USE OF SOIL MOISTURE BY PLANTS 115 



water, direct evaporation from the soil occurs much more 

 freely. 



To prevent the accumulation of water in the upper 

 foot, and the consequently greater loss of soil moisture, 

 the land should be plowed deeply, so that the irrigation 

 water may move easily and rapidly to the lower soil 

 layers. For the same reason, the soil should be kept 

 moist enough to permit water to descend quickly. The 

 limiting of root-development in the upper foot by deep 

 cultivation may also be advantageous. Whatever device 

 the farmer may employ to distribute water uniformly to 

 comparatively great depths, and to prevent the excessive 

 development of roots in the upper soil layers, will tend to 

 reduce the rate at which plants will absorb water from 

 the soil. Under the law of distribution, as explained in 

 Chapter III, the proportion of water is normally greater 

 in the upper than in the lower soil layers; yet, by proper 

 cultural treatments, it is possible to effect the most com- 

 plete distribution in the shortest time, and thus to con- 

 serve the water. 



81. The effect of time. Closely connected with the 

 law of the initial percentage, and derived from it, is the 

 further law that as time goes on, the rate of loss of soil 

 moisture becomes smaller and smaller. In the beginning, 

 when the soil is moist, much water is lost. After the first 

 day, there is a smaller quantity of water in the soil, and 

 the rate of loss will be a trifle smaller, and so on, day after 

 day, until a period is reached which finds the soil so dry 

 that the plant can no longer draw water from it. On a 

 shallow soil, during two weeks after irrigation, more than 

 31 per cent, or nearly one-third, of the total loss of water 

 occurred during the first three days after irrigation; 29 

 per cent the next four days; 23 per cent the next three 



