SOIL FORMATION, COMPOSITION, ETC. 83 



by artificial means. One set of factors tends to impoverish 

 the soil; the other set is constructive. It is necessary 

 to distinguish those factors which exhaust the soil from 

 those which build it up, and to know how to minimize the 

 former and to magnify the latter. In the natural state, 

 there usually is a process of enrichment due to the accu- 

 mulation of plant food elements in the surface soil. These 

 elements are left in the residues of decaying organic matter. 

 In the process of decay, the organic matter furnishes food 

 for myriads of bacteria, some of which have the power of 

 fixing the nitrogen of the atmosphere in a form that plants 

 can make use of for their growth. These organisms must 

 not be confused with the bacteria that exist in symbiotic 

 union with legumes, and fix nitrogen in such a form that 

 either the legume or a companion crop may make use of 

 it. The bacteria on legumes grow on living plants, and 

 may be termed parasitic in their mode of life, while the 

 bacteria that live on dead tissues may be termed saprophytic. 

 There is a point reached in the accumulation of plant food 

 in the soil from the plant residues at which the increase 

 and the loss in plant food about balance, due to loss through 

 leaching. 



65. Importance of the Rotation of Crops. When land 

 is planted to clean-cultured crops, two sets of losses to the 

 soil are operative; one, due to the amount of plant food 

 removed in the crop, and the other due to the increased 

 rapidity of nitrification brought about by cultivation, and 

 consequently, increased losses through leaching. 



There are advantages incidental to rotation. It is a 

 well-established fact that some plants take up greater 

 amounts of some elements than do others; that some plants 

 possess the power of taking their food from compounds that 

 others are powerless to use. Some plants have a longer 

 growing season than others, and although they may take 

 as much food from the soil, yet the drain is lighter, owing 

 to the longer growing season. The root systems of plants 



