PHYSICAL PROPERTIES OF SOILS lO/ 



cultivated crops, though the land may possibly be used for timber 

 or grazing. 



The water table, -when too near the surface, converts the soil 

 into a swamp. When somewhat lower, the soil is suitable for 

 some plants, but is too wet for agricultural purposes. If from 

 4 to 6 feet below the surface, the soil is suitable for cultivation, 

 but the water table is often considerably lower than this. 



In arid climates a proper substratum is much more important 

 than in humid climates, since the roots must be able to penetrate 

 deeply in order to endure drought. Hilgard 1 gives the following 

 examples of faulty substrata found in California. 



A. The surface soil of about 12 inches is underlaid by horizon- 

 tal layers of shale. This soil might possibly be rendered useful 

 by blasting with explosive. 



B. The surface soil of 12 inches is underlaid by a heavy red 

 clay, which can hardly be penetrated by roots. After blasting 

 with dynamite this soil has been used successfully. Without 

 blasting, orchards die in about three years. 



C. This soil has a calcareous hard pan at the depth of about 

 four feet. On account of the arid climate, the roots of trees must 

 be able to penetrate to a greater depth than this. 



D. The water level is about three feet. This prevents root 

 penetration and restricts the use of the soil to shallow rooted 

 crops. This condition may arise from the leakage of water from 

 irrigation ditches. 



E. The soil is underlaid by a layer of coarse sand or gravel at 

 the depth of about four feet, through which roots will not 

 penetrate to the water below. A large number of orchards have 

 died from this cause. 



Soil Temperature. Processes of plant and animal life can go 

 on only between certain limits of temperature. The temperature 

 to which the plant is subject depends upon both the soil and 

 atmospheric conditions, the latter being perhaps the controlling 

 factor. 



1 The Soil, p. 177. 



