PART II. 
BACTERIA IN SOIL AND WATER. 
CHAPTER III. 
NATURE'S FOOD-SUPPLY. THE CARBON CYCLE. 
THE CONTINUATION OF THE FOOD-SUPPLY. 
The farmer's primary occupation consists in converting soil, 
water, and air into human food. This he does 'through the agency 
of plants that grow in the soil and furnish the food necessary for 
his stock, in addition to a part of his own food. So long as plants 
find in the soil proper conditions for growth, the food-supply will not 
fail. The problem of keeping up the food-supply of plants thus 
becomes the one problem of supreme importance. 
By far the largest part of the plant food, in weight, comes from 
the air in the form of carbonic dioxid and water, and these two 
substances are practically inexhaustible. But, in addition, some 
foods are obtained from the soil. These last are present in the soil 
in limited quantities only, and some of them are found only in the 
upper layers. They are constantly being used by successive genera- 
tions of plants. This constant use, in the course of centuries, 
would have quite exhausted the soil were there not some means by 
which these supplies were replaced. That there is some such means 
is evident from the fact that plants have continued to grow on 
the same soil for countless generations, the soil remaining as fertile 
as ever. Clearly the problem for agriculturists is to find out the 
factors that have kept up the fertility of virgin soil and to apply 
them properly to cultivated soil. In this way only can the continued 
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