USE OF SOIL MOISTURE BY PLANTS 117 



the deeper the soil the more rapidly will the soil lose its 

 moisture. It does not follow from this law that the deep 

 soil will dry out more rapidly than a shallow one. On the 

 contrary, since, in the case above suggested, there is only 

 half as much water in the shallow soil as in the deep soil, 

 the shallow soil, with a smaller rate of loss, will dry out 

 very much more quickly than will the deep soil with a 

 larger rate of loss. This must be understood and remem- 

 bered by the farmer who is dealing with shallow soils. 



Various kinds of shallow soils occur in the irrigated 

 district. In some cases a hardpan has been formed a few 

 feet below the surface, which does not readily disintegrate 

 under the influence of irrigation. This leaves a compara- 

 tively shallow soil in which to store moisture, which dries 

 out quickly and must be irrigated frequently. Many soils 

 are underlaid at a depth of a few feet with coarse, loose 

 gravel, through which water percolates and is lost. Such 

 shallow soils must be treated as are soils with hardpan 

 near the surface. However, where an impervious hardpan 

 underlays the soil, if too much water be applied, there is 

 greater danger of water-logging; whereas, on soils under- 

 laid with loose gravel there is little such danger, for the 

 excess moisture percolates downward and is lost. From 

 another point of view, also, this is important. On shallow 

 soils of any kind, a given quantity of water cannot dis- 

 tribute itself over considerable depths. As a consequence, 

 the percentage of soil moisture is higher, which causes a 

 more rapid loss of soil moisture. From the point of view 

 of the conservation of soil moisture, such soils are, there- 

 fore, less economical than deep ones. 



83. Physical composition of soils. The rate of loss 

 of soil moisture from cropped fields varies with the physical 

 composition of the soil. In a fine soil, a given quantity of 



