40 THE WORK OF THE LEAF [cHAP. 



equivalent to 9 inches of rain. It is often impossible to 

 lay drains in a clay soil in the autumn on land which 

 has been cropped, because the subsoil is so dry as to be 

 unworkable by the ordinary drainage tools. Again, the 

 drying action of the crop explains the difficulty that is 

 often met with in the south and east of England in 

 attempting to secure a second catch crop by sowing on 

 the stubbles immediately after harvest ; unless there is 

 timely and continued rain, both soil and subsoil are too 

 dry to support any satisfactory growth. Even if a stand 

 of vetches or crimson clover is established, it is apt to 

 leave the land in spring so depleted of water that the 

 succeeding crop of roots is jeopardised unless there is 

 an abundant early summer rainfall. Another example 

 of the drying power of a growing plant is seen in the 

 wide circle of stunted growth which extends into a corn- 

 field round a tree in a hedgerow, should the season have 

 been a dry one. By its roots the tree has robbed the 

 subsoil of water much more than it has deprived the 

 crop of either light or nutriment. The fact that the 

 growth of a satisfactory crop necessitates the evapora- 

 tion of a certain amount of water is nowhere more 

 distinctly recognised than in the practice of alternating 

 one or two years of crops with a year of bare fallow 

 which prevails in some of the western districts of North 

 America, and also in Australia, in regions where there 

 is but a low annual rainfall of 8 to 12 inches. This 

 amount of rain is insufficient for more than a very small 

 crop year after year ; but by leaving the land without 

 crop for a season and cultivating the surface of the soil 

 so as to check evaporation, the rainfall of the fallow year 

 can be so far accumulated that when combined with the 

 normal rainfall of the following year a profitable crop 

 can be grown. Over a wide area of semi-arid country 

 farming can be made profitable without irrigation if the 



