82 SOILS 



Influence of Subsoil on the Water-holding Ca- 

 pacity of Soils. The amount of water held by a 

 soil depends not only upon the character of the 

 upper two or three feet of surface soil, in which the 

 roots of most farm plants chiefly feed, but also upon 

 the character of the subsoil and upon the distance 

 to the water table. Some subsoils are retentive, 

 others are leachy. A layer of gravel or sand three 

 or four feet below the surface may provide perfect 

 natural drainage, thereby increasing the amount of 

 film water that the upper soil can hold. A hard- 

 pan of impervious clay, or of rock close to the sur- 

 lace, will greatly reduce the water-holding capacity 

 of the soil, strange as it may seem. One might 

 think that if the water could pass down only three 

 or four feet before it strikes hardpan, the soil 

 above would be wetter than if the water could pass 

 down through many feet of soil. But the fact is 

 that the shallow soils are dryest; because, in times 

 of abundant rains, the water soon fills the soil, and 

 then flows off as surface drainage; whereas it 

 sinks down into the deep soil for many feet and is 

 stored there for the future use of the crop. The 

 first five feet of a strong loam may contain enough 

 water to make a layer ten to twenty inches deep 

 over the field. 



Height of Water Table. The distance below the 

 surface at which free water is found has an im- 

 portant influence on the amount of film water held 

 by the soil above it. Generally speaking the nearer 

 the water table is to the area in which the roots of 

 cultivated plants forage, the larger will be the 

 amount of film water held by this soil; for a large 

 part of this film water is drawn directly from the 

 iree water, and the nearer this is, the more abun- 

 dant and equable will be the supply. The roots 



