LEGUMES AND NITROGEN 351 



an excellent means of combating these pests by means of a 

 rotation of crops (Sec. 662). On account of the deep rooting 

 habit of many of the plants of this family, they improve the 

 physical condition of the soil, penetrating the lower layers 

 and leaving channels to carry off surplus water and admit 

 air when the roots decay. They add some available plant 

 food to the upper layers of the soil by bringing it up from 

 below. They materially increase the fertility and improve 

 the physical condition of the soil by adding a large supply of 

 organic matter. Lastly, they are very important in the 

 rotation because they have the power, through the medium 

 of bacteria which live on their roots, of taking nitrogen from 

 the air ana leaving it in the soil where it can be used by other 

 plants. 



448. How the Legumes Gather Nitrogen. Bacteria and 

 other forms of minute life often live on our useful plants as 

 parasites and do considerable injury, as in the case of the 

 grain smuts and rusts, fruit rots, and other fungous diseases. 

 In the case of the nitrifying bacteria, however, the relation 

 toward the host plant is a helpful rather than a harmful one. 

 If a healthy clover or pea or bean plant is dug up very care- 

 fully and the dirt washed away from the roots, many little 

 knots or bunches will be found on them. These knots or 

 tubercles, which vary greatly in shape and size according to 

 the plant on which they grow, are filled with thousands of 

 bacteria, too small to be seen without a very powerful 

 microscope. 1 These bacteria take the nitrogen from the 

 air and change it into a form which can be used by the 

 plants. Nitrogen is the most expensive fertilizer to pur- 

 chase; and as the legumes add it to the soil and at the same 



1A number of illustrations of typical forms of tubercles on leguminous and 

 other plants will be found in the Yearbook of the Department of Agriculture for 

 1910, pp. 213-218. 



