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Fluid Journal : Fluid Journal 1993-1995
2 Fluid Journal Summer 1994 common. Seed and fruit are small and shriveled. Unfilled, chaffy ears are a sign of potassium deficiency in corn. White/yellowish dots around outer edges of leaves are an indication of K hunger in alfalfa. Leaf then turns yellow and tissue dies. Yellowish green leaves that curl upward in fruit tree crops could mean a shortage of K. Crinkled upper leaves that are usually smaller and a darker green than normal are signs of K starvation in potatoes. Other signs of K deficiency in small fruit are premature dropping and poor storage capability. Availability in soil Although most soils contain thousands of pounds of K--- often 20,000 lbs/A or more--- just a small percent is available to plants over the growing season, probably less than two percent. Soil K exists in three forms: Unavailable K. Unavailable K is found in minerals (rocks). The K is released as soil minerals are weathered, but so slowly as to be unavailable to growing plants in a particular crop year. The weathering process is so slow, in fact, it could take hundreds of years to add significant amounts of available K to the soil. Generally, soils in the eastern parts of the U.S. are more highly weathered than those in the Midwest and West. Less weathered soils are richer in K than those that have undergone more extensive weathering. Slowly available K. Slowly available K is "fixed'' or trapped between layers of certain soil clays. Such clays shrink and swell during dry and wet soil conditions. Potassium ions (K+) can be trapped between these clay layers, becoming unavailable or only slowly released. Highly weathered soils don't contain many such clays. Sandy soils contain lower reservoirs of slowly available K than do those containing greater amounts of clay. Available soil K. Readily available potassium is made up of K found in the soil solution, plus K held in exchangeable form by soil organic matter and clays. Most soils contain 10 lbs/A or less of solution K. This will supply a growing crop barely a day or two. But, as the crop removes solution K, some of the exchangeable potassium (K+ cation) moves into solution. It is replaced on the soil colloid (negatively charged) by some other cation. This movement continues until a new equilibrium is established. So, through the cation exchange process, K is continuously available for plant growth if the soil contains enough K at the beginning of the growing season to supply the crop's needs. Slow mover It is crucial to maintain adequate K fertility levels in the soil because soil K does not move much except in sandy or organic soils. Unlike N and some other nutrients, K tends to remain where fertilization puts it. When K does move, it is usually by diffusion on slow, short trips through water films surrounding soil particles. Dry conditions slow this movement. High soil K levels speed it up. Adding to the problem are crop roots. Crop roots usually contact less than three percent of the soil in which they grow. So the soil must be well supplied with K to ensure that plant needs at every stage of growth are met until harvest. Total root mass of corn occupies less than one percent of the soil volume. This means corn roots contact less than one percent of the available nutrients in the soil. As mentioned earlier, plant roots can take up either soluble K or exchangeable K for use in meeting their needs. Question is: How does K act when it is applied in the soil? In other words, what form does K take when it is applied to the soil and dissolved in the soil solution? The K in fertilizer (commercial fertilizers, organic matter, crop residues, cover crops, etc.) takes on the ionic form (K+) when it dissolves. Thus, K from all sources is the same. Once the K ionizes to the K+ form, it makes no difference what the original source was. 190 180 170 160 150 140 130 120 110 Figure 1. Yield differences between K (120 lbs/A) and no K on Illinois corn yields over four-year period. NoK K (120 lbs/A) Yield (bu/A) Year 1 Year 2 Year 3 Year 4 High K Soil
Fluid Journal 1996-1998