108 TILLAGE— MOVEMENTS OF SOIL WATER [chap. 



round the roots of the grass after the surface has been 

 lifted by the winter frosts and by the action of worms, 

 it should be borne in mind that it is easy to do harm 

 by injudicious rolling in wet weather on soils that are 

 at all heavy. Even on grass land the clay may become 

 so puddled or tempered that it dries round the roots 

 with a very harsh caked surface, little permeable to 

 air and water. This sort of damage is perhaps most 

 often seen on lawns and cricket grounds which are 

 often rolled repeatedly with heavy rollers when the 

 ground is thoroughly wet; a smooth, pasty surface is 

 produced to the ultimate great detriment of the growth 

 of the grass. Of course upon arable land the greatest 

 care must be taken never to roll when the top is at all 

 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 



