370 HOW CROPS PEED. 



Boussingault does not state the kind or concentration, 

 or temperature of the acid employed to extract the soil 

 for the above analysis. These are by no means points of 

 indifference. Grouven {Iter db 3ter Salzm under Herichte) 

 has extracted the same earth with hydrochloric acid, con- 

 centrated and dilute, hot and cold, with greatly different 

 results as was to be anticipated. In 1862, a sample from 

 an experimental field at Salzmiinde was treated, after be- 

 ing heated to redness, with boiling concentrated acid for 

 3 hours. In 1867 a sample was taken from a field 1,000 

 paces distant from the former, one portion of it was treat- 

 ed with boiling dilute acid (1 of concentrated acid to 20 

 of water) for 3 hours. Another portion was digested for 

 three days with the same dilute acid, but without applica- 

 tion of heat. In each case the same substances were ex- 

 tracted, but the quantities taken up were less, as the acid 

 was weaker, or acted at a lower temperature. The follow- 

 ing statement shows the composition of each extract, cal- 

 culated on 100 parts of the soil. 



EXTRACT OF SOIL OP SALZMtJNDE. 



The most interesting fact brought out by the above fig- 

 ures, is that strong and weak acids do not act on all the 

 ingredients with the same relative power. Comparing the 

 quantities found in the extract by cold dilute acid with 

 those which the hot dilute acid took up, we find that the 

 latter dissolved 5 times as much of oxide of iron and 

 j^lumiua, 4 times as much potasli, 3 times as much soda, 



