4 * INTRODUCTORY 



chemical nature deal with the power of various soils to 

 retain manure, the causes of sterility or .fertility, and 

 the measures that can be adopted for the amelioration 

 of soils. 



The soil is, however, not merely a storehouse of food 

 for the plant, since water is equally indispensable to 

 its existence and is immediately derived from the soil ; 

 hence it is of prime importance to study the causes 

 which underlie the movement of water in the land, 

 and its supply to the growing crop. In the relation 

 between soil and water the cultivation to which the 

 land is subjected plays a prime part, hence it will be 

 necessary to trace the effect of each of the main opera- 

 tions of tillage upon the structure of the soil. Again, 

 the texture of the soil and the proportions of water 

 and air it retains, affect its temperature and that 

 responsiveness to change of season which we roughly 

 indicate by the terms " early " and " late " soils. The 

 general consideration of these questions may be termed 

 soil physics. 



Finally, the soil is not a dead mass, receiving on 

 the one hand manure, which it yields again to the 

 crop by purely mechanical or chemical processes ; it 

 is rather a busy and complex laboratory where a 

 multitude of minute organisms are always at work. 

 By the action of some of these organisms, vegetable 

 residues and manures are reduced, we might almost say 

 digested, to a condition in which they will serve as food 

 for plants ; others are capable of bringing into combina- 

 tion, or " fixing," the free nitrogen gas of the atmosphere, 

 and therefore add directly to the capital of the soil ; 

 others again are noxious or destructive to the food 

 stores in the soil. 



The work of these organisms is much affected by 

 cultivation ; in fact, it would not be too much to say 



