148 PHOSPHATIC FERTILISERS [chap. 



of calcium humate, with which the superphosphate 

 reacts. 



When superphosphate is applied to the soil, the 

 soluble phosphoric acid it contains is rapidly repre- 

 cipitated ; to some extent the clay provides the 

 necessary base, but on most soils the calcium carbonate 

 takes the chief part in the reaction, with the pro- 

 duction of di-calcium phosphate. As this precipita- 

 tion takes place all throughout the soil, the phosphate 

 is very finely divided and thoroughly disseminated, 

 hence the great effectiveness of superphosphate. 

 Though di-calcium phosphate, like tri-calcium phos- 

 phate itself, is probably eventually converted in the soil 

 into a compound approaching the composition of tetra- 

 calcium phosphate, it remains effective as a fertiliser 

 because of the fine state of division in which it 

 continues to exist. 



How thorough is the precipitation of the phos- 

 phoric acid within the soil may be seen from Dyer's 

 examination of the soils from the Broadbalk wheat 

 field, which had been receiving 3I cwts. per acre of 

 high grade super for fifty years previously. He found 

 that though the surface soil to the depth of 9 inches 

 had been enormously enriched in phosphoric acid 

 soluble in i per cent, solution of citric acid, the subsoil 

 below had practically gained none, so complete had been 

 the precipitation in the layer stirred by the plough. 

 Again, the drainage waters from these plots show a 

 most trifling amount of phosphoric acid, so that losses 

 by washing out must be negligible. Still more cogent 

 evidence of the retention of phosphoric acid by the soil 

 has been obtained more recently by applying the method 

 of successive extractions with a i per cent solution of 

 citric acid, until the phosphoric acid going into solution 

 has fallen to the low constant figure indicating the 



