Soil Clay Types
Potassium only takes one form (K+), but there are three basic soil clay types that hold or trap potassium and control its availability. These three soil clay types are shown below.

Muscavite and mica-clay minerals are rich in potassium with concentrations of 80-100,000 ppm. However, the potassium contained in this clay is imprisoned between the clay layers, making it virtually unavailable to the plant roots. You can picture this kind of soil as a brick wall with the potassium being the mortar that holds the bricks together.

Illitic clay types are the product of weathered muscavite and mica clays. Using the brick-wall analogy, an illitic clay would be a muscavite or mica brick wall needing repair. As shown in the figure, the edges of the illitic clay are frayed and wedged open, exposing the interior potassium that is deeper within the clay layers. The frayed edges of the clay can be "repaired" when potassium is applied and captured within the clay walls. Soil-test potassium levels are difficult to increase when illitic clays are present because of this entrapment process.
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The third clay type is montmorillinite or vermiculite. These clay types are lacking all of the interior potassium that normally binds the clay layers together. This allows these clay types to expand and contract during wetting and drying cycles. These clay types hold onto the potassium in a manner that makes the potassium readily available for plant roots.

All three clay types can be present in the soil at the same time. The amount of available potassium will be dependent on the dominate clay type present in the soil. For example, for soils dominated by illitic clays, it would be difficult to increase soil-test potassium levels by applying potash. Most soils have a mixture of clay types; therefore, soils vary in their ability to hold and trap applied potassium.

You should keep your soil-test potassium records and yearly potash application rates to determine if your soil-test levels are building as expected. Due to potassium trapping or "fixation," you will need 8 pounds of K 2O per acre to increase the soil test 1 ppm on the average. If soils contained only montmorillinite clay, about 2.2 pounds of K 2O per acre would be required to increase the soil test 1 ppm. If a soil requires more than 8 pounds of K 2O per acre, then the soil contains more trapping types of clays, and the economics of building soil-test levels become increasingly cost prohibitive.

Cation Exchange Capacity (CEC)
Clay and organic matter are negatively charged and therefore have the ability to hold positively charged cations such as potassium (K+), calcium (Ca++), magnesium (Mg++), sodium (Na+), and hydrogen (H+). In contrast to the imprisoning caused by soil clay types, holding potassium is a good thing. The ability to hold these positively charged cations is called the soil's cation exchange capacity and is an important measure of the soil's fertility. CEC measures the number of available exchange sites--the quantity of cations that can be held by the soil.
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The availability of potassium increases as the percentage of the exchange sites occupied with potassium increases. Therefore, the interpretation of a soil-test report requires knowing the soil-test potassium levels and the CEC. Ideally, the potassium should occupy about 3 to 5% of exchange sites.

For example, a soil-test result of 125 ppm of potassium availability may be adequate or inadequate depending on the CEC of the soil. You determine the percentage of exchange sites by taking the potassium ppm, dividing that number by 390, and then dividing by the CEC of the soil. For a sandy-textured soil with a CEC of 5, 125 ppm of potassium calculates to be 6% of the exchange sites occupied with potassium (125/390/5 = 6%) which would be more than adequate for most crops. On the other hand, a heavy textured soil with a CEC of 30 and a soil-test result of 125 ppm potassium would only represent about 1% of the exchange sites, which would be inadequate.

Conclusion
Clay type and CEC control the amount of potassium that is available for plant uptake. It is difficult to increase the soil's potassium levels in soil that contains a high percentage of clay types that trap the potassium. For such soils, the build philosophy is uneconomical.

Trapping clay types can be identified by tracking soil-test levels and the amount of applied potash. On average, soil-test levels for potassium should increase 1 ppm for every 8 pounds of K 2O applied per acre. The amount of potassium in soil solution increases directly in proportion to the relative amount of potassium occupying the exchange sites (based on CEC). Ideally, 3 to 5% of the exchange sites should be occupied by potassium to ensure adequate potassium is available for plant uptake.

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