188 



SOIL PHYSICS AND MANAGEMENT 



Water Transpired by Growing Plants for One Part of Dry Matter Produced 



* Hellriegel* used quartz sand in small amounts and supplied the necessary plant food 

 in solution. Figures include some loss of water by evaporation, which was not prevented 

 at first, but later reduced by means of covers. 



t Wollny' used small quantities of sand well supplied with organic matter. Perforated 

 covers materially reduced evaporation; this, however, was checked up on soil growing no crop. 



I King 4 used about 400 pounds of normal soils, in cans, some set down in the earth. Some 

 were run in the field, others in greenhouses. Water was added from beneath, so that evap- 

 oration was very slight. 



We see from the above table that the water required to pro- 

 duce one pound of dry matter varies from 214 pounds in the case of 

 beans, as determined by Lawes and Gilbert, of England, to 912 

 pounds for rape, as determined by Wollny, of Germany. The average 

 of all determinations shown above is 428 pounds. King's deter- 

 minations in Wisconsin probably apply better for the humid sec- 

 tion of this country than any others that have been made. The 

 number of trials made by King was as follows : Peas, 1 ; barley, 

 5; potatoes, 14; oats, 20; clover, 46, and corn (maize), 52. With 

 the water requirements as determined by King, a 100-bushel crop 

 of corn would require approximately 16 inches of water to produce 

 it, or 18 tons per bushel of grain ; a 100-bushel crop of oats, about 

 18* inches, or 20 tons per bushel ; a 50-bushel crop of wheat, 12.7 

 inches, or 28.7 tons per bushel, and a four-ton crop of clover, 20 

 inches of water. 



Briggs and Shantz have made determinations of water require- 

 - ments of plants in the arid regions of the United States, and their 

 results are given in the table, page 242. 



Dependent Upon Transpiration. - The amount of water 

 required by plants is dependent upon the amount of transpiration, 

 which in turn depends upon several factors, as follows : 



(1) High temperatures increase and low temperatures retard 

 transpiration. 



(2) Movement of the air increases transpiration from the plant, 



