ROTATION OF CROPS. 



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crop; but if this crop is changed to a distant locality, they 

 often perish for want of their proper food. So, many 

 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 perpendicular roots 

 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 

 of its ammonia, phosphates, etc., than is drawn by all 

 other parts of the plant. Root crops are generally less 

 exhausting, and plants cultivated for their leaves are 

 usually still less so. 



Enough has been stated to show the necessity of a 

 change of crops, and the following are found the best 

 rules to observe in practice: 



1. Crops of the same species, and even of the same 

 natural order, should not succeed each other. 



2. Plants with perpendicular roots should succeed 

 those with spreading and superficial roots, and vice versa. 



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