THE MAIN TYPES OF SOIL 55 



contain only organic nitrogen, that is to say, 

 nitrogen in a form in which it can directly be 

 utilized by animals, but not immediately 

 by farm crops. Such substances require 

 to decay, and to have their organic nitrogen 

 converted into some other form, before higher 

 plants can make use of them, and this change 

 is intimately associated with abundant 

 supplies of air, and no soil contains more air 

 than sand. Then, again, the process of 

 conversion is facilitated by heat, and in this 

 respect sandy soil offers more favourable 

 conditions than clay. The result is that the 

 class of manures referred to is specially adapted 

 for use on sandy land, and, conversely, is 

 ill-adapted for use on clay. That sandy land 

 offers conditions favourable to decay, is well 

 illustrated in the behaviour of fencing posts, 

 which, in light soil, decompose very rapidly, 

 and probably require renewal within a few 

 years of the erection of the fence ; whereas 

 the same class of post inserted in clay soil 

 may have a " life ' three or four times as 

 long, and come out of the ground compara- 

 tively sound after twenty or thirty years. 



The characters of clay are, to a large 

 extent, the opposite of those that are associ- 

 ated with sand. Clay in itself has a much 

 more complex composition, and although 



