66 IRRIGATION PRACTICE 



constitution of the soil; (2) the water content, and (3) 

 the presence of various salts. The finer the soil is, the 

 more firmly the dry particles are held together. As the 

 soil water increases, clay is less firmly, and sand more 

 firmly, held together. The presence of soluble salts 

 tends, in general, to reduce the force with which soil par- 

 ticles stick together, though lime and other substances 

 have the opposite effect. 



Of chief importance to the irrigation farmer is the 

 knowledge of how varying amounts of water affect the 

 cohesion of soil particles, since it is within his power to 

 regulate the quantity of water in the soil. Cameron and 

 Gallagher have done some excellent work on this subject. 

 They concerned themselves only with the percentages of 

 soil water which are found in actual agricultural practice; 

 for, large additions of water, beyond the saturation point 

 of the soil, always cause the soil crumbs to fall apart into 

 their constituent particles; and, likewise, at moisture 

 contents below the wilting point, the cohesive powers of 

 the soil grains have little agricultural meaning. 



Sand, loam, clay and humus soils were studied. In 

 all of these, save the clay, as the soil moisture increased, 

 the force with which the soil crumbs were held together 

 at first decreased up to a definite point, then increased, 

 and, by the addition of more water, decreased again to 

 the point of minimum cohesion. In other words, as water 

 is added to a dry soil, the soil first gradually softens; then 

 gradually hardens; then rapidly softens until it is a mushy 

 mass. The point of low cohesion, or easy penetration at 

 which tillage implements may be passed through the soil 

 with small resistance, corresponded, generally, with the 

 so-called point of optimum water content in the soil; 

 that is, the degree of wetness at which, according to the 



