The Release of Potassium from Non‐Replaceable Forms in Illinois Soils

Of the various forms of potassium which occur in the soil it is difficult to point to one of them as being of greatest importance to plant growth under all conditions. The plant physiologist would consider the K occurring in the soil solution as being the most important, pointing out that only dissolved materials can pass through the membranes of the feeding roots and that K is not available to the plant root until it has dissolved or been brought into the soil solution. The soil chemist, familiar with soil solution research and the base-exchange equilibria, would point out that the amount of K occurring in the soil solution at any one time is negligible compared to the total amount needed by the plant and might justly claim that ths replaceable K is of greaterimportance since without it the soil solution K could not be continually renewed at a rate sufficiently fast to supply all the K needed by the plant. At this point, therefore, we have the picture of s.everal pounds per acre of K in the soil solution being continually renewed through release of the replaceable K, at first practically as fast as the soil solution K is removed by the plant. As the replaceable K immediately surrounding the feeding root tip is diminished, replenishment in this area from the replaceable form becomes more difficult and the K concentration in the soil solution is lowered. Diffusion of K from more distant areas helps in part to overcome this decrease since in effect a diffusion potential for K is created. This diffusion, however, will not be rapid enough in most cases to supply all the K needs. The root tip, however, is continually pushing into new areas which contain less diminished amounts of replaceable K. In effect the soil solution E is of direct immediate importance to the plant but the replaceable K is essential to the continued renewal of the soil solution K. It so happens that the amount of K in the soil solution, because of base-exchange equilibria, is not directly proportional to the amount of replaceable K or to the K-fertility of the soil and its measure does not give a correla-' tion with K needs (5). On the other hand, the replaceable K value in many studies has been found to be definitely related to the Kfertility of the soil and its measurement has, therefore, been used as a measure of the "available K" in the soil (g), (4), (5), (7). The above soil processes, however, take care only of the more immediate needs of the plant. The water-soluble K is present in amounts sufficient at most for a few hours' to a few days' feeding by the plant and the replaceable K is present in amounts sufficient for a few months to a few years. In spite pi' this, the decrease in replaceable K over a period of years caused by growing crops is not the same as the amount of K removed in the crops but usually much less. Often, the replaceable K level will have decreased only slightly during crop growth and the level will rise significantly between the time the crop is removed and the time the next crop is planted. Of -the existence of this renewal process or processes there can be no doubt, for without it we could not have the yearly removal of large amounts of replacealbe K caused by cropping, without bringing about its complete exhaustion. The renewal process or processes are, therefore, necessary for the continued maintenance of replaceable K which must be kept at a sufficiently high level to supply the soil solution with adequate amounts for plant growth, failing which, the soil becomes K-deficient. It is not possible, therefore, to decide which form of K is the most important without greatly qualifying the decision. The man who buys a farm, however, should not be interested in the amount of K in the soil solution in his soil nor in the level of replaceable K which may happen to be present at the time of sampling except insofar as it represents the soil's ability to maintain that level. Instead, he should be interested in whether or not the non-replaceable K can