140 GEOLOGICAL SURVEY OF NEW JERSEY. 



From a statement made in the Strasburger botany, as to the 

 quantity of water required by an oak tree, we must draw the 

 conclusion that a forest of such trees requires from 16 to 19 inches 

 of water during the growing season. The researches of Burger 

 indicate that a mixed deciduous forest, which will cut 40 cords 

 of wood per acre, will require 19 inches of water during the 

 growing season, agreeing well with the above. Von Petten- 

 kofer has stated that during the growing season an oak tree may 

 require eight times the amount of rain falling upon it, and that 

 its roots will draw the required surplus from a great depth. 



As to farm crops we have perhaps no more reliable data than 

 the interesting experiments made by Professor King, at the 

 University of Wisconsin.* Based upon these experiments we 

 find that a crop of two tons of clover to the acre requires but 10 

 inches of rain-fall. This is heavier than the normal field crop. 

 Taking the normal field crop, his results indicate that during 

 the growing season Indian corn requires 4.8 inches, barley 3.6 

 inches, oats 3.6 inches, wheat 4.5 inches, and potatoes about 10 

 inches. The average for these crops is 6 inches as against the 

 1 6 to 19 inches above estimated to be necessary for forests. We 

 may state here that it is probably true that a very light crop 

 requires much more water per ton of dry matter produced than 

 a very heavy crop, for the reason that with the light crop there 

 will be a larger amount of water evaporated directly from the 

 soil than is the case with a heavy one which keeps the ground 

 well shaded. 



Now as to fallow ground we have the careful experiments of 

 Lawes & Gilbert, at Rothamsted, England, which were con- 

 ducted during a period of twenty years. These show an annual 

 evaporation from soil kept hoed but uncropped amounting to 

 16.68 inches, with 30.29 inches rain-fall. From April to Sep- 

 tember the evaporation was 11.46 inches, which may be com- 

 pared with the above estimates of the requirements for forests, 

 while the evaporation from fallow earth during the growing 

 period for field crops was no less than the amount shown by 

 Prof. King to be the requirement of farm crops. 



Taking the annual evaporation from fallow soil, as above 

 determined by Lawes and Gilbert, and comparing it with the 



*" Irrigation and Drainage," by F. H. King. 



