THE CHEMISTRY OF THE SOIL. 73 



incomparably less value than the original substance out of which they are 

 derived, owing to the absence of humus material and nitrogen, both of 

 which have been lost in the process of burning. 



Instead of a heap the compost may be conveniently prepared in a pit. In 

 either case the bottom should be cemented, or so drained that the liquid 

 escaping from the mass can be collected and returned to the compost. 



It will be found advantageous to prepare a second heap while the first 

 one is ripening and being used. It will also be found that if it is desired 

 to use more concentrated fertilisers, such as superphosphate, potash, and 

 ammonium salts, these can be mixed with advantage with the compost 

 manure before being applied to the land. Used in this way they will be in 

 less danger of leaching away, and will be of greater benefit than if applied 

 directly to the land. 



(c). — Green Manuring. 



Amongst the most effective methods of supplying humus to the soil and 

 increasing its fertility is the practice of green-manuring — that is, the plough- 

 ing under of a green crop. The beneficial action of this operation is a twofold 

 one: it enriches the soil, in the first place, by supplying it with a consider- 

 able proportion of readily-available plant-food; and in the second place, by 

 adding humus, and thus improving the soil's texture and its power of absorb- 

 ing and retaining moisture. When such a crop is buried, the surface soil 

 becomes enriched by the nourishing materials which the crop, during the 

 period of its growth, has drawn from the air and from the lower portions of 

 the subsoil, and this material is now placed within the reach of the succeed- 

 ing crop. 



During the growth of the plant the soil has, in addition, been stirred up 

 and disintegrated by the development of the roots. When ploughed under, 

 provided that sufficient moisture and warmth are present, the buried mass 

 decomposes with more or less rapidity, and the succeeding crop gets the 

 benefit of the fertilising ingredients contained in the decaying mass of vege- 

 tation in a readily-available form. The resulting humus is of the greatest 

 value, not only as a source of plant-food, but in improving the soil's texture, 

 in preventing too rapid evaporation, and in enabling the soil to absorb and 

 retain water, thus rendering it less liable to suffer during dry spells. 



A further important result is the formation of carbonic acid by the decom- 

 position of the buried crop. Carbonic acid is given off abundantly in the 

 fermentation of the mass and assists in the disintegration of the soil and in 

 rendering available the plant-food contained in it. 



Green-manuring is effective both in sandy and in heavy clay soils, and, 

 indeed, on all soils deficient in humus. On sandy soils the effect of green- 

 manuring is to consolidate the soil, the humus formed binding the particles 

 together. On clay soils, the effect of the addition of humus and the produc- 

 tion of carbonic acid is to loosen and aerate them. When conditions as to 

 warmth and moisture are favourable, and the crop decomposes fairly rapidly, 

 the production of soluble plant-food proceeds with considerable rapidity. 

 This is especially the case in respect of nitrogen, which is the principal 

 manurial ingredient. Nitrification (that is, the conversion of the nitrogen- 

 ous material of the plant into soluble nitrates) takes place quite rapidly. In 

 sandy soils, green manure nitrifies more rapidly than manures like dried 

 blood, bone-dust, &c, and only less slowly than ammonium sulphate; while 

 in stiff clay soils the green crop nitrifies very much more rapidly than either 

 sulphate of ammonia or animal manures. 



