AMELIORATION OF SOIL 135 



improving the condition of the surface soil, and obtaining a 

 good tilth in spring. If a subsoiler follows the plough in the 

 furrows, and penetrates and stirs the subsoil without inverting 

 it, still more benefit will be obtained. By such deep tillage, 

 the permeability of the surface soil is much increased, and 

 rain passes through freely to the subsoil. The improvements 

 effected by frost and tillage are not however very permanent ; 

 every storm of rain tends to bring the clay back to its original 

 condition. 



For practical purposes it is useless to attempt to improve 

 the texture of a stiff clay by admixture with sand, the effect 

 produced being far too small to render the measure economical. 

 Clay-burning is a more practical scheme ; it may be seen 

 frequently in operation in railway cuttings where it is desired 

 to prevent the erosion of the banks by storm water. In the 

 field, the dried clods of soil are made into heaps with hedge 

 cuttings, or other combustible matter, and slowly burnt at a 

 low temperature. The burnt clay is then spread over the 

 land and turned in with the plough. Sixty to eighty cubic 

 yards of burnt clay per acre is a usual dressing. It should be 

 recollected that the nitrogenous matter in the soil is lost 

 by burning ; the potash in the clay becomes, however, more 

 soluble by this treatment, if the burning has not been con- 

 ducted at too high a temperature, and especially if the clay 

 contains some lime. Clay-burning is to be regarded as an 

 extreme measure, to be adopted only when ordinary methods 

 have not proved sufficiently efficacious. 



A method far more frequently adopted is the application of 

 chalk or lime to the clay. The action of lime is peculiar. We 

 have already seen (p. 31) that it brings about the coagulation 

 of the colloid clay, and completely alters its physical character. 



