120 RELATION TO PHYSICAL CONDITIONS 



should be so arranged that the roots may easily secure 

 food and moisture. 



Soils vary greatly in their tilth. Those made up of 

 coarse-grained particles are less affected in structure by 

 various agencies than those composed of fine-grained 

 particles. With coarse-grained soils the keeping of a good 

 tilth presents no serious problem. With fme-grained soils, 

 on the other hand, the maintaining of a good structure 

 requires constant attention. It may be affected by several 

 factors, one of which is the presence of soluble salts. 



The ideal structure is usually one in which there is a 

 maximum of air space. This condition also favors the 

 various cultural operations. If fine soil particles are 

 packed tightly together, there is not sufficient air space 

 for the best root development and the soil is difficult to 

 till. When plowed, it becomes cloddy instead of mellow. 

 In order to secure the best condition, the fine particles 

 should be clustered together, or flocculated. This gives 

 air space between the groups of particles as well as be- 

 tween the individual particles in the group and establishes 

 lines of weakness in all directions. This enables the soil 

 to break up readily into a crumb-like structure when cul- 

 tivated instead of into clods. Anything that promotes 

 flocculation improves tilth; likewise anything that pro- 

 motes deflocculation injures tilth. 



The effect of soluble salts on tilth has been a subject of 

 considerable study. Sachsse and Becker (17) showed that 

 nitrate of soda not only prevented flocculation but also 

 separated floccules that had already been formed. It 

 was thought that this result might be due in part to the 

 formation in the soil of carbonate of soda, which in turn 

 acts upon the hydrated silicates, producing colloidal sili- 

 cates which reduce the permeability of the soil to water. 



