506 THE PRINCIPLES OF SOIL MANAGEMENT 



in the rotation. The presence of a grass crop upon the 

 land for several years favors the action of non-symbiotic 

 nitrogen-fixing bacteria, as already explained (see page 

 429). The grass crops also leave a very considerable 

 amount of organic matter in the soil, which by its gradual 

 decomposition contributes both directly and indirectly 

 to the supply of available nutrients. As the organic 

 matter left by the legumes and grasses decomposes 

 slowly, these crops should be followed by a coarse 

 feeding crop, like corn or potatoes, and one which 

 is at the same time a cultivated crop, as are these. 

 Stirring the soil at intervals during the summer greatly 

 facilitates decomposition, and leaves a supply of easily 

 available food for more delicate feeders, like wheat or 

 barley, that may follow the cultivated crop. The intro- 

 duction of cultivated crops in the rotation thus serves 

 to prepare food for the non-cultivated ones. Although 

 practical difficulties sometimes make it impossible to 

 follow the cultivated crops with winter wheat, the prac- 

 tice, where proper preparation of the seed-bed is pos- 

 sible, is a good one. 



360. Crops differ in their effect upon soil structure. 

 Plants must be included among the factors affecting 

 the arrangement of soil particles. The result of practi- 

 cally all root growth is to improve the physical condi- 

 tions of the soil, to a greater or less degree. In general, 

 crops with rather shallow and very fibrous roots are 

 most beneficial, at least to the surface soil. Millet, buck- 

 wheat, barley, and to a less extent wheat, leave the soil 

 in a friable condition. It is upon heavy soils that this 

 property is most beneficially exercised. 



