PRINCIPLES OF AGRICULTURAL CHEMISTRY 



10 grams soil are digested with 100 cc. hydrochloric acid 

 of 1.115 specific gravity for 8 hours at the boiling tem- 

 perature. Dr. E. W. Hilgard digests the soil on the 

 steam bath for five days, a method which appears to give 

 nearly the same results as the official method, with the exception 

 of potash, and oxides of iron and alumina. In the one comparison 

 made by Dr. Loughridge, 1 0.35 per cent, potash was dissolved by 

 the one method, and 0.63 per cent, by the other, or nearly twice 

 as much. This difference must be considered in comparing the 

 analyses made by Hilgard's method with the analyses made by the 

 methods of the Association of Official Agricultural Chemists. 



The proportion of the constituents of the soil which are dis- 

 solved by strong hydrochloric acid varies considerably with differ- 

 ent soils. For example, Veitch 2 determined in 16 Maryland soils 

 the total quantity of each constituent and the quantity dissolved 

 by strong acid by the A. O. A. C. method. The results are pre- 

 sented in the following table, expressed in percentage of the total 

 quantity of each ingredient present : 



PERCENTAGE OF TOTAL PHOSPHORIC ACID, ETC., DISSOLVED BY 

 STRONG ACIDS. 



The average order in which the constituents of these soils were 

 dissolved was as follows, beginning with the most soluble: oxide 

 of iron, phosphoric acid, magnesia, alumina, lime, and potash. On 

 an average, only 17 per cent, of the total potash of the soil was 

 dissolved by the A. O. A. C. method. None of the soils in ques- 

 tion were highly calcareous, otherwise a much greater proportion 

 of lime and magnesia would have been dissolved. 



1 Hilgard, The Soil, p. 341. 



2 Maryland Bulletin No. 70. 



