THE CHEMISTRY OF THE SOIL. 71 



It far exceeds in importance the question of manuring, and it is sate to 

 say that unless the conditions as to moisture are satisfactory, the application 

 of manures is not likely to be of any benefit, and the money expended on 

 their use is practically thrown away. 



Apart from the question of cultivation and drainage, the maintenance of 

 the best conditions as to water within the soil depends to a very large 

 extent upon the presence of humus. Humus, which is derived from the 

 gradual decay of animal or vegetable matter within the soil, is one of the 

 most important of the soil's constituents, and any great variation in its 

 amount affects profoundly the value of the soil for agricultural purposes. 



Functions of Humus. 



The presence of humus in the soil increases the fertility in the following 

 ways : — 



In the first place it absorbs and retains moisture in the soil, and prevents 

 surface evaporation. A surface soil, fairly rich in humus, exercises much 

 the same influence on the underlying soil as does a mulch of dead leaves or 

 other vegetable matter. During dry spells, and under the influence of hot 

 winds usually prevalent under such conditions, the loss of moisture from the 

 soil by surface evaporation is enormous, and in soils destitute of humus this 

 loss is so rapid as to result in the drying up of the soil and the wilting of the 

 crops. The final result of such conditions is the formation of scalded spots 

 and the complete removal of the fine surface soil by the wind in the form of 

 dust. 



The humus in the soil is the ingredient which is most subject to alteration 

 and destruction, and under dry conditions it is more or less rapidly 

 destroyed. As soon as it has lost its moisture and become dry it is rapidly 

 burnt out by the combined action of sun and air. So that it is exactly in 

 those circumstances where its presence is most essential that it is most liable 

 to destruction, and the necessity for renewing it most urgent. 



The presence of humus in the soil tends to improve its texture, 

 lightening and loosening it, and preventing compaction of the surface, so 

 that it is of special value in the amelioration of stiff soils. 



It is the principal source of nitrogen in the soil, and by its decay under 

 the influence of soil organisms, ammonium salts and nitrates are produced, 

 which are the forms in which this important element is assimilated by the 

 plant. It is of interest to remember that the humus of arid or semi-arid 

 regions is richer in nitrogen than that of the moister districts. This is a 

 point of great importance with reference to the potential fertility of these 

 soils. In point of fact, from a variety of causes acting together, the soils of 

 the dry climates are richer in plant-food of all kinds than are the soils in 

 regions of greater rainfall, consequently nothing but the absence of water 

 prevents these from being extremely reproductive. There is, therefore, no 

 problem which exceeds in importance that of retaining in the soil the little 

 moisture that it receives, and any operation that succeeds in arresting even 

 partially the unavoidable loss of that moisture deserves the highest con- 

 sideration. 



Methods of supplying Humus. 



There are three ways of supplying humus to soils in need of this constitu- 

 ent, namely, by the application of generous additions of farmyard manure 

 (in cases where this is available), by the application of compost manure, and 

 by green-manuring, or the ploughing under of a quickly-growing green crop 

 (leguminous for choice). 



