6 



these and all the other ash constituents essential for the growth of plants. 

 Nitrogen, on the other hand, is derived from the air and is incorpor- 

 ated into the soil largely by means of plants. Consequently, the natural 

 richness of a soil in nitrogen is almost entirely dependent upon the 

 amount of decaying organic matter present. Through careless cultiva- 

 tion, this original supply of nitrogen may be depleted ; or by growing 

 plants, particularly legumes, the nitrogen gatherers, it may be increased. 

 There is an almost unlimited supply of nitrogen in the atmosphere and 

 man has been given the means of gathering this and incorporating it in 

 the land. As a result, the amount of this element in the soil, more than 

 any other plant food constituent, is within the control of the farmer. 

 Moreover, the addition of organic matter to a soil has a very much wider 

 bearing than the simple addition of nitrogen ; for, in its decay the vege- 

 table acids and the carbon dioxide formed tend to bring the insoluble 

 potash and phosphoric acid into an available form. Humus, which has 

 such a wonderful effect on the mechanical condition of the soil, and which 

 so increases its water-holding capacity, is also a product of the decay 

 of organic matter. In fact, the presence of an abundance of decaying 

 organic matter is practically indispensable. It is the source of nitrogen ; 

 the acids liberated in its decay make available the important ash materials 

 which would otherwise be useless ; it warms the soil ; increases its capa- 

 city to hold water needed to dissolve the plant food; and improves its 

 physical condition. Without the presence of organic matter and the 

 associated germ life and the proper conditions for their action, a soil 

 cannot produce its best results, no matter how rich it may be in all the 

 essential constituents of plant growth. In one sense it may be correct to 

 speak of a soil as a reservoir of plant food, to be drav/n on for the growth 

 of successive crops, but it is equally correct to regard it as a busy, complex 

 manufacturing establishment in which all the various parts must work to- 

 gether under proper conditions to bring the store of plant food into a form 

 available to plants. To bring this about is the object of cultivation. 



Losses of Plant Food by Leaching. 



But these combined agencies, while beneficial, are destructive unless 

 means are taken to prevent loss by drainage. They tend to bring nitro- 

 gen, lime, magnesia, potash, etc., into a soluble form, which, unless 

 taken up by plants, is lost in the drainage water. As proof of this, we 

 have the familiar fact that water taken from underground drains or from 

 wells is "hard" because of the lime which it holds in solution. Conse- 

 quently, a surface soil is generally poorer in lime, and frequently in pot- 

 ash, than the subsoil. The complete impoverishment of the soil is pre- 

 vented by the presence of crtain constituents which combine chemically 

 with the liberated plant food substances, and by the conservative action 

 of vegetation. The plant is continually collecting from the soil and sub- 

 soil dissolved or easily soluble matter, storing these in its tissues, and at 



