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 FV, 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 gro\si:h. If shallow- 

 rooted crops are grown continuously, only part of the 



