152 SOILS: PROPERTIES AND MANAGEMENT 



symbiotic action of the nodule bacteria. Green-manure 

 crops may also protect the soil from loss of plant-food by 

 leaching. The addition of barnyard manure is a common 

 method of raising the organic content from external 

 sources, and on decaying this manure performs the same 

 function as natural soil humus. Muck, peat, straw, or 

 leaves may be used in a similar manner. 



Improper soil conditions not only prevent the proper 

 decay of organic matter, but also tend to encourage the 

 production of products inimical to plant growth. There- 

 fore, in order that organic materials added to any soil 

 may produce the proper humous constituents and per- 

 form their normal functions, soil conditions in general 

 must be of the best. Tile drainage should be installed, 

 if necessary, in order to promote aeration and granulation. 

 Lime should be added if basic materials are lacking, for 

 it promotes bacterial activity as well as plant growth. 

 The addition of fertilizers will often be a benefit, as will 

 also the establishment of a suitable rotation. The. rota- 

 tion of crops not only prevents the accumulation of 

 toxic materials, but also, by increasing crop growth, 

 makes possible a larger addition of organic matter by 

 green-manuring. 



Good soil management seeks to adjust the addition 

 of organic matter, the soil conditions, and the losses 

 through cropping -and leaching, in such a way that paying 

 crops may be harvested without impairing the humus 

 supply of the soil. Any system of agriculture that tends 

 to permanently lower the organic matter of the soil is 

 impractical, and improvident, as well as unscientific. 



