CEREALS AND FORAGE PLANTS 363 



are three great forage plants, which are used also for other 

 purposes. They are clover, alfalfa, and cow-pea, and any 

 outline of agricultural operations which does not include these 

 great crops would be incomplete. They all belong to a single 

 great family (Leguminosse) , and associated with them are 

 such familiar plants as sweet peas, common peas, beans, 

 peanuts, and such trees as the locusts and redbuds. 



These three forage plants have a very important character 

 in common, which can be described for all of them. They are 

 all able to use the free nitrogen of the air by their peculiar 

 association with the nitrogen-fixing bacteria of the soil. 

 This means that instead of drawing upon the very important 

 nitrates of the soil, they can add to the nitrates and thus 

 enrich the soil. For this reason they can be used to restore 

 soil that has become impoverished in its nitrogen supply by 

 other crops. It is customary, therefore, to alternate crops of 

 these clover-like plants with other crops, notably the cereals, 

 the process being called " rotation of crops." In other 

 words, these forage plants are very commonly used as the 

 " alternating crop " which restores the soil to good condition. 



These plants are not only useful in adding nitrogen com- 

 pounds to the soil, but they are also remarkably deep rooted 

 and leave the soil in better physical condition. The deep- 

 rooting not only puts the soil in better physical condition, 

 but it facilitates the movement of salts towards the surface, 

 so that the result of such a crop is not only an accumulation 

 in the superficial soil of nitrogen compounds, but also of other 

 important soil salts. 



Another very common use to which these plants are put, a 

 use which depends upon their rich contents of valuable salts, 

 is what is called " green manuring," which means that the 

 plants are ploughed into the soil and contribute their whole 

 bodies to enriching it. 



70. Clover. There are a good many clovers, but the 

 most valuable one as a forage plant is the red clover, whose 



