104 SOIL WATER AND ITS RELATION TO SOIL FERTILITY 



immediately checked, hence the loosened layer soon dries out. 

 When dry, each soil particle and crumb aids in lessening the escape 

 of moisture from below by offering resistance to the rise of the 

 water films (Figs. 40 and 44). 



Cultivation Is Necessary to Maintain Mulch. In order to 

 maintain both a loosened and dry condition of a soil mulch it is 

 necessary to rework the ground even though no rain falls, because 

 the loosened layer, owing to its own weight, gradually reestablishes 

 a greater or less degree of compactness which in turn favors 

 capillary rise of film moisture. Rain generally destroys the effec- 

 tiveness of a soil mulch; so that if the mulch is further desired, it 

 should be reestablished through cultivation (Fig. 44). 



Mulch of Greatest Value During Dry Periods. A soil mulch 

 is of the greatest value during dry periods. When frequent rains 

 keep the soil well supplied with moisture the main object of 

 cultivation during such periods is to kill weeds and to aerate the 

 soil, and not so much to establish a mulch to conserve moisture. 



Aside from the fact that a mulch conserves moisture, it is 

 important to mention here that heavy soils are much easier to work 

 during dry periods especially, when a mulch is maintained. 



Self-forming Mulches and Protective Crusts. Very rapid 

 evaporation causes the surface few inches of a sandy soil, peat or 

 muck to become quickly dried out even though no cultivation be 

 given it, and because of this rapid drying out, the films of soil 

 water become broken, so that the dried surface zone acts as a 

 mulch. This explains why some uncultivated soils lose more 

 water through evaporation in a humid than in a semi-arid climate. 



On certain loams and silt loams of crummy structure, self- 

 formed crusts, which break away from the soil beneath, become 

 effective in lessening water losses. These detached crust-like 

 layers vary from a fourth to about two inches in thickness, and 

 are formed as a result of rapid evaporation. Crusts not so loosened 

 from the soil beneath would increase evaporation. 



Mulches of Other Kinds. Straw, manure, leaves, grass and 

 other materials are used to a more limited extent for mulching; 

 in many instances to conserve moisture, to keep down weeds, for 

 fertilizing, to avoid soil washing, and in other cases to prevent 

 injury from repeated freezing and thawing during winter and 

 early spring. 



Weeds Are Moisture Robbers. The water requirement of 

 weeds is greater than that of corn; thus a growth of weeds in a 



