258 Bacteria in Relation to Country Life 



ents of plant-food. It has been shown that when various 

 organic materials are mixed with soil and kept in the 

 open for a year or more, the organic substances in chang- 

 ing to humus gain phosphoric acid from the soil. When, 

 however, the organic materials are themselves rich in 

 phosphorus, there may be a loss instead of a gain in the 

 resulting humus. 



Leguminous green-manures and nitrogen in the soil. — 

 All green-manures, then, may be valuable because they 

 prevent the loss of soluble plant-food, and because they 

 add humus to the soil. Humus in its turn may be valu- 

 able because it improves the physical properties and 

 chemical composition of the soil. Over and above all 

 these advantages, common to all green-manuring crops, 

 those of the legume family possess the additional ad- 

 vantage of adding nitrogen to the soil. Broadly speak- 

 ing, therefore, the non-leguminous crops, in so far as 

 they are used as green-manures, affect the fertility of 

 the land by modifying the transformation of the soil- 

 nitrogen. They take up the soluble nitrates and build 

 them again into insoluble protein compounds. When 

 turned under, they undergo decay and become the food 

 of vast hosts of soil bacteria. They are resolved again into 

 simpler substances and yield ammonia and nitrates, 

 as well as a humus richer in phosphoric acid, potash and 

 lime. The green-manuring crops of the legume family 

 accomplish all this and further' modify the composition 

 of the soil by the addition to it of nitrogen. 



From the bacteriological standpoint, the differences 

 are of great moment, since there seems to be a certain 

 relation between the crop growing on the land and the 



