The Beneficial Effect of Preliming upon PO4 Uptake from Incorporations of Monocalcium Phosphate¹

Abstract

I N previous consideration of the joint use of superphosphate and liming materials the transitions that occur in mixtures of superphosphate with limestone and with dolomite outside the soil (2, 3, 5, 6, 7) 3 have been integrated with the changes that water-soluble monocalcium phosphate undergo in prelimed soils (1, 4). It is obvious that some dicalcium phosphate is formed when that phosphate is incorporated with a prelimed soil. In contrast, such an addition to a soil of depleted alkaline earth content involves undesirable reactions with the amphoteric components of the soil colloids and resultant fixation of P04. The extent of the fo_rmation of the dibasic phosphate is governed by rate, type, and particle size of the liming material and by the thoroughness with which it and the soluble phosphate are disseminated throughout the soil. It is not to be expected, however, that ideal conditions as to uniform distribution of a relatively light incorporation of superphosphate and a heavier incorporation of limestone are to be attained in practice.The_ results given in the present paper were obtained by a plant-ash study to determine the effect of preliming at rational rates upon the uptake of P0 4 by rye seedlings from subsequent incorporations of water-soluble calcium phosphate, the principal phosphatic component of superphosphate. The values for additive phosphate and the analytical results are expressed by the conventional symbol, "P 20 5 ". EXPERIMENTAL PROCEDUREPrecipitated calc{um carbonate was used for the limings and the stipulated rates are in terms of pounds of CaCO, per 2,ooo,ooo pounds of air-dry soil. The uptake of P,0 5 was measured by the Neubauer technic described by Thornton (8) and with the following technic as to preparatory treatments: A 4so-gram charge of 2-mm sifted acidic soil was used to supply four 100-gram cultures and a so-gram surplus for chemical tests. Two charges received dry preparatory incorporations of precipitated CaC0 3 and then were moistened and aged 2 weeks. Initial pH value was used as the guide for the liming rate required to induce neutrality. An incorporation of 2S mgms of P,0 5 , as CaH 4 (P0 4 ),.H,O then was made with each unlimed and with each limed culture of roo grams of soil, diluted with so grams of quartz. Duplicate cult~Jres then were seeded with rye and kept at 20°C for r6 days, with a maintained relative humidity of so%.The phosphate incorporation corresponded to a P,0 5 rate of soo pounds, to a monocalcium phosphate rate of 887 pounds, and to a r6% superphosphate rate of 3,12S pounds per 2,ooo,ooo pounds of soil. The transition of such an incorporation to dicalcium phosphate would require 3S2 pounds of CaC03• Upon the lA cooperative