USE OF SOIL MOISTURE BY PLANTS 125 



moisture is great. Sunshine is next in importance. The 

 more abundant the sunshine throughout the growing 

 season, the more favorably affected is plant-growth, and 

 the more rapid is the loss of the soil moisture. Third in 

 importance is the relative humidity of the air. The drier 

 the air, the more rapidly does water evaporate from the 

 plant, and, therefore, the more steadily does water move 

 through the plant from the soil. Following these three 

 factors temperature, sunshine and humidity are winds 

 and all manner of air movements. These dry out the 

 soil and increase the rate at which water passes through 

 the plant to supply the more rapid evaporation from the 

 plant. Winds are always a serious factor of water-loss, 

 largely beyond the control of the farmer. Rains, especially 

 slight ones, during the growing season are a menace, for 

 they keep the top soil moist and make possible a rapid 

 direct evaporation; however, they tend to diminish trans- 

 piration, from the reduction in the relative humidity 

 which follows them. These factors, fundamental in 

 determining the season, determine largely the evaporation 

 of water from the soil itself. Experiments have shown that 

 the rate of loss of soil moisture due to plant-action is 

 frequently varied as a result of the seasons. 



The factors of water-loss discussed in this chapter 

 are those of most importance to the irrigation farmer. 

 Many of them may be controlled more or less perfectly 

 and, therefore, they should be well understood. 



REFERENCES 



BOUYOUCOS, GEORGE J. Transpiration of Wheat Seedlings as 

 Affected by Soils, by Solutions of Different Densities, and by 

 Various Chemical Compounds. Proceedings of the American 

 Society of Agronomy, Vol. Ill, pp. 130-191 (1911). 



