136 THE PRINCIPLES OF SOIL MANAGEMENT 



rainfall is from fifty to sixty inches, crops suffer more 

 from a lack of moisture than they do in some of the 

 states of the northern Mississippi valley, with only 

 a third of the rainfall. The light truck soils of the At- 

 lantic coast suffer much more from a lack of water 

 than do the interior soils of heavy texture which are 

 under the same rainfall and general temperature con- 

 ditions. Plants in a dry greenhouse use more water than 

 in the more moist outside air. These illustrations serve 

 to emphasize the three factors which determine the 

 amount of moisture a soil contains. These are (a) the 

 available supply of water; (b) the retentive capacity 

 of the soil for water; (c) the rate and amount of loss of 

 water from the soil. Each of these factors depends 

 on many conditions. 



65. The supply. The supply of water is obviously 

 controlled by conditions external to the soil. These are 

 the precipitation in the forms of rain and snow, under- 

 ground seepage, and irrigation. 



66. Retentive capacity of the soil. The retentive 

 capacity of the soil varies greatly according to its 

 physical properties, As soils ordinarily occur in the field, 

 they show the presence of moisture. This moisture 

 is held quite intimately. Two soils may appear equally 

 moist, yet have very different capacities to maintain 

 crops. Plants suffer much more quickly from dry 

 weather on sand soil than on clay soil, even when the 

 soils appear equally wet at the outset. 



67. Statement of water content. Five different 

 methods are commonly used in stating the moisture 

 content of soils. These are: (1) In terms of per cent 



