ROTATION SYSTEMS FOR GRAIN FARMING 229 



For southern Illinois and other Southern states, a four-year rota- 

 tion of (i) corn, (2) cowpeas (or soy beans), (3) wheat (or oats), 

 and (4) clover is very satisfactory; and a three- year rotation, 

 in which it is more difficult to maintain the nitrogen, is (i) wheat, 

 (2) corn, and (3) cowpeas; or (i) cotton, (2) corn and cowpeas, 

 and (3) oats and cowpeas, in either of which soy beans may be sub- 

 stituted, and should be substituted in case of danger from cowpea 

 wilt or other disease; and similarly, alsike or sweet clover may be 

 sometimes substituted for red clover in case of clover sickness, 

 which is more fully discussed later on. In these rotations consider- 

 able use can be made of legume catch crops. Thus red clover or 

 sweet clover may be started with the wheat and plowed under the 

 following spring as green manure for corn, or cowpeas can be grown 

 after the wheat is harvested. Clover or vetch or cowpeas (or a 

 mixture of legumes) can be seeded in the corn at the time of the 

 last cultivation and plowed under late the following spring before 

 seeding the regular cowpea crop; and, where cotton is to follow, 

 some legume catch crop could be seeded after the regular cowpea 

 crop is harvested, allowed to grow during the late fall, winter, and 

 early spring, and plowed under for cotton. 



If necessary, not only the cotton stalks, but also the cotton seed 

 may be returned to the land, the lint of itself being of much greater 

 value than any grain crop. (Two bales of cotton, or 1000 pounds 

 of lint, worth $100, is no larger crop, comparatively, than 100 

 bushels of corn, worth $40, as a ten-year average price in Illinois.) 



Any one who is familiar with agricultural practice can estimate 

 closely the probable or possible crop yields, and with the yields 

 determined and with the disposition of the crops, catch crops, and 

 crop residues decided upon, any one can compute very closely 

 from the data given in Table 23 as to the probable maintenance 

 of the nitrogen supply. 



Two factors of opposite effect (i) the loss of nitrogen, espe- 

 cially by leaching, and (2) the addition of nitrogen in rain and by 

 fixation of free nitrogen independent of legume plants, especially 

 by the azotobacter (factors which tend to counterbalance each 

 other) are discussed on another page. 



From all of the facts it will be understood that there is just as 

 much reason and as much satisfaction in computing that a 50- 



