504 THE PRINCIPLES OF SOIL MANAGEMENT 



the incentive arising from diminishing productiveness 

 of the soil consequent upon long-continued cultivation, 

 coupled with an increasing population. Countries 

 having undepleted and uninfested soil, or an unpro- 

 gressive people, have done little with crop-rotations. 



Another condition that discourages the use of crop- 

 rotation is the suitability of a region to the production 

 of some one crop of outstanding value, combined, per- 

 haps, with a relatively cheap supply of fertilizing ma- 

 terial. The abundant use of fertilizers may postpone 

 for a long time the recourse to crop rotations. 



356. Principles underlying crop-rotation. There are 

 many benefits to be derived from a proper rotation of 

 crops that are not directly concerned with soil-produc- 

 tiveness. The practice of crop-rotation must depend 

 upon certain principles in soil management, some 

 of the most prominent of which are mentioned below, 

 and are modified by climatic, topographic, geographic 

 and economic features, and many other factors, that 

 cannot be treated here. 



357. Nutrients removed from the soil by different 

 crops. Some crops require large amounts of one 

 fertilizing constituent, while others take up more of 

 another. As before pointed out (see page 294), cereal 

 crops are able to utilize the potassium and phosphorus 

 of the soil to a considerable degree but have less ability 

 to secure nitrogen. They are, therefore, usually much 

 benefited by the application of a nitrogenous manure and 

 leave a considerable residue in the soil. A number of 

 other crops, as, for instance, beets and carrots, can 

 utilize this residual nitrogen. Grasses remove compara- 



