26 SCIENTIFIC REPORTS OF THE AGRICULTURAL RESEARCH 



agricultural chemists throughout the world as a means of 

 evaluating the manurial requirements of soils, and 

 undoubtedly has proved of great value, but whether or not 

 the method. can be applied uniformly to all soils is a ques- 

 tion which, since its introduction, has exercised the minds 

 of chemists, and in particular a considerable controversy 

 has arisen regarding the values obtained for available phos- 

 phoric acid in highly calcareous soils. 



A short time ago Mr. Sen published the results of a 

 series of experiments carried cut in this laboratory, which 

 showed that the addition of increasing proportions of 

 carbonate of lime to a non-calcareous soil, rich in available 

 phosphate, had the effect of greatly decreasing the values 

 obtained, and that the values obtained when the calcium 

 carbonate content reached about 20 per cent, were of the 

 same order as those obtained from Pusa soils. An increase 

 in the proportion of calcium carbonate over this value did 

 not materially affect the values obtained. 



The greatest change in the order of the values obtained 

 occurred at the point where the calcium carbonate content 

 was sufficient to neutralize fully the citric acid of the 

 solvent solution, and Mr. Sen concluded that the effect of 

 the calcium carbonate in giving rise to low available phos- 

 phate values was due to the neutralization of the acid. In 

 addition, as the values obtained continued to decrease after 

 the point of neutrality was reached, he concluded that a 

 certain amount of phosphoric acid was " absorbed." 



These experiments have been repeated in greater detail, 

 and no evidence has been obtained that absorption occurs 

 to any appreciable extent, but, on the contrary, the values 

 obtained from the various mixtures appear to be determined 

 mainly by the composition of the liquid after its reaction 

 with calcium carbonate. With increasing amounts of 

 calcium carbonate the composition of the liquid gradually 

 changes, and the phases may approximately be defined as 

 follows : — (a) 1 per cent, citric acid solution, (b) a saturated 

 solution of calcium citrate with decreasing amounts of 

 acidity, (c) a saturated solution of calcium citrate and 



