384 THE PRINCIPLES OF SOIL MANAGEMENT 



for these crops, and on rich soils manure is likely to cause 

 small grain to lodge. 



Farm manure, in judicious amounts, may be plowed 

 under in orchards to great advantage. 



253. Functions. The useful function which farm 

 manures perform in the soil are as follows: (1) To 

 improve the physical condition of the soil by the intro- 

 duction of organic matter, with its favorable influence 

 on the structure and moisture content. (See page 129.) 

 (2) To add a certain quantity of plant-food in a compara- 

 tively readily available condition. (3) To introduce 

 a new bacterial flora capable of increasing the rapidity 

 of decomposition of organic matter, and of thereby in- 

 creasing the amount of available fertility. 



254. Green Manures. Crops that are grown only for 

 the purpose of being plowed under to improve the soil 

 are called green manures. They may benefit the soil 

 in one or all of four ways: (1) By utilizing soluble plant- 

 food that would otherwise escape from the soil. (2) By 

 incorporating vegetable matter with the soil. (3) Le- 

 guminous crops, when used, add to the nitrogen content 

 of the soil through the fixation of atmospheric nitrogen. 

 (4) Plant-food from the lower soil may be brought to the 

 surface soil. 



A large number of crops may be used for this purpose, 

 but certain ones are more useful than others, while the 

 climate determines to some extent which crops should 

 be used. Leguminous crops have the great advantage 

 of acquiring nitrogen from the air. Crops that can be 

 planted in the fall and grow during the cool weather 

 can be utilized when otherwise the land would frequently 



