210 



ROCKS AND SOILS 



through coarse sand is much less than in fine sand or 

 clay. 



The chemical elements which plants need are dis- 

 solved in very small amounts in soil water and enter 

 the plant with the soil water which is taken in by the 

 roots of plants. It is necessary that large amounts of 

 water be available for plants. Much of this water is 

 evaporated from the leaves of plants into the atmos- 



\vell cultivated, capillary tubes near the surface are 

 broken up. Cultivation also loosens the surface soil 

 and provides a good circulation of air around the roots 

 of plants. The prevention of evaporation of soil water 

 is sometimes accomplished by mulching. In this proc- 



William Thotnpso 



FIG. 338. VEGETATION OF A DESERT 



phere while the elements dissolved in the water re- 

 main in the plant. Also a large amount of soil water 

 rises to the surface of the ground and is evaporated 

 directly from the soil into the atmosphere. This means 

 that as much as possible of the water which gets into 

 the soil must be conserved. 



Evaporation of soil water may be reduced to a mini- 

 mum in several ways. The one most commonly used 

 by farmers and gardeners is cultivation. This is done, 

 in part, to prevent the growth of weeds, but more im- 

 portant still, it is done to conserve soil water. If the 

 surface layer of the soil becomes hard, capillary ac- 

 tion causes water to rise to the surface, where it is 

 evaporated and lost to plants. However, if the soil is 



Photos by Bureau of Reclamation, Department of Interior. Courtesy 

 Scientific American. 



FIG. 339. IRRIGATION PROJECTS 



Above Boise River diversion dam, power plant, and intake to 



irrigation canal 

 Below Irrigating from concrete-lined ditches 



ess the farmer spreads a layer of hay, straw, or leaves 

 around the plants and between the rows. This surface 

 covering retards the evaporation of capillary water. 

 In some arid regions of the country the average 

 rainfall is sufficient only to grow a crop once in two 

 years. In these places it is common for the farmers to 

 continue cultivation during the period in which no 

 crop is grown. This tends to accumulate and conserve 



