ISSUED: 5-73
by Lloyd Murdock, and Kenneth Wells, Extension Specialists in Agronomy, University of Kentucky College of Agriculture

Potassium in Kentucky Soils
Thirteen of the sixteen elements essential for plant growth must come from the soil. And, except for nitrogen, potassium is required by plants in much greater amounts than all the other soil-supplied nutrients. For profitable crop production, there must be an adequate supply of potassium within the root zone of the plants being grown, and this potassium must be in a form which can be used by the plant.
The "total" potassium content in Kentucky soils is much greater than that of nitrogen or phosphorus. Unfortunately much of the "total" potassium is in a mineral form which is unavailable for use by plants. So, even though total potassium content is high, "plant available" potassium levels may be low.

Potassium Content of Kentucky Soils
The potassium content of Kentucky soils can be attributed to two sources--native and fertilizer potassium.
As previously indicated, the quantity of total potassium found in Kentucky soils is relatively high. Table 1 shows the total potassium content of the plow layer (the surface 7 inches) of some typical Kentucky soils.

Table 1.-Total Potassium Content of the Surface 7 Inches of Soils on Experiment Fields in Kentucky*
Soil Class Location of Experiment Field Total Potassium 
Content (lbs/A)
Maury silt loam Lexington 29,000
Crider silt loam Princeton (limestone) 32,600
Tilsit silt loam Princeton (sandstone) 30,000
Monongahela silt loam Berea 19,000
Welston silt loam Fariston (Laurel Co.) 24,400
Bedford & Dickson silt loam Campbellsville 13,000
Tilsit catena silt loam Greenville 24,600
Grenada silt loam Mayfield 29,700
*Kentucky Agricultural Experiment Station Bulletin 397, "Soil Management Experiments."(Out of print; copies available only at libraries.)

Native Potassium -- This is the potassium that was in the parent material (rocks) from which the soil was formed. The potassium in the parent material is contained primarily in two minerals, mica and feldspar. Where the parent material has a relatively high content of mica, potassium is released to an available form at a more rapid rate. On the other hand, where soils are derived from parent materials relatively high in feldspar, as in the sandstone formations, the total potassium will be released at a much slower rate.
Data from a greenhouse study, shown in Table 2, illustrate differences in the rate of release of potassium. On an Eden soil having 300 pounds of "available" potassium per acre, four millet crops removed 1100 pounds of potassium, while on a Tilsit soil with 54 pounds of "available" potassium per acre, the four millet crops removed only 50 pounds of potassium.

Table 2.--Potassium Uptake by Millet Grown on Six Kentucky Soils*
Soil Type Parent Material
Available Potassium (lb/A)
-----Soil Test Level-----
Potassium Removed 
in 4 Millet Crops (lb/A)
After 4 
Eden Calcareous Shale, Siltstone, and Limestone 300 193  1100
Pembroke     Limestone 173  74 275
Maury Phosphatic Limestone 114  73 125
Bedford Limestone 91 49  125
Grenada Loess 78  54  75
Tilsit Sandstone and Shale 54 45  50
*Data from paper by Paul Sutton and W.A. Seay (1958), "Relationship Between Potassium Removed by Millet and Red Clover and the Potassium Extracted by Four Chemical Methods from Six Kentucky Soils." SSSAP22:110

Alluvial (water transported) soil materials may have been carried great distances from their area of origin and, because of this, soils formed from such material may be quite variable in potassium content, depending on the mineralogical composition of the parent material.
Fertilizer Potassium -- Potassium fertilizer has been applied, sometimes in large amounts, to many Kentucky soils. While these applications increase the level of "available" potassium, particularly in and just below the plow layer, they have relatively little affect on the total potassium content of the soil. Much of Kentucky's tobacco land has had heavy applications of potassium fertilizer. On such soils where the level of available potassium has been increased to a high level, the soil will supply all the potassium needed for good plant growth. However, on soils where the available potassium is low, the potassium requirement by crops may be greater than the rate at which a particular soil can supply it, and crop yields will be lowered without the addition of potassium fertilizers. The soil test result of a representative soil sample is the best guide to follow in determining the potassium fertilizer needs of a particular soil.

