Factors Affecting the Availability of Potassium in Soils of the Lower Mississippi Deltas

r I "HE total potassium contents of the soils of the A lower Mississippi deltas are relatively high, being in practically all cases over i%. The responses to potash fertilizer in these soils are very variable. Chemical tests for available potassium in the soils also show widely varying results. The amount of available potassium in a soil is largely limited by the rate at which it is liberated from the primary minerals, by the amount of potassium that the soil can hold in soluble and exchangeable forms, and by the tendency for the potassium to be changed from the soluble and exchangeable forms to the more fixed states from which it is not released by the exchange reactions. It has been the purpose of this work to study (a) the relationships between characteristics of representative soils of the lower Mississippi deltas and the availability and fixation of potassium in the soils; and (b) to determine effects of the decomposition of organic matter, the presence of calcium salts, the variation in soluble potassium, and high moisture content on the availability and fixation of potassium. A review of the literature on the soil conditions or the factors affecting the availability of potassium, especially those bearing upon the fixation and liberation of native and added potassium, shows a wide variation of opinion as to the relative importance of the different causative agents. Attention has been paid to the effect of calcium compounds in liberating potassium in soils. Using a small leaching set-up and working with purified soil colloids, permutits, artificial soils, and natural soils, Jenny (5) concluded that in all these systems lime liberated absorbed potassium in large quantities. McMiller (8) demonstrated that the addition of i% of gypsum gave marked increases in the content of water-soluble potassium after the soils had been kept at approximately optimum moisture for 3 months. , Some of the more recent investigators, on the other hand, have reported entirely different results from the study of the lime-potassium relationship in soils. Dean (2), Harris (4), and Maclntire and his coworkers (7) have presented data which show that lime did not liberate potassium but in some cases it reduced the availability of potassium. Peech and Bradfield (n) studied the effects of lime and neutral calcium salts upon the solubility of potassium and concluded that calcium does not facilitate the conversion of the exchangeable potassium of colloidal clay into less exchangeable forms. They further showed that increases in absorbed calcium favor the absorption of potassium by clays from neutral potassium salts, while increases in the calcium ion concentration result in the liberation of absorbed potassium. Other factors bearing on the release of potassium have been studied. Bartholomew (i) and others (3, 9) have shown that crops removed considerably more potassium, in addition to that added in the manure, from plots fertilized with manure than they did from unfertilized plots, thus indicating that the decomposition of the organic matter converts some of the insoluble minerals of the soil into forms which can be used by plants. Jenny (5), however, found that microorganisms reduced the leaching of potassium as compared with sterile systems. He demonstrated that this reduction was more pronounced in the presence of calcium carbonate. Explanations as to the nature of potassium fixation have been offered and investigated (6, 12, 10). Walker (13) showed that wetting and drying greatly increased the fixation of potassium in soils. Heating the soil to 105 °C resulted in the fixation of 45% of the potassium which formerly was exchangeable. Air-drying the soils resulted in the fixation of 28% or more of the potassium added as potassium chloride. He further showed that additions of organic matter to the soils not only prevented the fixation of applied potassium but changed from nonexchangeable to exchangeable form more than 70 p.p.m. of native potassium. Walker's work indicated that after heating and drying there was a very slow tendency for the colloidal complexes to release the fixed potassium after the soil had been restored to optimum moisture and temperature. Acids weaker than 5 N hydrochloric failed to remove the fixed potassium.