EXTENSION COURSE IN SOILS. 29 
continue to evaporate at the upper portion of the layer containing 
moisture, but the surface layer of dry soil keeps the moist soil below 
somewhat cooler, so that loss by evaporation is greatly lessened. A 
dry, loose layer of soil or other material is called a mulch. The 
development of a soil mulch is by all means the cheapest and usually 
the most effective way of reducing the water loss by evaporation. 
The common farm method of developing a soil mulch is by cultiva- 
tion, which also kills weeds and promotes the circulation of air in 
the soil. 
The loss of water from surface evaporation, other conditions being 
equal, is greater in fine-textured than in coarse-textured soils; like- 
wise, the firmer the soil surface, the greater is the loss. This explains 
an objection to leaving a rolled surface in preparing a seed bed or 
after planting. Experimental results from cultivation to depths of 
1, 2, and 3 inches, respectively, and at intervals of one-half, one, and 
two weeks have shown, in general, that within these limits the deeper 
the mulch and the more frequent the cultivation, the greater are the 
quantities of soil moisture preserved. A general average from these 
same results ' shows that a soil mulch prevents the evaporation of 
about 3,500 pounds of water per day over each acre of land, which is 
about one-tenth of the quantity required during three months of the 
growing season to produce a 90-bushel crop of corn. While these 
results vary considerably with climate, soil, and season, yet they are 
significant in showing the means of retaining moisture in the soil by 
cultivation. 
The depth of mulch which is desirable depends on circumstances. 
Under most conditions a mulch of 3 inches has at least three-fourths 
the efficiency of a mulch of 5 or 6 inches in depth, and in the case of 
such crops as corn, in which the roots are apt to come close to the 
surface, so that cultivation to a depth of 5 or 6 inches would cut off 
many of them, it is unwise to attempt to produce a mulch more than 
3 or 4 inches in depth. In many cases the cultivation of the soil 
from 2 to 3 inches in depth is to be preferred. Deep cultivation is 
generally undesirable in the Mississippi Valley and the eastern part 
of the United States. Farther west, where the rainfall is 25 inches 
or less annually, and the roots of plants are forced to grow deeper, 
a greater depth of mulch is considered desirable, and it is a common 
practice to cultivate to a depth of 5 or 6 inches. 
Dry farming (Ref. No. 10). — Dry farming is a term which has 
come to be applied to the practice of agriculture in the arid lands of 
the West and Northwest. Where irrigation is impracticable, and 
where the annual rainfall is so low that it is impossible to grow a 
crop each year, land is fallowed every other year by keeping up a 
thorough cultivation which prevents the growth of vegetation and 
