244 SOILS AND FERTILIZERS 



tion of crops is, therefore, less likely to cause a deficiency 

 of some one constituent than is a continuous growth of 

 one crop, and it utilizes more completely the available 

 nutrients. 



303. Some crops or crop treatments prepare nutriment 

 for other crops. — It is quite evident that leguminous crops 

 not only leave in the soil an accumulation of organic nitrogen 

 transformed by bacteria from atmospheric nitrogen, but that 

 they leave part of the nitrogen in a form readily available 

 for use by other plants. The presence of a grass crop on the 

 land for several years favors the action of non-symbiotic 

 nitrogen-fixing bacteria. 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. 



Stirring the soil at intervals during the summer greatly 

 facilitates decomposition, and leaves a supply of easily avail- 

 able food material. The introduction of intertilled crops in 

 the rotation thus serves to prepare nutriment for those that 

 receive no intertillage. 



304. Crops differ in their effect on soil structure. — Plants 

 must be included among the factors that affect the arrangement 

 of soil particles. The result of root growth is usually to im- 

 prove the physical condition of soil. In general, crops with 

 rather shallow and very fibrous roots are most beneficial, at 

 least to the surface soil. Millet, buckwheat, barley and to a 

 less extent, wheat leave the soil in a friable condition. It is on 

 heavy soils that this property is most beneficially exercised. 

 Tap-rooted plants, and others with few surface roots, do not 

 exhibit this action. Alfalfa and some root crops are likely 

 to leave the soil rather compact as compared with the crops 

 mentioned above. The effect of sod is nearly always bene- 

 ficial to heavy soils, and this is one of the reasons for using 

 a grass crop in a rotation. 



