INSTITUTE, PUSA, FOR 1917-18 2§ 



but no attention has hitherto been drawn to the danger of 



employing a water containing calcium bicarbonate in 



solution for the irrigation of lands showing white alkali, 



and consequently this question is being subjected to a close 

 investigation. 



(c) Mode of action of superphosphate in calcareous 

 soils. The mode of action of superphosphate in calcare- 

 ous soils is somewhat obscure, and it appeared desirable to 

 obtain information regarding the combinations formed, the 

 rapidity of their formation, and the consequent effective 

 range of superphosphate applications. With this object 

 in view a preliminary series of experiments have been 

 carried out in relation to the reactions occurring between 

 phosphates of lime and calcium carbonate under varying 

 conditions. It has been found that superphosphate reacts 

 very rapidly at ordinary "temperature with calcium car- 

 bonate (CaC0 3 ), the product of theo'eaction being the com- 

 paratively insoluble dicalcicphosphate, and that the latter 

 in turn slowly reacts with more CaCO, forming tricalcic-' 

 phosphate. At higher temperatures the intermediate 

 stage practically disappears and tricalcicphosphate is 

 formed with great rapidity. 



The reaction between solid dicalcicphosphate and calci- 

 um carbonate (CaC0 3 ) in the presence of water is very slow 

 owing to the slight solubility of the former, but as dicalcic- 

 phosphate is very rapidly removed by CaC0 3 from a satu- 

 rated aqueous solution and also from a saturated solution 

 in 1 per cent, ammonium citrate the probability of its per- 

 sisting in solution in the presence of calcium carbonate 

 (CaC0 3 ) is very remote. 



Applying these facts to calcareous soils it would appear 

 that the range of action of superphosphate is limited owing 

 to the rapid formation of the comparatively insoluble 

 dicalcicphosphate and the formation in turn of the insoluble 

 tricalcicphosphate from any dicalcicphosphate which 

 becomes soluble. The final stage in which the phosphoric 

 acid (P 2 5 ) is in the fonn 4 of tricalcicphosphate would be 

 quickly reached, so that the fertility of. these calcareous soils 





