INFLUENCE OF FERTILIZERS 313 



On beans about twenty times as many tubercles were found in 

 the moist soil and microscopic examinations showed important 

 differences in the number of bacteria present and the structure of the 

 tubercles. Similar results were obtained with lupines and other 

 plants. This is what is to be expected, for when the root system is 

 not actively functioning the nodules are slowly destroyed by the 

 nodule-forming bacteria within and the saprophytic organisms 

 without. The nitrogen fixed by the plant is proportional -to the 

 number and size of the nodules. Hence, the gains made in combined 

 nitrogen are dependent upon the water applied to the legume. 

 This optimum will vary with different soils. Kalantarov found in 

 a loam soil that nodule bacteria require for their growth a minimum 

 moisture content of about 30 per cent., whereas Prucha found in a 

 sandy soil the optimum moisture content was from 20 to 40 per cent. 

 Wilson found that an increase in moisture content from 35 to 45 

 per cent, more than doubled the production of nodules, while with 

 an increase from 45 to 55 per cent, it was nearly doubled. There- 

 fore, water is necessary for the normal functioning of the plant and 

 bacteria, and it tends 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 atmosphere 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. 



Temperature. It is a well-known fact that the temperature of a 

 soil varies with its moisture content and a water-logged soil is slow 

 to heat up. Gain found the temperature of moist soil at midday 

 to be 7 degrees cooler than dry soil. This difference in temperature 

 persists throughout the day and night and is noted in a diminished 

 degree even to a depth of four feet. This difference may be suffi- 

 cient in some soils to greatly decrease or increase the metabolism of 

 the organism which Zipfel finds is at its optimum at a temperature 

 between 18 and 20 C. 



Influence of Fertilizers. The legume bacteria require the same 

 elements for their growth as do other plants, and the application 

 of fertilizers to a soil which increases the available potassium and 

 phosphorus is attended by an increased bacterial activity. How- 

 ever, it has long been known that nitrates inhibit nodule formation. 

 Wilson found that nodule development was prevented by the 

 presence of nitrates, sulphates, 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 prevent their 



