148 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



orchards it is often advisable to keep the soil cultivated and free 

 from weeds or crops in order to prevent injury to the trees from 

 want of water. 



Wet and Dry Soils. Dry soils are composed of coarse par- 

 ticles, with free percolation and little power of retaining water. 

 Wet soils are composed of very fine particles having an enormous 

 extent of intersurface and offering great resistance to the passage 

 of water. 



The character of the subsoil is also of great influence. A sandy 

 surface soil acts differently when it has a subsoil of loam or clay. 

 A clay soil is no longer wet when it has an open porous subsoil. 

 Whether the soil is level or on a hill side or receives the drainage 

 of higher land, influences the water held in the soil. 



The most suitable physical composition of a soil depends on the 

 climate and the situation in which it is placed ; soils of great value 

 in one situation may be of little value in another. A clay land 

 which can be used only for pasture with an available rainfall of 

 forty inches, may be used to great advantage where the rainfall is 

 only twenty-five. 



Effect of Cultivation and Manure. Shallow surface cultiva- 

 tion conserves moisture. Rolling compacts the soil and causes 

 water to rise to the surface. Fall plowing allows water to 

 penetrate the soil, and if followed by surface cultivation, may 

 allow a balance of water to be carried over to the next season. 

 Spring plowing, if followed by dry weather, causes loss o"f moisture 

 by evaporation ; it should therefore be followed by surface cultiva- 

 tion. Manure or straw, spread as a mulch, prevents loss of water 

 by evaporation. 



