AMELIORATION OF SOIL 137 



standing at an injurious height. A few pipes generally suffice 

 for this work, and the effect they produce is immediate 1 . 



The second case is that of a deep clay. Here there is no 

 water reservoir to be tapped ; the whole soil is equally full of 

 water, and holds it firmly. Drain-pipes are usually laid in 

 such a soil, 15-20 ft. apart, and 3 ft. below the surface. 

 Little or no water is at first delivered by the pipes, and no 

 improvement in the soil may be perceived for some time, the 

 effect being very gradual. The amelioration consists first and 

 chiefly in an alteration in the physical character of the clay, 

 which commences round the pipe and gradually extends from 

 it. The clay subsoil, being opened up by the introduction of 

 pipes, is in fact placed under the influence of the changes 

 of temperature and changes in the condition of dry ness, which 

 we have already seen determine the gradual disintegration of 

 a tenacious soil, and the formation of a looser texture. The 

 forces thus brought to bear upon the subsoil are far weaker 

 than those which ordinarily affect the surface ; but, on the 

 other hand, the transformation in the subsoil is not hindered 

 by the occurrence of tillage operations, or by the puddling 

 effect of heavy rain. As this change in the texture of the 

 clay proceeds, the soil is able to part more freely with its 

 excess of water, and the drying thus effected serves to extend 

 still further the alteration in the character of the clay. Drain- 

 age operations will be especially successful when lime is at 

 the same time applied to the surface. 



1 If the impervious layer is quite near the surface, and is of the nature 

 of a ' pan ' (see pp. 28, 29, 46), the most effectual remedy will be the destruction 

 of this pan by a subsoil plough. Steam cultivation is in such cases of special 

 value. In parts of the United States it has been found practicable to shatter 

 the pan, and thus ensure the drainage of the surface soil, by means of 

 dynamite cartridges. 



