EOTATIOX OF CROPS. 61 



parasites -leave their seeds or spores in the soil, to the in- 

 creased injury of the succeeding crop, if of the same 

 species. 



Again, different plants derive their principal nourish- 

 ment from different depths of soil. The roots of plants 

 exhaust only the portions of soil with which they come 

 in contact. Perpendicular rooted plants throw out few 

 side roots, and derive most of their nourishment from a 

 considerable depth, while fibrous-rooted plants seek their 

 food near the surface. Plants of the same species extend 

 their roots in a similar direction, and occupy and exhaust 

 the same strata of earth. 



Different plants by means of their roots act differently 

 upon the physical nature of the soil. Surface roots 

 spread abroad their tufted fibers, which in their decay 

 break up and lighten the surface soil, while the roots of 

 clover have a somewhat similar effect upon the deeper 

 strata. 



The most exhausting crops are, in general, those which 

 are allowed to perfect their seeds, as they extract from 

 the soil all the essentials of the plant, from the root to the 

 seed. The seeds of many species draw from the soil more 

 largely its ammonia, phosphates, etc., than the total 

 amount extracted in the formation of all other parts of the 

 plant. Root crops are generally less exhausting, and 

 plants cultivated for their leaves are usually still less so. 



A rotation was formerly thought necessary from an 

 idea that each plant throws off from its roots into the soil 

 certain matters which are injurious to others of the same 

 species afterward grown upon the soil. It was also thought 

 that there were some tribes of plants, the fig for instance, 

 of which the acrid juices from the root injured the soil 

 and the plants grown near them, while of others, as le- 

 gumes, the sweet juices were beneficial to the soil and the 

 adjacent or succeeding crops. These views are not now 

 considered tenable, 



