88 WATER REQUIREMENT OF PLANTS. 
CONCLUSIONS. 
The measurement of the water requirement under field conditions 
is uncertain, owing to the difficulty of determining what proportion 
of the rainfall during the growing season is actually used by the crop. 
This uncertainty arises from a lack of knowledge regarding the 
amount of run-off and the amount of rainfall that is evaporated from 
the soil surface without becoming available to the crop. The pro- 
duction of the crop upon water stored in the subsoil during a period 
without rain, as in Leather's experiments, is an ideal condition for 
field measurements of this kind. Under these conditions Leather 
obtained a lower water requirement in the field experiments than in 
the pot cultures. The writers found that the water requirement was 
higher under field conditions than in pot cultures if the rainfall dur- 
ing the growing period was taken into consideration, but that the 
determinations agreed closely if the light rainfall that occurred dur- 
ing the growing season was ignored. 
SUMMARY. 
The term " water requirement " is used in this paper to indicate the 
ratio of the weight of water absorbed by a plant during its growth to 
the dry matter produced. 
The experiments made in connection with the effect of soil-moisture 
content on the water requirement show, as a rule, an increase in the 
water requirement when the soil-moisture content approaches either 
extreme. Many of these experiments were conducted in open pots 
and the direct evaporation estimated from check pots without plants. 
Owing to the uncertainty of this method when a wide range in soil- 
moisture content is employed and to the difficulty of securing a uni- 
form distribution of soil moisture when the soil is maintained at a low 
water content, it would appear that the direct effect of soil-moisture 
content on the water requirement is still an open question. 
Many investigations have been made regarding the effect of fer- 
tilizers on the water requirement, and with few exceptions the ex- 
periments show a reduction in the water requirement accompanying 
the use of fertilizers. In highly productive soils this reduction is 
slight. In poor soils the water requirement may be reduced one-, 
half, or even two- thuds, by the use of fertilizers. Often a high water 
requirement is due to a deficiency of a single plant-food element. 
As the supply of such an element approaches exhaustion, growth 
practically ceases, while the transpiration continues, which leads to a 
high water requirement. This emphasizes the importance of main- 
taining the fertility of the soils of the semiarid regions if the most 
effective use is to be made of the limited water supply. 
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