474 THE PRINCIPLES OF SOIL MANAGEMENT 



Combining these principles, then, it may be said that 

 if a clay soil must be plowed when too wet, it should be 

 plowed with a sod plow, and to as shallow a depth as 

 is permissible. But, on an over-dry soil, the opposite 

 conditions should be fulfilled, that is, steep mold- 

 board and increased depth. Likewise, on sandy soil, 

 where the aim is generally to compact the structure, 



Fig. 133. The modern garden seeder. It modifies the soil structure 



this may be furthered by deep plowing with steep 

 moldboard when the land is over-wet. 



In connection with this phase of the subject, it is 

 important to consider what Professor Roberts called 

 the plow sole. That is, the soil at the bottom of the 

 furrow which bears the weight of the plow and trampling 

 of the team, and which, under uniform depth of plowing, 

 does not become loosened. In clay soil, especially, it 

 gradually becomes more compact, in time developing 

 something of a "hard-pan" character, which is detri- 



