MAINTENANCE OF ORGANIC MATTER 131 



63. Chemical effects. The chemical effects are of 

 two sorts: (a) Vegetable and animal remains contain 

 all of the essential elements of plant food, and by their 

 decay these are given back to the soil in a form readily 

 available as food for other plants. It is therefore a 

 direct source of food elements. 



(b) The products of the decay of organic matter are 

 many forms of organic acids, the simplest and most 

 abundant of which is carbon dioxid. In the soil moisture 

 these act powerfully upon the mineral soil particles 

 to bring their elements particularly the bases into 

 solution. Because of their presence, the soil water 

 must be regarded as a weak solution of all of these 

 products and by their presence its dissolving power is 

 greatly increased. 



64. Maintenance of organic matter. Two conditions 

 are necessary to maintain an adequate amount of 

 organic matter in the soil. These are, first, an adequate 

 supply, and second, avoidance of a too -rapid loss, 

 together with the maintenance of those soil conditions 

 which promote the proper form of decay. 



The organic matter derived from the higher plants 

 is supplemented by that from bacteria and fungi, which 

 are generally abundant in the soil. Much may be accom- 

 plished by good soil management to favor the develop- 

 ment of the lower forms, so that they may be a very 

 important source of humus. In fact, it has been sug- 

 gested that they may sometimes be the chief source 

 of supply. 



Any plant may be used as a green manure, to furnish 

 organic matter to the soil. Plants which have been 



