SOIL STRUCTURE 183 



Since the soil has low cohesion, plowing when it is too 

 dry will not clod, while, because of low plasticity, little 

 puddling will occur if tillage is in progress when the soil 

 is wet. Being always rather loose, sucfy a soil is often 

 benefited by plowing when it is slightly wet, the parti- 

 cles being brought into closer contact and excessive per- 

 colation being stopped while at the same time the water 

 capacity is raised. 



119. Control of cohesion and plasticity. — It is evi- 

 dent not only that cohesion and plasticity control the 

 successful tillage of the land, especially where the soil 

 texture is fine, but also that these same factors vary with 

 the moisture and the granular structure of the soil. It 

 has been shown that there is a moisture zone in all soils — 

 this being narrower the finer the soil texture — at which 

 neither cohesion nor plasticity is excessive. In this zone 

 a heavy soil may be successfully plowed, with results 

 favorable to the structural condition of the soil. Since 

 the processes of granulation have already been shown to 

 lower cohesion and plasticity, it is evident that as a crumb 

 structure is developed the moisture zone for proper plow- 

 ing will be widened, especially in a heavy soil. This is a 

 very important point in the handling of clays and clay 

 loams, since it not only opens a way for the elimination 

 of the dangers of bad structural relationships, but also 

 provides for putting the soil in a condition for easier and 

 more convenient tillage. In a sandy soil, particularly 

 where humus is the granulating agent, a soil with 

 more cohesion, more plasticity, and a greater water- 

 holding power is developed, all of which tend toward a 

 better medium for plant growth. Methods of develop- 

 ing this granulation thus become the logical topic for 

 further discussion. 



