INSTITUTE, PUSA, FOR 1919-20 37 



tent of the soil gases at a minimum and thus permit of a 

 greater concentration of the phosphoric acid in the soil 

 solution is apparent. 



From what has been noted above about the action of cal- 

 cium carbonate on a solution of monocalcic phosphate it 

 would appear probable that this reaction must play a promi- 

 nent part in calcareous soils in retaining those forms of 

 soluble phosphates which are capable of reacting, but that, 

 on the other hand, the effect of this chemical retention would 

 be at a minimum in non-calcareoue soils, and in these cir- 

 cumstances adsorption may be the limiting factor. It is 

 of the utmost importance to determine which of these modes 

 of retention is the predominant factor in the two types of 

 soil for not only must the " availability " of the retained 

 phosphoric acid be very dissimilar in the two cases, but the 

 distribution through the mass of soil must also be very 

 different. 



With this object in view parallel series of experiments 

 were carried out with a Pusa soil, representing highly cal- 

 careous soils, and a soil from Kalianpur which is almost 

 devoid of lime. These showed that the phosphoric acid of 

 superphosphate is mainly retained through adsorption in 

 non-calcareous soils, whereas the retention is mainly due to 

 other causes in the case of calcareous soils. Support to 

 the conclusion that the retention of superphosphate by cal- 

 careous soils is due to chemical combination is lent by the 

 behaviour of solutions of di- and tri-sodium phosphates 

 with the type soils. In these latter cases no chemical re- 

 action could be demonstrated between calcium carbonate 

 and the sodium phosphates and the retention here obeys the 

 adsorption laws. 



The rapidity of the reaction between superphosphate and 

 calcium carbonate lends very strong support to the hy- 

 pothesis that this reaction must be the determining factor 

 in the retention of phosphoric acid under conditions in 

 which it can occur. If this is correct, then it would appear 

 that when superphosphate is applied to a calcareous soil 

 comparatively insoluble calcium phosphates are im- 



