REASONS FOR INTERCHANGE OF CROPS. 273 



the soil. If wheat exhaust four degrees, rye will exhaust but 

 three and a quarter degrees, barley but two and one fourth, 

 and oats but one six-tenth degrees per bushel of the products. 



2. The leguminous plants, such as peas, beans and other 

 pulse, exhaust the soil much less than the preceding class, 

 because their leaves are more numerous, and their stalks 

 more vigorous. They are therefore able to derive more 

 nourishment from the atmosphere, while their roots divide 

 the soil more perfectly, and leave it in a better state for suc- 

 ceeding crops ; hence they have been said to impoverish the 

 soil. 



3. Root crops, such as potatoes, turnips, beets,, carrots, 

 onions, cabbages and clover, exhaust the soil less than either 

 of the preceding classes, and are hence called ameliorating 

 crops. This class are provided with large fleshy and porous 

 leaves, by means of which they obtain a large portion of their 

 nourishment from the atmosphere, in the form of ammonia, 

 carbonic acid and water. As these plants are seldom culti- 

 vated for their seeds, they rarely mature them the first season ; 

 hence they derive but little nutriment from the soil. Theii 

 bulbous or tap roots divide the soil more perfectly, and pre- 

 pare it for succeeding crops. 



The reason why some plants foul the soil more than oth- 

 ers is also due to their structure. Plants which have small 

 leaves, permit the weeds to grow, and to appropriate to them- 

 selves the nutriment which belongs to the crop. They also 

 exhaust the soil most, while plants with broad leaves cover up 

 and prevent the weeds from growing, and these also exhaust 

 the soil the least. 



II. The composition of plants explains the reason why 

 some plants exhaust the soil more than others, and hence 

 may aid us in forming a judicious system of rotation. 



We have seen, p. 169, that different plants require different 

 quantities and kinds of alkalies and salts, such as potash, soda, 

 ammonia, magnesia, etc. to complete their growth ; and when 

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