THE ORIGIN OF SOIL. 55 



at present, large returns without trouble. But these crops are 

 using up the foods in the soil, and in time the wheat-grower 

 must face the problem of the renewal of the supply. If we 

 can learn the method of bringing back to the soil the lost 

 material, we shall have solved the great problem of agriculture. 

 If \ve cannot learn these methods and find out how to control 

 them, we cannot believe that agriculture will continue after the 

 few mines of fertilizers (the nitrate beds of Chili) upon which 

 we are now drawing shall be exhausted. 



The importance of any discoveries looking toward the con- 

 tinuation of this food supply is evident. It is in the study of 

 the phenomena of this food cycle with which we are chiefly 

 concerned in considering the problem of the relation of bac- 

 teria to the soil ; for we shall learn that the most vital factor 

 in this food cycle is the action of soil bacteria. The study of 

 nature's food cycle, therefore, is largely the study of the func- 

 tions of bacteria in the soil. 



BACTERIA IN THE SOIL. 



We must first notice that the soil is full of living organisms. 

 Not only are the higher plants numerous, but all types of the 

 fungi are abundant. Bacteria are extremely numerous in all 

 soil which is supplied with moisture. 



The very superficial layers of earth are extremely rich in 

 bacteria, the number varying according to conditions, from a 

 few thousands (10,000) to several millions (5,000,000) per 

 gram. In sandy soil the number is very small. When the 

 soil is polluted with decomposing organic matter the number 

 may rise to 100,000,000 per gram. As we pass below the 

 surface the number rapidly diminishes. At a distance of 

 three or four feet the bacteria are few, and at six feet they 

 have mostly disappeared. Below this they are rarely found 

 and in the strata of soil at moderate depths of ten or fifteen 

 feet they are practically absent ; in lower layers they are 



