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 tjiem 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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