Potassium Categories
Just as with all the other nutrients, potassium must be present in the soil in a form that is available to plants. Even though the total potassium content of most Kentucky soils is far above the amounts required or removed by crops, many of these soils will not release sufficient potassium for highest crop yields during the growing season. This is because only a very small amount of the total potassium is in the readily available form during a cropping season.
Terms commonly used by soil scientists to describe the different categories of potassium in the soil are "nonexchangeable" or "fixed," "exchangeable" or "available," and "readily available" (see Fig. 1).
Exchange reactions in the soil are such that a balance is maintained between the three categories shown in Figure 1. As the growing crops remove readily available potassium from the soil solution, which we may define as the soil water and the minerals dissolved in it, some of the exchangeable potassium will move into solution. And then some of the nonexchangeable or fixed potassium will move into the exchangeable form. However, nonexchangeable potassium does not become exchangeable as fast as exchangeable potassium becomes readily available. This process is reversible and when the readily available potassium is increased, for example, through fertilization, some of it will revert to exchangeable potassium.
In addition to plant uptake of potassium from the soil solution, some of the exchangeable potassium on the soil colloids is also absorbed directly by roots which touch those colloids. Plant roots possess a negative charge and attract the positively charged potassium (K+) which is held on the clay mineral surfaces and edges.
Soil tests reflect the amounts of exchangeable and readily available potassium present when the soil sample is tested. Tests do not reflect the rate at which the nonexchangeable or fixed potassium can move into the exchangeable form. This explains why some soils may have a relatively low test level and yet supply enough potassium for relatively high crop yields. This is largely influenced by the mineral form of the soil colloids; and, for this reason, consideration must be given to the clay mineralogy of a particular soil in addition to recent potassium fertilization, crop management, and soil test results when planning potassium fertilizer programs.

