146 SOILS: PROPERTIES AND MANAGEMENT 



that no mean portion of the soil phosphates is held in 

 organic combination. The promotion of favorable hu- 

 mous decay must thus liberate a considerable amount of 

 phosphorus for plant utilization. 



98. Organic content of representative soils. — The 

 organic content of soils varies widely according to climatic 

 conditions. The following average data show the limits 

 of variation as well as the comparative content of the 

 important soil sections of the United States : — 



Organic Content of United States Soils 



It is at once apparent that the subsoil contains con- 

 siderably less organic matter than do the surface layers. 

 Also, the areas of the United States that have been 

 glaciated are noticeably richer in organic material than 

 the residual, coastal plain, and arid regions. This is 

 largely a climatic and geochemical relationship. Some 

 soils, particularly alluvial soils, very often run higher 

 than the average data given above. An organic content 

 of 5 or 6 per cent is not an uncommon figure with such 

 materials. Muck and peat soils are of course not to be 

 classified with the above, as their organic content may 



