198 SOILS 



make it more moist at the times when moisture is 

 needed most. This is a feature of drainage that 

 many people find hard to understand, yet the 

 explanation is very simple. Drainage lowers the 

 water-table, thus increasing the volume of soil 

 above it in which the roots of plants can feed, for 

 they can use only film water. The larger the area 

 of soil above the water-table, the more film water 

 there is for the plants to use. They root deeper 

 and so are farther away from the dry surface soil. 

 Furthermore, a soil is more mellow after being 

 drained than before, so it can absorb and hold 

 more water as film moisture, and its ability to 

 draw up water from the water-table is increased. 

 Under-drainage simply carries off free or standing 

 water, thus leaving more room for the film water 

 that plants use. Hence it is that a drained soil is 

 dryer in a wet time and more moist in a dry time 

 than before it was drained. 



In humid regions under-drainage maybe equiva- 

 lent to irrigation as a means of supplying water to 

 the crop. The farmer who drains his land owns 

 more soil than he did before ; for until the water- 

 table was lowered he had the use only of the soil 

 above it, the only part in which the roots of his 

 plants can feed. If he lowers the water-table 

 two feet he adds a layer of soil two feet thick to 

 his property. He has two feet more of soil in 

 which the roots of his plants may find nourishment. 

 This is the cheapest way of increasing the size of 

 the farm. 



After a soil has been drained the roots of plants 

 penetrate it deeper, earthworms burrow deeper 

 in it, air follows these channels and the ventilation 

 of the soil is still further improved. A system of 

 tile drainage is itself very effective in aerating the 



