30 BULLETIISr 721, U. S. DEPAETMENT OF AGRICULTURE. 



tion, it would be poor policy to sell them, for the reason that by so 

 doing a large amount of plant food would be shipped away. Live 

 stock, if properly handled, enable the farmer to keep a larger propor- 

 tion of the plant foods on the farm than could be done if the crops 

 themselves were removed. Feeding the crops on the farm is the best 

 practice, and will generally yield the largest returns per unit of 

 land and per unit of labor, especially if the proper relations between 

 crops, live stock, land area, and labor are established. It is apparent, 

 therefore, that several objects may be accomplished by proper crop 

 rotation, all of which must be kept in mind in order to reap the 

 greatest returns from the sugar-beet farm. 



Effect on the soil. — As has been previously noted, all plants 

 require certain plant foods, and these elements are utilized by 

 diiferent plants in different proportions. The rotation of crops 

 insures a better utilization of these plant foods than can be obtained 

 by growing a single crop. Certain crops are deep rooted while 

 others are more shallow. The deep-rooted crops tend to stir the 

 soil to a greater depth and in this way make the plant foods more 

 readily available for the shallow-feeding crops. Certain crops aid 

 in the production of certain plant foods, as, for example, the 

 leguminous crops store nitrogen, which is rendered available to the 

 other crops grown in rotation with the legumes. Again, certain 

 crops require more or less cultivation, as is the case with sugar 

 beets. This stirring of the soil tends to expose the plant foods to 

 the action of the elements, thereby rendering the mineral material 

 available for the use of the beet plants and the plants of succeeding 

 crops. 



Relation of pests to crop rotation. — ^The rotation of crops tends to 

 reduce or to destroy plant pests which depend upon certain plants 

 for their existence. As is well known, some plant pests live and 

 thrive only on certain plants. If these plants are grown year after 

 year in the same field, they furnish favorable breeding conditions 

 for the propagation and increase of these pests. By changing to 

 other crops, plants upon which the pests can not live or upon which 

 they do not thrive may be grown and the pests thereby destroyed or 

 reduced to a minimum. Frequentl}^ the pests have resistant forms 

 or stages in which they can exist in a dormant condition for several 

 years, as is notably true of the brown-cyst stage of the sugar-beet 

 nematode and the resting-spore stage of certain fungi. In such 

 cases it is necessary to plan the rotations with a view to starving 

 out these pests. To do this the rotations must be of such a length 

 that crops upon which these pests can not thrive may be grown for 

 several years in succession. In some cases other methods must be 

 resorted to in order to control the destructive pests, but a large 



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