IV.] 



WATER EVAPORATED BY CROPS 



121 



wet or even damp, lest a pasty surface be developed, 

 which will dry to a glazed baked crust. It is necessary 

 even to wait until the dew has been dissipated before 

 rolling strong land that has been well worked and 

 drilled for roots. 



The Drying Effect of Crops. 



Since a crop transpires about 300 lbs. of water for 

 each pound of dry matter .produced, any land which 

 is carrying a heavy crop must contain much less water 

 than the adjoining uncropped land, unless there has 

 been such an excess of rainfall as to saturate the soil 

 in either case. Any summer growing crop, however, 

 especially one of roots, transpires so large a proportion 

 of the customary rainfall during the period of growth, 

 that it must leave the soil much drier for its growth. 

 As an example of this removal of water by the growing 

 crop, the following figures obtained at Rothamsted 

 during the very dry summer of 1870 may be quoted, 

 showing as they do the water present in successive 9 

 inches of fallow and of adjoining land carrying a barley 

 crop : 



The total difference between the cropped and un- 

 cropped land down to the depth of 54 inches, amounted 

 to more than 900 tons of water per acre, or 9 inches 



