The effect of mono-, di-, and tricalcium phosphates on the reaction of soils of different degrees of acidity

The effect of six different phosphates on the reaction of three soils of different degrees of acidity was studied in small tumblers over a period of 18 months. The changes in pH values after 6 weeks and after 18 months were determined and were compared with the effect obtained by additions of small amounts of calcium carbonate. It was found that the original reaction of the soil materially influenced the effect of the various phosphates on soil reaction; the more acid the soil, the greater the basic action of the different phosphates or the less their acidic effect. On a soil of pH 4.28, monocalcium phosphate was found to have a basic action approximately equivalent to its calcium content, whereas on a soil of pH 5.63, it had no effect, and on a soil of pH 6.40 it had a slight acidic action. The action of superphosphate on the most acid soil was about tbe same as for monocalcium phosphate, but on soils of the two higher pH values it had a slightly greater acidic action than the latter. This is believed to be temporary and due at least in part to the higher salt concentration in the soil resulting from the calcium sulfate of superphosphate. Dicalcium phosphate was found to have a basic action approximately equivalent to its total calcium content on the very acid soil of pH 4.28 and to approximately one-half its calcium content on a soil of pH 5.63, whereas it had only a very slight basic action on a soil of pH 6.40. Tricalcium phosphate showed a slightly lower neutralizing value in the soil than did dicalcium phosphate, possibly due to its lower solubility. Monosodium phosphate was found to have a greater basic action than monocalcium phosphate, probably as a result of the high content of sodium introduced into the exchange complex. It is concluded from these results that for most soils of the humid regions, the pH values of which lie mostly between 5 and 6, superphosphate, rock phosphate, and monocalcium phosphate can be considered to have no appreciable effect on soil reaction, whereas hydrated dicalcium and tricalcium phosphate can be considered basic. The relation between phosphorus fixation and the results obtained in this study are briefly discussed.