The Rotation of Farm Crops 1615 



form humus, the supply of nitrogen available for other crops in the rotation 

 may be increased. The clovers, alfalfa, vetches, peas, and beans are able 

 to use free nitrogen from the air through the aid of bacteria living in 

 nodules, or tubercles, on their roots, when their special requirements are 

 fulfilled in the soil. Rotation gives a better opportunity to alternate 

 nitrogen-gathering plants with those that depend on the soil for the 

 nitrogen that they consume. 



5. Good physical condition is a valuable asset in soils. Freedom from 

 clods and ability to work well under tillage implements constitute what 

 is called " good tilth " in soils. A properly managed rotation of crops will 

 keep soils in this desirable condition. 



6. Soils left bare during part of the year tend to grow weeds or to wash 

 away on the slopes. Either a productive crop or one intended as a manure 

 should always occupy the land. By seeding new meadows in ripening 

 grainfields or by sowing winter wheat and rye after the removal of an 

 intertilled simimer crop, the land is kept occupied by the successive crops of 

 the rotation. 



7. Most of our farm crops have enemies in the form of insect pests or 

 plant diseases, or both. These enemies are parasites. Their perpetuation 

 usually depends on the presence of their particular host plants. Many 

 of them are not readily transferred from field to field except on fragments 

 of their hosts. If farm crops follow one another in rotation and sufficient 

 care is taken to plant seeds free from disease, the crop enemies are starved 

 out for lack of their own host plants and are thus kept in control. Potato 

 scab and the leaf spot of mangels are plant diseases that crop rotation 

 helps to control. 



8. It is thought that some crops during their growth leave in the soil 

 organic substances that are injurious to successive crops of the same kind 

 or to certain other crops that might follow them. By rotating the proper 

 crops, or by making the rotation sufficiently long and properly relating the 

 members in it to one another, the soil may be freed of such harmful substan- 

 ces and no injury noticed. This may be the best explanation for the failure 

 of red clover when grown in a continuous cropping scheme, or for the in- 

 different growth of corn when planted after buckwheat in a rotation. 



9. Weeds are a serious pest on many farms. Some kinds of weeds are 

 most troublesome in meadows and pastures but are easily destroyed in 

 plowed fields. Some weeds spread entirely through seed formation; 

 others have rootstocks that multiply and prosper when undisturbed by 

 the plow and cultivator. The alternation of hoed crops with those that 

 form sods or harbor weeds is the only practical remedy for many weed 

 problems. A good rotation of crops, thoroughly tilled, should keep harmftd 

 weeds under control if not exterminate them,. 



