414 THE HOME, FARM AND BUSINESS CYCLOPEDIA. 



should take place at a low temperature. This treatment by burning is a 

 very extreme one, and can be recommended only in few cases ; it must 

 always be attended with an entire loss of the nitrogen in the soil burnt. 

 The ploughing in of burnt clay is of use in improving the texture of 

 heavy land. 



THE CHEMISTRY OF CROPS. 



To understand the chemistry of crops we must first inquire as to their 

 composition. The following table gives the average composition of ordi- 

 nary farm crops. The quantities of carbon, hydrogen, and oxygen present 

 are omitted, also some of the smaller ash constituents. By " pure ash " 

 is understood the ash minus sand, charcoal, and carbonic acid. 



The composition of grain and of all seeds, is tolerably constant ; but the 

 composition of straw, leaves, roots, and tubers, will vary very considerably 

 according to the character of the soil, manure, and season. 



