THE MAIN TYPES OF SOIL 71 



atmosphere in the course of a year, and it 

 has been found that the quantity is not 

 usually much, if at all, in excess of 5 Ib. 

 or 6 Ib. per acre. 



Besides the bacteria associated with the 

 roots of leguminous and a few other plants, 

 a certain number of organisms living inde- 

 pendently in soil also possess the power of 

 inducing free nitrogen to enter into combina- 

 tion, and so of becoming available for the use 

 of the higher plants. This side of the question 

 has not yet received so much experimental 

 attention as the other, but enough has been 

 done to show that such organisms are of high 

 importance. As in the case of the nitrify- 

 ing organisms, so in the case of these free 

 nitrogen-fixing organisms, thorough aeration 

 of the soil, and a non-acid condition, com- 

 bined with a proper degree of moisture, 

 appear to be favouring factors. 



In considering the organisms that play a part 

 in plant nutrition, one must not overlook the 

 action of worms, which are present in practi- 

 cally all cultivated soils. Attention was 

 specifically drawn to the importance of worms 

 by Darwin, who attributed no inconsiderable 

 part of the fertility of our fields to the action 

 of these creatures. It is probable, however, 

 that their effects have been considerably 



