458 SOILS: PROPERTIES AND MANAGEMENT 



certain crops, as clover, alfalfa, peas, beans, and some 

 others, improve the soil, making it possible to grow larger 

 crops of cereals after these crops have been on the land. 

 Within the past century the benefit has been traced to 

 an increase in the nitrogen content of the soil, and the 

 specific plants so affecting the soil were found to be, with 

 a few exceptions, those belonging to the family of legumes. 

 It has furthermore been demonstrated that under certain 

 conditions these plants utilize the uncombined nitrogen of 

 the atmosphere (see Fig. 61), and that they contain, 

 both in the aerial portions and in the roots, a very high 

 percentage of nitrogen. In consequence, the decomposi- 

 tion of even the roots of the plants in the soil leaves a 

 large amount of nitrogenous matter. 



378. Relation of bacteria to nodules on roots. — It has 

 also been shown that the utilization of atmospheric nitro- 

 gen is accomplished through the aid of certain bacteria 

 that live in nodules (tubercles) on the roots of the plants. 

 These bacteria take free nitrogen from the air in the soil, 

 and the host plant secures it in some form from the bac- 

 teria or their products. The presence of a certain species 

 of bacteria is necessary for the formation of tubercles. 

 Leguminous plants grown in cultures or in soil not con- 

 taining the necessary bacteria do not form nodules and do 

 not utilize atmospheric nitrogen, the result being that 

 the crop produced is less in amount and the percentage 

 of nitrogen in the crop is less than if nodules were formed. 



The nodules are not normally a part of leguminous 

 plants, but are evidently caused by some irritation of the 

 root surface, much as a gall is caused to develop on a leaf 

 or a branch of a tree by an insect. In a culture contain- 

 ing the proper bacteria, the prick of a needle on the root 

 surface will cause a nodule to form in the course df a few 



