THE SOIL 



317 



fields where alfalfa has been growing. Two or three 

 bushels of soil taken from an alfalfa field and scattered 

 over an acre of ground are enough to inoculate it with 

 alfalfa bacteria. Clover, beans, peas, and other podded 

 plants have a special kind of 

 bacteria which live in nod- 

 ules on their roots. These 

 bacteria in the nodules can 

 take free nitrogen from the 

 soil air and make nitrates 

 which the plants can use for 

 food. 



"The different chemical 

 changes produced by soil 

 bacteria are quite numerous. 

 Some kinds are specialized 

 for one series of changes, NQDUIES C 4 TAINING NlTROGES 

 others for changes of a dif- Qn the roots rf a Ieguminous plant 

 ferent sort. Some will at- 

 tack by preference carbohydrates like starch or sugar, 

 some will decompose woody tissue, some will cause the 

 decay of proteins, some of fats, etc. This division of labor 

 allows an effective decomposition of humus. Various 

 gases and acids are produced in the course of decay, and 

 help to decompose the rock particles in the soil and to 

 render the mineral plant food contained in them available. 

 The insoluble protein compounds in the roots and stubble 

 are broken down and their nitrogen changed partly to 

 ammonia. The particles of ammonia, as they are thus 

 generated by bacteria of many kinds, are at once pounced 

 upon by a special class of germs whose function it is to 

 change the ammonia into nitrate. Thanks, therefore, to 

 the activities of many species of bacteria, the nitrogen 



