NUTRITION 91 



gum mastic, and gum tragacanth; of the resins; and of 

 turpentine, rubber, vegetable dye-stuffs, perfumes, and 

 various other articles of commerce. Other plant secre- 

 tions may play an important role in practical agricul- 

 ture, in connection with the rotation of crops, as 

 indicated in paragraph go(b), below. 



90. Rotation of Crops. Closely connected with nutri- 

 tion and secretion is the question of crop rotation in 

 agriculture. Farmers have known for ages that if one 

 kind of plant is grown in the same soil year after year the 

 yield is greatly diminished. Under such conditions, for 

 example, the yield of wheat will diminish from 25 or 30 

 bushels to 12 or 15 bushels per acre, and also deteriorate 

 in quality. Various hypotheses and theories have been 

 proposed from time to time to account for this fact, but 

 only three of these theories are here noted, as follows : 



(a) Nutrition Theory. We have seen above that grow- 

 ing plants withdraw from the soil various so-called mineral 

 "nutrients," in solution in the soil-water. These are 

 essential to the healthy, vigorous growth of the plant. 

 Different kinds of plants absorb these compounds in 

 different proportion, and one theory of crop rotation is 

 based upon this fact. It is argued that, by following one 

 kind of crop with another, different demands are made on 

 the soil, and the compound of which the soil was im- 

 poverished or " exhausted " by the first crop is renewed 

 by capillary action from lower or adjacent regions. Its 

 renewal is also hastened by the application of suitable 

 fertilizer. Especially is this true in the case of nitrogen, 

 which is renewed by alternating with non-leguminous 

 plants, leguminous crops, whose root-tubercle organisms 

 renew the nitrates, as explained in paragraph 82. This 



