SOIL WATER 69 



called the transpiration ratio, because the water given off 

 by the leaves of the plants is said to be transpired. The 

 quantity of water required to produce a pound of dry matter 

 varies from 200 to 500 pounds in humid regions to almost 

 twice that amount in arid regions. There are a number of 

 factors that influence the transpiration ratio. Among these 

 are the following : 



1. The kind of plant. 



2. The quantity of water in the soil. 



3. The humidity, wind and temperature of the air. 



4. The natural fertility and manurial treatment of the soil. 



76. Transpiration by different crops. — Some kinds of 

 plants require much more water to produce a pound of dry 

 matter than do others. Oats, rye, peas and potatoes are 

 crops that have a high transpiration ratio. Wheat and 

 barley have medium ratios and corn and millet low ratios. 

 This, in a way, is a guide to the adaptability of these crops 

 to growth on dry soils. 



77. Effect of soil moisture on transpiration. — An increase 

 in the water content of any soil usually results in an increased 

 transpiration ratio for any crop grown on it. This is well 

 brought out by an experiment in which corn was grown in soil 

 contained in pots to which different quantities of water were 

 added and so maintained during the entire period of growth of 

 the plants. The results are expressed in the following table : 



Table 14. — Effect of Soil Moisture on Transpiration 



