12 SOILf^: PROPmiTIES AND MANAGEMENT 



This agrees in general with the distribution of these 

 minerals in the earth's surface and accounts for their 

 universal presence in all soils. 



8. Organic matter. — The minerals as listed account 

 for all the elements of plant-food obtained from the 

 soil except nitrogen, which, as already indicated, is found 

 very largely locked up in proteid and other nitrogenous 

 material. The incorporation of organic matter in any 

 soil, either by natural or by artificial means, besides 

 tending to better its physical condition also enriches 

 it in its total, or gross, nitrogen content. Though this or- 

 ganic matter is so necessary in a fertile soil, its addition 

 and thorough incorporation occurs late in the process of 

 soil formation. Through the agency of bacteria and 

 other organisms the organic compounds are slowly sim- 

 plified, new compounds are split off, and nitrogen is in- 

 troduced into the soil solution mainly as nitrate, which 

 is one of the principal forms in which it may be used by 

 plants growing on the soil. 



9. The soil and the plant. — Observed from the agri- 

 cultural standpoint, then, the soil becomes purely a 

 medium for crop production. Its composition, both 

 mineral and organic, is of vital importance hi the further- 

 ance of such a use. All the physical, chemical, and 

 biological agencies become directed toward this end. 

 The study of the soil and a better understanding of its 

 function will allow the great class of landowners not 

 only to increase their crops, and consequently their 

 profits, but at the same time to maintain as far as possible 

 the fertility of our greatest national resource. A rational 

 study of the soil should ultimately lead to a study of 

 conservation in its bearing both to present prosperity 

 and to the welfare of posterity. 



