WATER OF SOILS 191 



in April or May, but it occurs more often in July or August, at the 

 time when the growing crops are in greatest need of moisture. At 

 the University of Illinois' 1 the distribution of rainfall is so irregular 

 that in the past twenty-five years seven Aprils have been dry or 

 have had less than two inches of rainfall, and during this same 

 time four Mays, eight Junes, five Julys, six Augusts, and eleven 

 Septembers have been dry, or a total of 41 out of 150 growing 

 months for the 25 years. In the southern third of the State the 

 distribution is still more irregular, and drouth is more injurious 

 there, because of the greater evaporation and the character of the 

 soil. This illustrates quite well the conditions generally in the 

 humid area. 



2. Soil. The supply of moisture for the crop depends upon 

 the character of the soil itself. An open porous soil, such as a 

 coarse sandy loam or sand, will lose a great deal of moisture by per- 

 colation, and hence will not have a large supply for crops. Fre- 

 quent rains are necessary for 'such a soil. However, the "firing" 

 of the crop on sandy soil is not always an indication of lack of 

 moisture. On finer grained soils, however, the moisture is retained 

 much better and an abundant supply is usually present for the 

 growing crop. The retentive power of the soil is increased very 

 materially by the presence of organic matter. Probably no one 

 constituent plays a greater part in maintaining the supply of moist- 

 ure in the soil than that of organic matter. 



3. Loss by Evaporation. In a soil deficient in organic mat- 

 ter, consisting of medium-sized soil particles, the movement of 

 moisture to the surface and its evaporation may reduce the supply 

 sufficiently to injure the crop. This factor is of especial importance 

 in semi-arid and arid sections. (See Chapter 18.) Mulrhes, good 

 tilth, and a fair supply of organic matter reduce evaporation to a 

 large extent. 



Ways of Expressing Moisture Content. The moisture con- 

 tent of soils has been expressed in a number of different ways, some 

 of which have been discontinued because of their impracticability. 

 Some of the methods are as follows: (a) per cent based on weight 

 of soil; (b) in cubic inches or per cent of volume, and (c) in acre 

 inches. 



(a) Per Cent of Weight of Soil. In expressing the moisture 

 content in per cent various methods have been used. A few inves- 

 tigators have based it on the weight of the wet soil as taken from 

 the field. This is not satisfactory, because the base varies from day 

 to day as the moisture content changes. In some cases the per cent 



