SOIL CHANGES DUE TO IRRIGATION 65 



water is applied to a soil it forms a film around each of 

 the particles of widely differing sizes; and further, many 

 small and large particles may form a larger composite 

 particle or crumb with one continuous film surrounding 

 it. The soil should possess a well-devel- 

 oped crumb structure; for the plant has 

 then a better chance to develop than if 

 the individual particles remain separate 

 in single-grain structure. 



44. Cohesion of soil particles. By 

 direct examination, every good farmer 

 may determine whether the soil is in 

 proper condition for plowing or for other 

 cultural operations. Usually this condi- 

 tion means that the proportion of mois- 

 ture in the soil is such that a plow or a 

 cultivator may be passed through it with 

 the least resistance and without destroy- 

 ing the crumb structure or tilth. The 

 question of the force with which dry or 

 moist soil particles stick to each other is 

 not of itself of very great importance; FIG. 15. Adhesion of 

 but it is of interest in showing the effect hairs due to water * 

 of various proportions of water on the properties of the 

 different soils. Pure clay dries to a very hard mass, 

 difficult to break. If to the clay be added sand, humus, 

 gypsum or lime, the resulting mass, when dry, may be 

 broken with less than one-fifteenth the force necessary 

 to break the pure clay. In fact, coarse sands or soils rich 

 in gypsum or lime, as they dry, often fall apart into a 

 coarse mass, which forms a natural mulch over the soil. 



The force with which soil particles are held together 

 depends, primarily, upon three factors: (1) the physical 



