36 SOILS AND FERTILIZERS , 



the United States, shallow plowing is more generally 

 practiced than in the eastern states. Deep plowing in 

 the fall gives better results than in the spring. It is 

 not a wise plan to plow to the same depth every year. 

 Prof. Roberts says: 16 "If plowing is continued at one 

 depth for several seasons, the pressure of the imple- 

 ment and the trampling of the horses in time solidify 

 the bottom of the furrow, but if the plowing is shallow 

 in the spring and deep in summer and fall, the objec- 

 tional hard pan will be largely prevented." 



In regions of scant rainfall deep plowing of silt soils 

 should be done only at intervals of three or five years. 

 With an average rainfall, deep plowing should be the 

 rule on soils of close texture. The depth of plowing 

 should be varied to meet the requirements of the crop, 

 of the soil and the amount of rainfall. 



36. Permeability of Soils. The rapidity with 

 which water sinks into the soil after a rain depends 

 upon the nature of the soil, and upon the cultivation 

 which it has received. Shallow surface cultivation 

 leaves the soil in good condition to absorb water. 

 When the surface is hard and dry a large per cent, of 

 the water which falls on rolling land is lost by sur- 

 face drainage. Soils of close texture which contain 

 but few non-capillary spaces, offer the greatest resist- 

 ance to the downward movement of water. 



A soil is permeable when it is of such a texture that 

 it does not allow the water to accumulate and clog 

 the non-capillary spaces. Cultivation may change 

 the tilth of even a clay soil to such an extent as to 

 render it permeable. Deep plowing increases per- 



