214 MODIFICATION OF THE SOIL POPULATION 



rapid to result in the conservation of most of the original nitrogen 

 which is present. 



Green manure added to the soil is comparatively free from 

 microorganisms, except those which are carried down by the dust 

 and may be adhering to the plants, or those organisms which may 

 actually be pathogenic upon the living plant. Also, some microbes 

 may have begun an attack of those parts of the plants which have 

 died off. Leguminous plants are, however, very rich in the spe- 

 cific nodule-forming bacteria, which become liberated on the dis- 

 integration of the nodules; the soil thus becomes heavily inocu- 

 lated. 



Stable manure is very rich in bacteria. It has been suggested 

 that the favorable effect of manure upon soil fertility is due not 

 so much to the nutrients in the manure (about 10 pounds of nitro- 

 gen, 2 pounds of phosphorus, and 8 pounds of potassium in one 

 ton of fresh manure containing 80 per cent of mositure), as to 

 the presence of large numbers of bacteria. The introduction of 

 these bacteria into the soil was believed to bring about a modifica- 

 tion in the soil population of such magnitude as appreciably to 

 influence the soil processes. If that were the case, the frequent 

 additions of small quantities of manure might suffice, since even 

 a hundred pounds of manure would be more than sufficient to 

 properly inoculate the soil with the various bacteria. However, 

 the effects of the manure do not appear to be the result of such 

 modifications of the soil population. In the first place, the kinds 

 of bacteria in the manure are of a distinctly different nature from 

 those in the soil. The numerous cells of Bacterium coli and of 

 other intestinal bacteria present in such abundance in the manure 

 soon die off, after the manure is plowed into the soil. On the other 

 hand, the extent of microbial development in the soil is associated 

 with the amount of available food and with the environmental con- 

 ditions. If the food supply is increased, or if the environmental 

 conditions are made more favorable, the numbers of micro- 

 organisms capable of using this energy will also increase. No 

 matter how many bacteria are added to the soil (the same is true 

 of fungi as well), the nature of the population will not be changed, 

 unless additional food is added or unless the soil is treated in such a 

 manner as to render the organic matter already present there more 

 available. There are certain exceptions to this rule, as in the case 

 when microorganisms are added to soils in which they do not exist, 



