PART I] 
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 gen- 
erations 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 
AI 