Potassium Fixation and Release
As previously indicated, only a very small percentage of the total potassium in a soil is in a form available to plants at any given time because the reactions shown in Figure 1, are constantly taking place. With the application of potassium fertilizer, potassium first goes into the soil solution, soon after which much of it goes into the exchangeable and some to the nonexchangeable forms. As crops remove the readily available potassium, the reactions are reversed and exchangeable potassium goes into the soil solution. As a result there is constant fixation and release of potassium in the soil.
Figure 2 illustrates the potassium cycle in the soil. During weathering, physical, chemical, and biological forces act on the parent materials and break them down into finer fractions, largely sand, silt, and clay size particles. This breakdown results in the release of several chemical elements, including potassium, and the formation of different clay minerals.
Most of the total potassium inherited from the parent material during the soil forming processes will be in the nonexchangeable and exchangeable forms. Note in Figure 2 that both exchangeable and nonexchangeable potassium are sources of readily available potassium and that the process is reversible.
The relative amounts of sand, silt and clay fractions found in a soil depend on the kind of parent material (sandstone, limestone, shale or mica) from which the soil was derived. Potassium fixation and release is greatly influenced by the relative amounts of these fractions and the kinds of clay minerals present in the soil.
Sand and Silt Fractions -- The sand and silt fractions of most soils are made up largely of quartz. Other minerals, mainly feldspars, in these fractions may also contain potassium and other nutrient elements but, since the particle size is relatively large, the particles dissolve very slowly and the rate of potassium release is low. Also, because of the physical and mineralogical nature of sand and silt, their ability to fix potassium is low.
Clay Minerals -- Clay minerals (the dominant materials in the clay or colloidal fraction) in a soil are relatively active in fixing and releasing potassium. The different types of clay minerals vary in their capacity to fix and release potassium.
Generally there are four dominant clay minerals in Kentucky soils. Listed here in order of their abundance, they are kaolinite, soil mica or illite, vermiculite, and montmorillonite. No soil is composed of only one of these and, usually, a soil will contain as many as three or four. Each clay mineral has its own characteristics with respect to potassium fixation and release. In addition, each clay mineral contains different amounts of native potassium, which is bonded between the clay layers.
Because of their crystal structure and the location and amount of negative charges within the crystals, illite and vermiculite clays are capable of absorbing potassium from the soil solution and entrapping it between layers of the clay particle (see Fig. 3). The potassium cations are fixed or entrapped in this way because of the relationship of their size to the hexagonal cavities in the silica sheets of two adjoining mica or vermiculite layers. This fixed, or nonexchangeable, potassium is not available to plants but is slowly released as the levels of exchangeable and soil solution potassium become lower.
It will be noted in Figure 3 that kaolinite does not have potassium entrapped between the layers. Soils containing predominantly the kaolinite clay mineral have less exchangeable potassium to release than soils which have a higher percentage of the mica and vermiculite type clay minerals.
Few Kentucky soils, except those in the Purchase Area and those occurring in slack water bottom positions, contain appreciable quantities of montmorillonite clay. The montmorillonite mineral can hold large amounts of exchangeable potassium, but will fix only a small percentage of it. Therefore, most of the potassium held by montmorillonite clay is in an available form.
Influence of Parent Material on Types of Clay Minerals -- Soils derived from calcareous shales are high in exchangeable potassium. These soils contain some illite and vermiculite in their clay and fine silt fraction as well as some readily weatherable potassium-bearing feldspars.
Soils derived from the limestone formations in Kentucky are medium in their ability to release potassium. This suggests that they are composed of a mixture of the clay minerals, with kaolinite predominating but with vermiculite and illite also present.
Soils derived from sandstone and acid shales are low in their ability to release potassium. The major factors being a low percentage of clay minerals in these soils and kaolinite being the predominate mineral present.
The parent materials from which loessial soils are derived are unknown. However, these soils contain montmorillonite and illite clays, and are medium in their ability to release potassium.
Organic Matter -- Growing plants obtain potassium from the soil for their nutrient supply. When the plant residues are returned to the soil, the potassium they contain is readily released and can then be adsorbed to the exchange sites in the soil (see Figure 2). Highly decomposed organic matter is called humus. Because of the negative charges on the humus particles, humus can also adsorb potassium cations in much the same way as the clay minerals and hold them in an exchangeable form for rapid release.
Cation Exchange
Ions with a positive (+) charge are referred to as "cations," while those with a negative (-) charge are referred to as "anions." The interaction of potassium and other cations, such as calcium and magnesium, with the soil colloids is referred to as "cation exchange." This is shown in Figure 4.
The importance of cation exchange capacity (CEC) is that it prevents or reduces the leaching of fertilizer components such as potassium, ammonium, magnesium, calcium, and other cations. Cation exchange is a means by which the soil can store potassium and other cations that may be released later to plants.
As pointed out previously, the soil colloids with negative charges attract and hold the cations. The contribution of the clay mineral fraction to the cation exchange capacity is dependent on both the kinds and amounts of minerals in the soil. The contribution of humus depends on the amount in the soil; though in most Kentucky soils the humus content is, on a percentage basis, very low. While the clay minerals and humus account for most of the CEC, the finer fractions of the silt can also have a limited number of exchange sites.
Of the clay minerals, kaolinite has the lowest CEC (5 to 15 me/100 grams). The CEC of illite is intermediate (10 to 45 me/100 grams), while montmorillonite and vermiculite clay minerals are relatively high (60 to 150 me/100 grams). The CEC of humus is about 140 me/100 grams. These values are for pure clay minerals or humus. The sand and silt fractions account for roughly 75 to 85 percent of the weight of silt loam soils and contribute little to the CEC. The 15 to 25 percent of clay minerals in silt loam soil along with the smell amounts of humus in the surface soil is largely responsible for the CEC. While CEC determinations are not routinely made on soil samples tested in Kentucky soil testing laboratories, most of the silt loams in Kentucky have a CEC of 8 to 12 me/100 grams.
Cations on the exchange sites are held rather loosely on the edges of the clay mineral or humus particles and are constantly being replaced by other cations. They occupy exchange sites because they are balancing the negative charges of the clay minerals and humus fractions in the soil. For this reason the reactions are reversible.

