30 BULLETIN 721, IT. S. DEPARTMENT 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 tlian 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 
different 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. Frequently 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 
