90 PLANT GROWTH 



example, that for every dry weight pound of oats, the plant 

 has absorbed six hundred pounds of water. Most of this has 

 been lost in transpiration. These figures vary widely from 

 year to year and with the locality because of the variation in 

 the environment. Of all the causes for variation in the water 

 requirement of a single species perhaps weather conditions 

 are most important. If the conditions for high transpiration 

 prevail when the plants are large they may lose enough water 

 in a week to give them a high figure for the year. Plants use 

 less water in comparison to their growth and therefore have 

 a lower water requirement if they have about optimum soil 

 moisture, but too dry or too wet a soil slows the growth of 

 the plants and causes a higher relative water requirement. 

 Plants grown in a fertile soil develop bigger plants and have 

 a lower water requirement than those grown in a poor soil. 

 Plants grown in a higher humidity have a lower water re- 

 quirement than those grown in a lower humidity. This 

 study has never been done extensively with trees nor with the 

 garden plants. The comparable water requirement for trees 

 would be exceedingly important as a guide in selecting shade 

 trees because of the difference in their competition for water 

 in the garden. Observations indicate that shallow-rooted 

 trees require more water than deep-rooted ones, but this may 

 be because the latter remove much of their water require- 

 ment from below the root level of the lawn or garden plants. 

 Transpiration and soil moisture are closely related. When 

 the soil moisture is low the transpiration rate is low and 

 growth is slow; when soil moisture is optimum, transpiration 

 is high and growth is good. It must be remembered that the 

 soil is able to hold several inches of the winter rainfall to be 

 used to supplement the summer rainfall during the growing 

 season. Recently the Soil Conservation Service has deter- 

 mined a soil moisture below which it is unwise to sow wheat 

 in some of the wheat growing states. They depend on using 

 a definite part of this reserve moisture and know that with 



