2/4 SOILS AND FERTILIZERS 



When remunerative crops can no longer be produced, 

 the soil is said to be exhausted. Soil exhaustion may 

 be due either to a lack of plant food, to bacterial 

 products, or to poor physical conditions arising from 

 the soil being temporarily out of condition because of 

 a one-crop system and poor methods of cultivation. 



329. Principles involved in Crop Rotation. There 

 are a few fundamental principles with which all rota- 

 tions should conform. Briefly stated these are : 



1. Deep- and shallow-rooted crops should alternate. 



2. Humus-consuming and humus-producing crops 

 should alternate. 



3. Crops should be rotated so as to make the best 

 use of the preceding crop residue. 



4. Crops should be rotated so as to secure nitrogen 

 indirectly from atmospheric sources and to promote 

 desirable bacterial activities in the soil. 



5. Crops should be rotated so as to keep the soil in 

 the best mechanical condition. 



6. In arid regions, crops should be rotated so as to 

 make the best use of the soil water. 



7. An even distribution of farm labor should be se- 

 cured by a rotation. 



8. Farm manures and fertilizers should be used in 

 the rotation where they will do the most good. 



9. Rotations should be planned so as to produce 

 fodder for stock, and so that every year there will be 

 some important crop to be sold. 



