Effect of Nitrogenous Fertilizers, Organic Matter, Sulfur, and Colloidal Silica on the Availability of Phosphorus in Calcareous Soils 1

ECENT chemical studies ( I ) ~ have shown that, in general, the R calcareous soils of southern California are well supplied in total phosphate and that this phosphate is readily soluble in very dilute acid (0.002 N sulfuric acid); however, it is difficultly available to plants. Results of a similar character have been secured by investigators in other western states (z,3,4). With soils well supplied with difficultly available but readily soluble phosphate, the question arises as to whether the availability of the phosphate can be measurably increased by physiologically acid nitrogen fertilizers, by sulfur, by organic matter, or by other materials such as colloidal silica. Conversely, i t is desirable to know whether physiologically alkaline fertilizers decrease phosphate availability. Considerable importance is attached to this problem inasmuch as there are large areas of such soils in the semi-arid regions of the world, many of them being intensively cropped and heavily fertilized with both nitrogenand phosphate-containing fertilizers. Although much previous study has been devoted to the question of t& effect of many materials on phosphate availability, particularly rock phosphate, insufficient work has been carried out with naturally calcareous soils to warrant general conclusions. It is unnecessary to review the extensive literature of this subject. It suffices to state that in the absence of calcium carbonate, physiologically acid fertilizers, organic matter, sulfur, and colloidal silica have often been shown to increase the availability of rock phosphate and, in some cases, that of the native soil phosphate. The results of some preliminary studies with calcareous soils are reported in this paper. ’