SOIL MOISTURE 47 



more than two hundred and fifty thousand pounds of water. The 

 enormous amount of moisture transpired by trees has a very con- 

 siderable influence in cooling the air in the vicinity of the leaves, 

 for a great amount of heat must of course be used up wherever 

 water or moisture is changed to the gaseous form. The tempera- 

 ture of a place may be perceptibly lowered by the evaporation of 

 water from vegetation. This cooling influence is noticeable when 

 one is passing a field of heavy green alfalfa. 



Conservation of Soil Moisture. When we consider the vast 

 amount of water needed by plants, it is evident that every means 

 possible must be practiced to conserve the moisture of the soil 

 in hot, dry weather. The shallow surface of the soil should be 

 stirred frequently. Widening the spaces between soil particles in 

 plowed ground, checks the capillary action, so that less water 

 reaches the top of the ground to be wasted by excessive evapora- 

 tion. A mulch of humus or of manure will answer the same pur- 

 pose. The water-holding power of different soils varies greatly. 

 According to Johnson's investigations an acre of surface soil one 

 foot deep may hold the following quantities of water: 



Sand 1,197,900 Ib. 



Prairie Soil .... 1,524,600 Ib. 

 Peat 2,047,300 Ib. 



Deep plowing at the proper time enables rain water to sink readily 

 into the earth instead of running off to be wasted. 



Irrigation. In the western part of the United States the rain- 

 fall is so uncertain and scanty and the ground water is so far from 

 the surface that there is rarely enough to supply crop needs. 

 Fortunately many of these regions are traversed by river courses, 

 and water is drawn off from these by means of numerous canals 

 or ditches to the fields in cultivation. This process is called 

 irrigation. Sometimes the supply of water is drawn from reser- 

 voirs fed by springs or artesian wells. The Salt River Valley and 

 the Gila Valley of Arizona are rich farming districts made produc- 

 tive by means of irrigation. In Meade County, Kansas, the Pecos 

 Valley in New Mexico, and in Redlands and other portions of 

 California irrigation is often carried on from reservoirs fed by 

 strong artesian wells. 



