USE OF SOIL MOISTURE BY PLANTS 119 



hairs. In general, the stronger the soil solution the less 

 rapidly will plants take water from the soil with a given 

 rate of growth. This is not an invariable law, however, 

 since it depends, in part, on the nature of the soil materials 

 that go into solution. If the soil solution is acid, the rate 

 of absorption by plants is accelerated; if alkaline, it is 

 retarded. In the vast majority of cases, soils are alkaline 

 rather than acid. Especially in arid regions is the occur- 

 rence of acid soils infrequent. 



The soil solutions of fertile soils are usually more con- 

 centrated than those of less fertile soils. It follows, there- 

 fore that, the more fertile a soil is, the less rapidly does the 

 plant absorb the soil moisture with a given rate of growth. 

 This law, which has been demonstrated in a number of 

 interesting experiments, teaches the farmer the great 

 importance of keeping the soil in a most fertile condition. 

 Bouyoucos has made some interesting observations on 

 this subject. As above stated, the more concentrated the 

 soil solution is, the less rapidly do plants take moisture 

 from the soil. Yet this concentration need not always be 

 due to plant-food, for Bouyoucos has shown that an 

 innocuous soluble substance, such as common salt or 

 sodium sulfate, if added to the soil, decreases the rate 

 at which the plants take water from the soil. This is 

 important because of the fact that in a great many 

 irrigated soils of the country, resulting from the peculiar 

 climatic conditions, are found considerable quantities of 

 common salt, and soluble salts of magnesium, calcium 

 and other elements which are not needed as plant-foods. 

 These accumulations, ordinarily known as alkali, when 

 present in large quantities, are a serious menace to suc- 

 cessful agriculture. The above law seems to show, how- 

 ever, that the presence of such materials in the soil may 



