PLANT FOOD AND SOIL FERTILITY 31 



they will be found swollen into knots in some places. These 

 knots are called root nodules and are characteristic of legumes. 

 Each nodule contains bacteria of a kind known as nitrogen- 

 fixing bacteria. They have the power of using the free nitrogen 

 of the soil-air and building it up or fixing it into simple nitro- 

 gen compounds, such as nitrates. 

 But the nodules do not always form on the roots of legumes. 



Photograph of roots of red clover showing nodules of nitrogen- 

 fixing bacteria. Nitrogen-fixing bacteria greatly enlarged. (Wis- 

 consin Agr. Exp. Station.) 



This may be the case when an attempt is made to grow the 

 legumes on a -soil where they have never been grown before. 

 In such cases the soil must be inoculated. 



By soil inoculation is meant the process of transferring to 

 the soil nitrogen-fixing bacteria enough to start nodules on 

 the roots of the young legumes. One method is to scatter 

 over the field to be planted some of the soil taken from an- 



