238 SOILS 



least amount of moisture that a soil must have in 

 order to grow a profitable crop, varies widely in 

 different parts of the country, and with different 

 soils in the same section. It depends upon the 

 minimum amount of rainfall, the nature of the soil 

 and especially of the subsoil, the contour of the 

 land and the kind of crop. 



The amount of water actually used in the 

 growth of the different crops is capable of fairly 

 accurate calculation. For example, it is estimated 

 that it requires at least four and one-half inches 

 of water per acre to produce fifteen bushels 

 of wheat, nine inches to produce thirty bushels, 

 and so on. These seem like small amounts, 

 but, as has been shown in Chapter IV, a large 

 proportion of the rainfall is lost, chiefly by sur- 

 face drainage, by evaporation and by leach- 

 ing, so that scarcely half of the water that falls 

 upon a soil may become available for crops. 

 Hence at least eight to twelve inches of rainfall are 

 needed to produce a profitable crop of wheat, 

 although the wheat uses less than half of it. 



It is not enough, however, that the amount of 

 rainfall should come up to a certain standard. 

 If it does not fall at the right time, even larger 

 amounts of rainfall do not save a region from the 

 necessity for irrigation. Moreover, if the soil is 

 not retentive a rainfall considerably in excess of 

 the amount needed to produce a crop on a more 

 retentive soil will not avail. Most of the arid 

 area of the United States has a rainfall of about 

 ten or twelve inches, but there is a wide variation in 

 the time when most of this falls, and the ability 

 of the various soils to hold it. 



There is a large area in the West where dry 

 farming, which is the profitable culture of crops 



