OBSTRUCTION 



97 



four feet, and since moisture can rise to heights of from at least 

 three to five feet in fine textured subsoils below the root zone, it is 

 reasonable to assume that a crop growing on a silt loam having a 

 deep silty clay subsoil draws its moisture supply from six to nine 

 feet of soil during a di ought. 



Compactness. — Compactness or firmness means good contact 



Fig 41. — Capillary rise of water in soils. I 



between the soil grains and crumbs within the seed bed. Since 

 capillary rise of water is possible because of the films which extend 

 from soil particle to soil particle, we can readily understand how 

 compactness or good contact between the soil grains facihtates 

 this movement of water. 



Obstruction. — ^Under field conditions practically the only 



obstruction that may at times interfere with the rise of soil water 



is too much coarse material plowed under; it may be too much 



coarse manure or litter, a rank growth of clover, or too much 



7 



