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Soils - Part 6: Phosphorus and Potassium in the Soil

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The behavior of each plant nutrient in soil is unique. Each nutrient’s behavior is a combination of attributes that depends on the parent minerals involved and the solubility and mobility of the nutrient in question. Potassium, unlike nitrogen and phosphorus, is not associated to any great extent with organic matter. Total amounts of potassium in soil will vary from 0.3 to more than 2.5 percent. While total content of potassium is important, it is of little value in determining how well a given soil can supply potassium to growing plants. The general terminology used to describe potassium is shown in Figure 6.3.

1. Relatively Unavailable Forms

Depending on soil type, from 90 to 98 percent of soil potassium is in this form. Minerals containing most of the potassium are feldspar and mica. These minerals are the source of soil potassium, and they release potassium very slowly to the more available forms as they weather and break down.

 

 Figure 6.3. Three forms of soil potassium
 

 2. Slowly Available Forms

Potassium in this form is part of the internal structure of the clay minerals forming the colloidal fraction of the soil. Slowly available potassium cannot be replaced by ordinary cation exchange processes and is referred to as “non-exchangeable” potassium. As shown in Figure 6.3, this form is in equilibrium with the available forms and, consequently, acts as an important reservoir of slowly available potassium. An equilibrium exists between “non-exchangeable,” “exchangeable” and “soil solution potassium,” as shown by the arrows in Figure 6.3. Because of this equilibrium, it is possible for some of the potassium applied as fertilizer to be temporarily converted to the “non-exchangeable” form. This is an important reaction in that it helps reduce leaching of potassium from applied fertilizer, especially on sandy soils.

3. Readily Available Forms

Readily available potassium is composed of exchangeable potassium and potassium in the soil solution. Exchangeable potassium is absorbed on the soil colloid surfaces and is available to plants. However, higher plants obtain most of the potassium from the soil solution phase.

The equilibrium between these different forms of potassium is “dynamic.” That is it is always changing; thus, that portion of the total potassium in the different forms ranges from one to two percent for readily available; one to ten percent for slowly available; and 90 to 98 percent in unavailable form.

Perhaps an example using the Thurman loamy sand from Table 6.1 may help clarify the relationships between the different potassium forms shown in Figure 6.3. There are about 247 pounds of available potassium per acre in the top six inches of this soil. If two percent of the potassium were in the soil solution and exchangeable forms, then the soil would contain 247 pounds/.02 = 12,350 pounds potassium per acre six inches of soil.

 Table 6.1.  Exchangeable potassium for crop producing soils from different areas1
 
 Table 6.2.  Relative proportions of total potassium in available and unavailable forms
 

Based on the figures above, the percentage of total potassium in the top six inches of the Thurman loamy sand is 0.61 percent (12,350/2,000,000), which is in the expected range for sandy soils of Nebraska.

One further point to bring out from this example is that Figure 6.3 shows 10 percent of the readily available potassium is normally in the soil solution. The potassium soil test gives readily available potassium, which is the potassium in solution plus the exchangeable potassium. Thus, from Table 6.1, there are 722 pounds of readily available potassium in the top three feet of the Thurman loamy sand. Assume this soil is capable of producing 150 bushels of corn per acre, which has a potassium requirement of approximately 200 pounds. Further, assume the corn will draw its potassium from the top three feet. Thus, at any given time, the amount of potassium available for plant use is ten percent of 722, or 72 pounds of potassium per acre. This is approximately 30 percent of the total potassium required to produce the 150-bushel corn crop. Thus, during the growing season, the soil is continually supplying the potassium required for crop production. It is for this reason that soil test results, properly interpreted using correlation and calibration data, provide an index of how well different soils can supply potassium, and not a measure of pounds of nutrient.

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