72 AGRICULTURE 



shows that it is best to plow deep unless there are reasons 

 for not doing so. If land is plowed two or three inches 

 deeper than it has ever been plowed before, there is danger 

 that the first crop after such deep plowing will be injured by 

 the subsoil which is brought to the surface. This subsoil 

 often dries and forms a hard crust that interferes with plant 

 growth. Moreover, the plant-food in this layer of sub- 

 soil may not be in such a form that the plant can immedi- 

 ately use it. But the longer it lies on the surface exposed 

 to the air, the more fertile it becomes. Generally, deep 

 plowing is beneficial to the second and the third crops, 

 even if not to the first crop. 



Subsoil plowing. The depth of plowing can be in- 

 creased without any danger of injuring the first crop if 

 each year the plowing is about one inch deeper than the 

 year before. The depth of the plowed soil can be suddenly 

 increased by the use of a subsoil plow, which simply loosens 

 the subsoil, but does not bring it to the surface. In using 

 a subsoil plow we must make sure that the lower layers 

 of soil are dry enough to be pulverized. Subsoiling is 

 usually best done in the fall, because at this time the sub- 

 soil is apt to be dry and capable of crumbling. Harm and 

 no good comes from plowing the subsoil when it is very 

 damp. 



When to cultivate. Most cultivation consists in de- 

 stroying the plants not needed and in forming a shallow 

 layer of loose soil at the surface of the ground. It is just 

 as important to form this mulch, or loose, light layer of 

 soil, as it is to destroy the weeds. Cultivation is often 

 needed when there are no weeds. We may be sure that it 



