78 THE ESSENTIALS OF AGRICULTURE 



that these bacteria had the power to penetrate the root, causing 

 the nodules to form, and then to take up free nitrogen gas, 

 which comprises four fifths of the volume of the air, and com- 

 bine it with other elements in a form which the plant could use. 

 It is therefore possible, through legume crops, to maintain and 

 even to increase the quantity of nitrogen in the soil. 



85. Other free-nitrogen-gathering bacteria. If a soil on which 

 no legumes are allowed to grow remains uncropped for a few 

 years, and if the amount of nitrogen in it is carefully determined 



FIG. 35. Red clover increases the available nitrogen of the soil 



at the beginning and the end of this time, the nitrogen supply 

 of the soil will be found to have increased slightly. This increase 

 is mainly due to the action of certain bacteria which have the 

 property of fixing free nitrogen from the air, independently 

 of legumes. Such bacteria are found in most soils, but under 

 ordinary conditions the amount of nitrogen supplied to the soil 

 by this means is not large. 



86. Inoculation for legumes. Not all soils are supplied with 

 the bacteria which cause the nodules on the roots of legumes. 

 This is particularly true where legumes are introduced which 

 have never been grown on the soil before. However, if a new 



