86 The Sugar-Beet in America 
the air and combining it in such a way that it can be 
used by other plants. The clovers, vetches, cowpeas, soy- 
beans, field peas, and alfalfa are all plowed under as green- 
manures. The small grains are also much used for this 
purpose. A worn-out or poor soil will usually produce a 
fair growth of rye which, when plowed under, puts the 
soil in a condition to raise other crops. For beet land 
under irrigation, probably no crop will be better as a 
green-manure than alfalfa which is used in a rotation 
wherein the last crop of alfalfa is plowed under. 
ROTATIONS 
Reasons for crop rotations. 
Some sort of crop rotation has been practiced for 
many centuries. The reasons for this practice were 
probably not at first understood; even today all the ef- 
fects of alternate cropping are not known, but so many 
reasons are now evident that no good excuse seems to 
exist for not practicing some kind of rotation on almost 
every farm. As pointed out in Table IV, all crops do not 
require the various foods in exactly the same propor- 
tions: some use more potash or nitrogen; others need 
relatively more phosphorus or lime. If one crop is 
grown continuously on the same land, the available supply 
of scarce elements is reduced and the yield will finally 
decrease; but if crops with different requirements are 
alternated, the food supply of the soil is kept in a more 
balanced condition. Each kind of plant has a differ- 
ent rooting system and manner of growth. If shallow- 
rooted crops are grown continuously, only part of the 
