SOIL BURNING. 29 



sent are destroyed, a part of the nitrogen being evolved as 

 gas ; the soil may thus suffer a considerable loss of plant 

 food. 



Natural drainage in stiff soils is effected by original 

 fissures, by cracks produced in dry weather, and especially 

 by channels left on the decay of deeply-rooted crops, and 

 by worms. The porosity of stiff soils is largely increased 

 by the two agencies last named. 



9. Burning. Burning is occasionally resorted to as a 

 means of increasing the available plant food, and improv- 

 ing the texture of a heavy soil. The soil is burnt in heaps, 

 which are then spread over the land. If the soil contains 

 limestone, it is easy to see that the phosphates of the lime- 

 stone may become more available by the complete 

 disintegration which attends the conversion into lime. 

 The lime will also attack the silicates of the soil at a high 

 temperature, and liberate a part of the potash from its 

 insoluble combinations. To produce the best results it is 

 essential that the burning should take place at a low tem- 

 perature. This treatment by burning is a very extreme 

 one, and can be recommended only in few cases; it must 

 always be attended with an entire loss of the nitrogen in 

 the soil burnt. The ploughing in of burnt clay is of use 

 in improving the texture of heavy land. 



