32 THE FERTILITY OF THE SOIL [CH. 



Finally the losses may be reduced by checking evapo- 

 ration, and this can be done by sheltering from the 

 wind with windbreaks or hedges and by maintaining 

 the fine tilth at the surface. Horticulturists some- 

 times adopt all these, agriculturalists sometimes only 

 one or two. 



Food supply. The supply of plant food in the 

 soil depends in the first instance on its mineral 

 composition. The great mass of inert material that 

 constitutes the framework of the soil and subsoil 

 affords but little food. The food constituents are to 

 be sought among the more soluble and reactive 

 substances, and, in some way that is not sufficiently 

 understood, their availability is increased when calcium 

 carbonate is present. A second factor, the accumu- 

 lation of plant residues, began to come into play 

 soon after the soil was formed and has in some 

 cases assumed so much importance that it now con- 

 trols the situation. And, as plant residues are not 

 themselves plant food, but have to be converted by 

 micro-organisms into simpler substances, we can add 

 as a third factor the activity of the food-making 

 organisms. 



It is a simple matter to increase the supply of 

 plant food by adding fertilisers to the soil. The 

 amount of nitrogen may be increased by adding 

 nitrate of soda, sulphate of ammonia, organic sub- 

 stances such as farmyard manure, guanos, certain 



