EFFECT OF CROP GROWTH ON THE REPLACEABLE BASES IN SOME CALIFORNIAN SOILS

Abstract

There are greater relative variations in the amount of replaceable bases in soils classified as of different origin than within a group of soils of uniform classification. The quantities of exchangeable bases in these clay or silty clay loam soils are generally much higher than in the sandy loam soils. When the soils were subjected to a prolonged period of cropping annually to barley, or were cropped twice with a long fallow period intervening, there were no appreciable changes in the content of total replaceable bases. There were significant decreases of replaceable K in all soils cropped annually for 12 yrs. Nine of the 12 soils, which supported 2 barley crops with a 10-yr. fallow period intervening, show significant decreases in K. The content of total bases is not significantly altered by these decreases in K, because Ca and Mg, which comprise 90% of the total, remain constant. The loss of K from the entire group of soils supporting an annual crop of barley for 12 successive yrs. is 32 % of that contained in these soils at the beginning of the experiment and is 82% of the K removed in the crops from these soils.. The loses of K from the same original group of soils, which have supported 2 crops of barley with an intervening fallow period of 10 yrs., are of questionable significance in several cases, but for the entire group make up 13% of the original content and constitute 64% of the crop withdrawal. The data seem to be especially significant, since this experiment has been conducted under conditions which preclude the loss of any of the 4 bases from the soil except by crop withdrawals. These relations between losses of K and crop withdrawals of this constituent suggest the importance of the replaceable base complex with respect to available K in soils. The 0.05 N HCl-extractable bases in these soils are compared to the ammonium acetate-replaceable bases and their relations discussed briefly.