38 SCIENTIFIC REPORTS OF THE AGRICULTURAL RESEARCH 



VII. Mode of action of superphosphate in calcareous 



AND NON-CALCAREOUS SOILS. 



In continuation of the investigation, reported last year, 

 dealing with the reactions between mono- and di-calcic 

 phosphates and calcium carbonate, attention has been 

 directed to the retention of P 2 5 in calcareous and non- 

 calcareous soils, Pusa and Kalianpur soils being taken as 

 the respective types. Formerly the phenomenon of reten- 

 tion by soils was explained as solely due to the formation 

 of comparatively insoluble phosphates, but in later years 

 the tendency has been to ascribe it largely to adsorption. 

 In view of the facts that Pusa soil contains a very large 

 proportion of CaC0 3 , and that the mono- and di-calcic 

 phosphates react with this substance, it seemed probable 

 -even if the laws of adsorption were the determining 

 factor in non-calcareous soils — that retention would be 

 mainly due to chemical combinations in calcareous soils. 



In order to test this, definite quantities of the type soils 

 were shaken with a definite volume of solutions of super- 

 phosphate of varying concentrations, and the distribution 

 of the P 2 5 between the solvent and the soil determined. On 

 plotting the logarithms of these values against each other 

 it was found that, in the case of the non-calcareous soil, 

 they lay upon a straight line, which is the criterion for 

 adsorption. In the case of the calcareous soil the points 

 lay along a sinuous curve; consequently adsorption is the 

 determining factor in non-calcareous soils but not in cal- 

 careous soils. 



A similar series of determinations with solutions of di- 

 and tri-sodium phosphates showed that adsorption 

 occurred in both calcareous and non-calcareous soils, and 

 consequently the conclusion to be drawn is that the reten- 

 tion of superphosphate in the former type of soils is one 

 of "chemical combination. 



A variation of these experiments was made by allowing 

 one kilo of soil to remain in contact for one week, without 

 shaking, with 450 c.c. of solutions of varying concentra- 

 tions of superphosphate, and then determining the clistribu- 



