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 90(6), 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 
