92 TILLAGE— MOVEMENTS OF SOIL WATER [chap. 



light and heat, manure, etc. Of this we can have no 

 better proof than the enormous crops grown by irriga- 

 tion on sewage farms. Where the conditions are 

 favourable, and the farm is situated on a free draining 

 sandy or gravelly soil, so that the water can be often 

 renewed and drained away to keep the soil supplied 

 with air as well as water, the production of grass, 

 cabbages, and other green crops is multiplied five or 

 even tenfold by the unlimited supply of water. Speak- 

 ing generally, over a great part of England, where the 

 annual rainfall is from 35 to 25 inches, a large proportion 

 of which falls in the non-growing season, it is necessary 

 to husband the water supply, and it will be found that 

 one at least of the objects of many of our usual tillage 

 operations is the conservation of the moisture in the 

 ground for the service of the crop. From this point 

 of view, the various operations dealing with the land 

 can now be considered, such as drainage, ploughing, 

 hoeing, rolling, and other cultivations. 



The Effect of Drainage. 



Drainage is usually regarded as a means of freeing 

 the land from an excess of water, but it also has an 

 important effect in rendering a higher proportion of the 

 annual rainfall available for the crop, so that drained 

 land will suffer less from drought than the same land 

 in an undrained condition. 



Land may require drainage for various reasons : it 

 may possess a naturally pervious subsoil, and yet be 

 water-logged owing to its situation, or the subsoil may 

 be so close in texture that percolation is reduced to a 

 minimum and the surface soil remains for long periods 

 almost saturated with water, especially if the slope is 

 gentle and water lies after rain until very large amounts 



