48 THE CHEMISTRY OF THE FARM 



increased production of nitrates must be ranked as 

 the most important. 



By means of pipe drainage the various chemical 

 actions just mentioned are carried down to a greater 

 or less extent into the subsoil, for as the water level 

 is lowered the air enters from above to fill the cavities 

 in the soil. By draining, the depth to which the roots 

 penetrate, and consequently the extent of their feed- 

 ing ground, is increased ; roots will not grow in the 

 absence of oxygen, and rot as soon as they reach a 

 permanent water-level. In a water-logged soil deoxi- 

 dation is active, the nitrates present are destroyed, a 

 part of the nitrogen being evolved as gas; the soil 

 may thus suffer a considerable loss of plant food by 

 lack of drainage. 



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 the roots 

 of crops, and by worms. The porosity of stiff soils 

 is largely increased by the two agencies last named. 



14. Clay Burning. — Burning is occasionally resorted 

 to as a means of increasing the available plant food, 

 and improving the texture of a heavy soil. The soil 

 is mixed with vegetable rubbish and burnt in heaps, 

 which are then spread over the land and ploughed in. 

 If the soil contains lime this will attack the silicates 

 of the soil, 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 temperature. 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. 



