it is in primary high-potassium minerals in the silt and sand fractions. These minerals include two micas (muscovite and biotite) and two feldspars (orthoclase and microcline). The minerals in soils of and regions are relatively unweathered and therefore are effective suppliers of potassium to plants. Biotite seems to decompose most readily. It can be an important source of potassium for crops in tropical soils when temperature and moisture are high.

Many soils also contain potassium-bearing, clay-size minerals, the hydrous micas or illites, which have less potassium and more water than the primary micas. Because of their smaller size, which gives them a greater exposed surface area, some forms of illite effectively supply potassium. The high potassium-supplying power of some soils in the Corn Belt is attributed to this source.

Such clay minerals as illite, montmorillonite, vermiculite, and kaolinite, and soil organic matter have cation-exchange capacity that is, the ability to retain on their surfaces cations that can be replaced rapidly by other cations. Potassium ions often constitute from 1 to 3 percent of the exchangeable cations in the soil.

Exchangeable potassium is the important reservoir of readily available potassium. It may be derived from potassium-bearing minerals or from fertilizers, other soil additives, or crop residues. It generally represents only a small part of the total potassium. For example, in a soil containing 40 thousand pounds of potassium (to convert to potash, K₂O, multiply by 1.2) an acre at plow depth, the exchangeable potassium might be 400 pounds; that is, 1 percent of the total.

In turn, the soluble potassium that is free to move with the soil water amounts to a small fraction of the exchangeable quantity, about 1 to 5 percent. The soluble and exchangeable forms are in equilibrium with each other. A reduction of the soluble form by crop removal or leaching is followed instantly by a transfer from the exchangeable form so as to maintain the equilibrium relationship.

When a soluble potassium salt is added to the soil, a transfer occurs in the opposite direction, from solution to exchange surfaces, and the equilibrium is reestablished rapidly at a higher available potassium level. Because of this relationship, no distinction is made between soluble and exchangeable forms in the usual determination of exchangeable potassium by salt or acid extractions or other methods.

When a soluble potassium fertilizer, such as muriate of potash, is applied to some soils that contain expandable lattice clay minerals (like montmorillonite and vermiculite and some forms of illite) a substantial part of it may be converted into a form which is not readily available. Drying appears necessary for this fixation by montmorillonite, but not by illite. G. Brown, of the Rothamsted Experimental Station, in England, has named a potassium-fixing form of illite in Irish soils "degrading illite."

Fixation and release in these minerals are viewed as the comparatively slow entry and exit of potassium ions within the layers of cations located between silica layers of the lattice. When a large fraction of these cation sites is occupied by potassium ions, the lattice is contracted. The degree of subsequent availability of the fixed potassium depends on such factors as the kind of crop, type of mineral, moisture content, and the levels of exchangeable potassium, calcium, and hydrogen.

The illite of some river sediment soils in the Netherlands, named "ammersooite" by J. Temme and H. W. van der Marel, fixes fertilizer potassium so firmly that potatoes and clover cannot be grown successfully without extremely high potash applications, although sugar beets, oats, barley, and wheat yield satisfactorily.

Fixed potassium should not be considered as a total loss but as an addition to the reserve supply forms, which helps to reduce leaching and luxury consumption of soluble and exchangeable forms. Its availability generally is intermediate between that of exchangeable and natural nonexchangeable forms. In the case of illites, fixation may be regarded as the restoration of potassium previously lost from the crystal lattice by weathering, leaching, and cropping.

Another equilibrium exists between exchangeable and clay lattice forms, but this equilibrium is more sluggish than that between soluble and exchangeable forms. The overall equilibrium may be represented as follows: Soluble <-> Exchangeable <-> Lattice (fixed; illitic).

The time required for equilibrium to be established between each pair of forms increases from left to right. An increase of soluble potassium, as from a fertilizer application, results in a movement of some potassium to the right, and a decrease, as from cropping, in a movement to the left.

The release of the nonexchangeable forms to the more readily available exchangeable and soluble forms has been increased by cropping, freezing and thawing, liming, and drying. Drying fixes potassium in some soils if the readily available level is high but releases it if the level is low.

The exchangeable form is never depleted completely by cropping but it often reaches a minimum level characteristic of the soil and the cropping situation. In soils containing all the various forms, the exchangeable potassium value is usually lowest at harvest time and highest in the spring before planting.

Other cations can affect some of the forms of potassium.

When water is added to a soil, a small fraction of the exchangeable potassium changes to the soluble form because of replacement by calcium and magnesium ions originally in solution. During a drying period, some soluble potassium replaces an equivalent amount of calcium and magnesium in the exchangeable forms.

Both the fixation and the release of potassium in illite are favored by an increase in the exchangeable calcium relative to exchangeable hydrogen, which is the basic process in liming. The increased calcium ions. evidently expand some of the clay mineral lattice interlayers sufficiently to facilitate the entry or exit of potassium ions.

Ammonium ions and hydrogen ions (actually hydronium ions, H₃O+) are of about the same size as potassium ions and therefore interfere and compete with potassium in fixation and release reactions involving the inter-layers of expandable lattice minerals. Ammonium added to a soil containing vermiculite, montmorillonite, or degrading illite thus may become fixed and thereby decrease the fixation of subsequently added potassium. The converse would occur where the potassium was applied first.

LOSSES OF SOIL POTASSIUM occur in cropping, leaching, and erosion.

Soils that cannot supply significant amounts of natural and fixed nonexchangeable potassium (such as organic soils; acid, coarse-textured soils; and acid soils that do not contain illites) have no reservoir of reserve potassium to maintain the exchangeable form at a moderate or high level. Potassium removed from such soils by cropping must be replaced frequently by potassium in fertilizers.

Exchangeable potassium is subject to leaching with water by exchange with hydrogen and other cations, and leaching losses in permeable soils in humid regions must be replaced. If clay is abundant in the subsoil, potassium leached from the surface soil may become concentrated there in exchangeable and fixed forms. Erosion of surface soil in extreme cases may cause an appreciable loss of available potassium by the removal of fertilizer particles and of soil particles and organic matter that have high exchangeable potassium content. Because the total content of potassium usually does not vary abruptly with depth, erosion does not alter appreciably the total Potassium of the surface soil.

Crops remove large amounts of potassium from soil, as compared with other nutrients except nitrogen and calcium. The actual amounts are affected by the species, variety, and size of plant and by such factors as level of available potassium, supplies of other elements, soil moisture, soil aeration, and temperature. The potassium contents needed for average to good acre yields therefore should be regarded only as approximate needs.

The grain portion of barley, oats, and wheat crops contains about to pounds. The straw contains about 30 pounds. Corn grain contains 15 pounds and the stover about 50 pounds. The aboveground part of a cotton crop may have a content of 40 pounds, of which about one-third is in the lint and seed. Various grasses contain 25 to 50 pounds. Alfalfa and sweetclover contain 100 to 150 pounds, and other legumes 50 to 75 pounds. Potatoes contain 150 pounds, 100 pounds in the tubers and 50 pounds in the vines. A 15-ton crop of celery may contain 200 pounds of potassium.

When a high supply of readily available potassium is present and other growth conditions are favorable, the uptake by crops may far exceed average requirements. That may result from an increase in the size of the plants and from the luxury consumption of potassium.

Annual crops do not take up potassium at a constant rate but approximately according to the size of the plant at each stage.