Potassium Fertilization
Crops require relatively large quantities of potassium. On soils where potassium is not released within the plant root zone at rates sufficient to meet the needs of a particular crop, applications of potassium fertilizers are essential if high crop production is to be maintained. The best guide to follow in planning a potassium fertilization program is the soil test result from a good representative soil sample. Soil test results along with past fertilization, cropping history, the crop to be grown, and management of crop residues are most helpful in determining if additional potassium is needed and how much should be applied.
The most common source of potassium is muriate of potash (KCl or potassium chloride). This source is satisfactory for all field crops grown in Kentucky except tobacco. A non-chloride source of potassium such as the sulfate or nitrate form should be used on tobacco because excessive amounts of chlorine lower the quality of tobacco and can cause "white stem."
Because of the reactions previously explained, all the potassium applied as fertilizer is not used by crops the year in which it is applied. Even under ideal conditions, only 40 to 50 percent of the potassium applied will be recovered by the immediate crop. The remainder-held in the soil - is slowly released to succeeding crops, if erosion is controlled and there is no sediment loss. Except for plant removal, erosion is about the only way potassium will be lost from the rooting zone of silt loam and heavier-textured soils.
Time of Application -- Since potassium is not subject to leaching except on very sandy soils, potash fertilizers can be safely applied in the fail if erosion is controlled. The advantages of applying needed potassium in the fall are: (1) avoiding the rush season of spring planting, (2) soil conditions are ideal for spreading, (3) fertilizer dealers are well-supplied with material, and (4) a price discount can often be obtained for fall purchases. On very sandy soils and where there is considerable risk of surface soil losses from erosion during the winter months, potash fertilizer should be applied nearer planting time.
Cation Balance -- The relative concentration of potassium, calcium, magnesium, ammonium, and other positively charged ions in the soil solution will influence the uptake of any particular positively charged cation. With an excessively high concentration of any one of these cations, the plant will take up large amounts of that particular ion. Since the plant can utilize only a certain amount of cations in proportion to anions, an excessive amount of one particular cation may result in inadequate uptake of other essential cations even though there is an adequate concentration in the soil solution. Very high concentrations of calcium in the soil solution may reduce the uptake of potassium. On the other hand, a high concentration of potassium may reduce magnesium uptake. So, even though it is important to keep cation concentrations high enough for good crop growth, it is just as important not to supply so much of any one cation that excessive amounts will limit the uptake of other cations. An adequate supply of all of the essential elements should be available for rapid root development and high yields.
Rate of Application -- When potassium fertilizer is applied to soils low in available potassium, some of the applied potassium will revert to the exchangeable form and some to the nonexchangeable form Heavy rates of application will build up the levels of potassium in the soil to a point where less and less will revert to the nonexchangeable form. At this point lighter applications of potash fertilizers will be sufficient to supply crop needs and maintain a high concentration of available K. As might be expected, a soil test is the best guide for planning the potassium fertilization program.
Crop Management Practices -- A 100-bushel mature corn crop (stover and cobs included) contains about 100 pounds of elemental potassium. Only about 22 pounds of this will be in the grain. That is, about 78 pounds of potassium will be returned to the soil if the stover is left on the field. Likewise, a 40 bushel crop of wheat contains 40-45 pounds of potassium, about 30 pounds in the straw and only 10-15 pounds in the grain. It is obvious that the management of these crop residues will influence the potassium fertilization on the succeeding crops in the rotation. When the whole crop is harvested, as is the case of corn or small grain silage, larger amounts of potassium are removed and heavier applications of potassium fertilizer will be required to maintain an adequate supply in the available form for the next crop.
Another consideration when planning a potassium fertilization program is the intensity of the cropping system. When double cropping is practiced, annual removal of potassium is increased, requiring adjustments in the annual potassium fertilization practices.