USE OF SOIL MOISTURE BY PLANTS 



111 



tion appears to be a process wholly necessary to plant life. 

 Our question is, To what extent may it be reduced with- 

 out injuring plant-growth? 



79. The initial percentage of soil moisture. The 

 most important factor in determining the rate of loss of 

 soil water is the average percentage of water found in 

 the soil at the beginning, known as the initial percentage. 

 All other conditions being the same, the loss of water 

 from two plants during a definite period of time varies 

 as the initial percentage. The following table, selected 

 from a great number of experiments on this subject 

 made at the Utah Station, illustrates the law: 



The soil which contained at the beginning of the 

 experiment 21.84 per cent of water, lost during ten days 

 more than twenty-five pounds of water to the square foot; 

 whereas the soil that contained 13.18 per cent of water at 

 the beginning of the period lost only about ten and one- 

 half pounds of water to the square foot. It seems very 

 clear that the rate of loss of water from a soil increases as 

 the initial percentage of water in the soil increases; that 

 is, the higher the initial percentage of water, the greater 

 the loss; the lower the initial percentage, the smaller the 

 loss. 



The reason for this effect of the initial percentage can 

 be fairly well understood. The fine root-hairs come into 



