160 Soil Analysis [268-270 



has been estimated. Hence we know how much dry soil the 

 200 grams contained. One quarter of this — i.e., something 

 less than 50 grams — is used for each estimation. 



268. Conclusion. — Dr. Dyer's conclusion from many 

 analyses was that 



(a) ' When a soil is found to contain so little as about o'oi 

 per cent, of phosphoric acid (P 2 5 ) soluble in a 1 per cent, 

 solution of citric acid, it would be justifiable to assume that it 

 stands in immediate need of phosphatic manure.' 



{b) It is difficult — " more difficult than in the case of 

 phosphoric acid — to give any plausible suggestion as to what 

 percentage of citric-acid-soluble potash may be regarded as 

 marking the non-necessity of special potash applications. 

 Probably this limit lies below 0*005 per cent.' 



269. Full Analysis of Soil. — In many cases a much fuller 

 analysis is required than the one just described. It is not only 

 necessary to discover what plant food is immediately available, 

 but also what stores of food are locked up in such a manner 

 that good tillage and weathering may at some future time bend 

 them to the use of the crop. This is generally done by two 

 separate sets of operations. The first is to analyse that portion 

 soluble in hydrochloric acid, and the second to analyse the 

 insoluble residue. 



FULL ANALYSIS OF PORTION SOLUBLE 

 IN HC1 



270. Preliminary Operations. — Dry, finely powdered 

 soil, prepared as directed in paragraph 132, is such a very 

 hygroscopic substance that it is rather difficult to weigh out 

 accurately. The following method is therefore recommended 

 as avoiding the difficulty : 



Place about 30 grams of the powdered soil in a porcelain 



