i62 BACTERIA AND SOIL FERTILITY 



to leach out the soluble nitrogen and thus stimulates to greater 

 action the legume bacteria, for it is known that the legume feeds 

 first on the combined nitrogen of the soil and turns to the atmos- 

 phere only when this is greatly reduced. 



Excessive quantities of water may exclude the nitrogen from 

 the roots and also favor anaerobic action, both of which would be 

 detrimental to the legume bacteria. 



Influence of Fertilizers. — The legume bacteria require the 

 same elements for their growth as do other plants, and the appli- 

 cation of fertilizers to a soil which increases the available potas- 

 sium and phosphorus is attended by an increased bacterial activity. 

 However, it has long been known that nitrates inhibit nodule 

 formation. Wilson found that nodule development was pre- 

 vented by the presence of nitrates, sulfates, and ammonium salts, 

 although the organisms retain their vitality in the presence of these 

 salts. It is thought by some that the addition of soluble nitrates to 

 the soil decreases by a kind of compensatory action the formation 

 of root tubercles by legumes. Legumes growing on soil rich in 

 nitrates may actually be immune to the nodule bacteria and pre- 

 vent their entrance into the roots. Small quantities of nitrates 

 tend to stimulate. 



Legumes Associated with Non-legumes. — For centuries it 

 has been the practice in China, Japan, Western Asia, Northern 

 Africa, as well as in ancient Rome, to grow legumes and non- 

 legumes in combination, and there is no doubt but that time and 

 again practical farmers have noted the more vigorous growth and 

 darker green of non-legumes when so grown long before the 

 investigations of Hellriegel and his associates established the fact 

 that legumes are capable of utilizing uncombined atmospheric 

 nitrogen, His discovery gave the key to the mystery — the non- 

 legume was getting combined nitrogen from the legume. This 

 was strikingly demonstrated by a very ingenious experiment con- 

 ducted by Lipman. He grew non-leguminous plants in soil in a 

 porous pot surrounded by earth in a larger glazed earthenware pot 

 in which leguminous plants were growing and found that under 

 favorable conditions non-legumes associated with legumes may 



