UNFAVORABLE FOOD SUPPLY 151 



267. Crop rotation economizes plant-food, because 

 some crops use more of a given food constituent than 

 others. The alternating of crops having different food 

 requirements tends to prevent the exhaustion of special 

 food substances. Crop rotation also aids in avoiding 

 damage from injurious insects and fungi. 



268. A growing crop tends to conserve fertility, be- 

 cause it reduces drainage by taking up water from the 

 soil, and at the same time appropriates the available 

 plant-food, thus preventing loss of the latter from leaching. 



269. Manures are, in part, the raw material from 

 which the cultivator turns out valuable products. They 

 should, therefore, be most carefully preserved and 

 applied. Leaching of the manure pile by undue 

 exposure to rain and over-rapid fermentation, by which 

 nitrogen escapes as ammonia or other gaseous nitrogen 

 compounds, should be stringently avoided. All refuse 

 organic matter should, so far as possible, be made to 

 increase the always too small stock of manure. 



