136 RELATIONS OF SOIL TO WATER 



The soils containing the smallest proportion of true colloid 

 clay are those most successfully treated by lime. The action 

 of the lime extends to a considerable depth, and endures for 

 some time ; it is not, however, strictly permanent, as lime is 

 continually being removed from the soil in the drainage water. 

 It is usual to apply ten to fifteen tons of chalk, or three to 

 nine tons of lime per acre. The land is best ploughed and 

 then limed in autumn, and afterwards harrowed in the spring. 



The enrichment of a clay soil with humus very much im- 

 proves its physical condition, and for garden purposes there 

 is no better treatment for a clay soil than the digging in of 

 large quantities of fresh stable manure. In the field, great 

 advantage is experienced by growing clover and grass for 

 several years, and then ploughing in the crop residues remaining 

 in the surface soil. 



The methods we have mentioned serve chiefly to ameliorate 

 the surface soil. If a deep clay soil is to be permanently im- 

 proved it is generally necessary to have recourse at the same 

 time to draining. The primitive methods of draining consisted 

 in throwing the land by the plough into high ridges or ' lands,' 

 or in cutting ditches ; the far more effective modern drainage 

 systems are carried out by the insertion of drain-pipes in the 

 subsoil, the water collected by these pipes being removed by 

 a main drain into which they deliver. 



There are two very distinct cases in which draining is a 

 remedy. If the surface soil is freely permeable to water, but 

 a bed of clay occurs in the subsoil, the water collects upon the 

 surface of this clay, and may rise to such a height as will 

 injure the fertility of the land. To remove this water it is 

 only necessary to tap the water reservoir above the clay bed 

 at such a distance below the surface as will prevent the water 



