USE OF SOIL MOISTURE BY PLANTS 113 



immediately after an irrigation, when the soil is moist, 

 the plant will of necessity use much more moisture per 

 unit of time than later when the soil is not so moist. 



A question of importance in this connection is this: 

 If two fields contain respectively 20 per cent and 10 per 

 cent of water, will the loss of soil moisture during any 

 definite period be twice as great from the one field as 

 from the other? From the data in our possession, it 

 may be answered that the losses are proportionally 

 larger from the wettest soils. This may be seen from the 

 table on page 111. The difference in the moisture per 

 cent is only 8.66 but the difference in the pounds of 

 water lost to the square foot during the same period was 

 14.54. That is to say, the wetter the soil became, the 

 more rapid did the proportional loss of moisture become. 

 This important phase of the law of the initial percentage 

 might have been foretold by recalling that the thinner the 

 soil-moisture film, the more firmly is it held by the soil. 

 Under the point of lento-capillarity, plants can absorb the 

 soil moisture only with the greatest difficulty; above this 

 point, the absorption goes on much more rapidly. Pre- 

 liminary experiments seem to show that, if the lento-cap- 

 illary water of a soil be subtracted from the percentage 

 of water held by each of two or more soils, and the cube 

 roots be taken of the remainders, that is, of the water in 

 true capillary condition, an approximately correct meas- 

 ure of the relative ease with which plants can abstract 

 water from the soil is obtained. 



The law of the initial percentage teaches the impor- 

 tant doctrine that moderate irrigations are in all proba- 

 bility more economical than heavy ones; and it may 

 explain why heavy irrigations, as will be shown later, do 

 not yield proportional increases of dry matter, 

 H 



