94 TILLAGE— MOVEMENTS OF SOIL WATER [chap. 



of both water and air is established in the soil above 

 the drain. In the first place, the introduction of the 

 air which follows the water drawn off by the drains 

 brings the whole depth of soil into activity, whereas 

 previously only the portion not water-logged was avail- 

 able. Plant roots cannot grow without oxygen from 

 the air, hence in a water-logged soil the roots 

 are confined to the surface layer only ; after drainage 

 the roots can penetrate as far as the air extends. 

 At the same time, all the fundamental chemical and 

 biological processes of the soil, such as nitrification and 

 weathering, are brought into action by the introduction 

 of the oxygen upon which they depend. Later it will 

 be seen that a water-logged soil results in the loss of 

 nitrogen to the land when such manures as nitrate of 

 soda are applied to it It is the extended root range of 

 the crop resulting from the introduction of air by 

 drainage which enables the drained land to resist a 

 drought better than before. In an undrained soil 

 the roots are confined to a shallow layer, which 

 they soon deprive of all moisture; further supplies 

 of water from the saturated soil move upwards very 

 slowly in a clay soil, so that the plant may suffer 

 greatly. In a drained soil, on the contrary, the roots 

 traverse the whole 3 feet or so into which air has been 

 admitted ; this mass of soil, even after it has given up by 

 percolation all the water it can, will still hold much more 

 than is contained in the shallow layer alone traversed by 

 roots before drainage. Following upon drainage, a slow 

 improvement in the texture of clay soil is always 

 manifest: by the drawing of air into the soil, by the 

 consequent evaporation and drying, a certain amount of 

 shrinkage and a clotting of the fine clay particles result, 

 which is never entirely undone when they are wetted 

 again. Roots, which afterwards decay and leave holes, 



