CLASSIFICATION OF SOILS 77 



been brought about by the growth of forests, which place large quan- 

 tities of organic matter, as leaves and twigs, on the surface of the 

 soil, yet very little becomes incorporated in it. This material either 

 completely decays or is burned by forest fires. The resulting soils 

 are light colored. 



The relatively heavy rainfall on timber soils has leached out the 

 soluble material, including limestone, so that they are neutral or 

 acid. It is likely true that the leaves have aided in this process. 



6. Color. One of the most important factors in distinguishing 1 

 soils is that of color. This is primarily due to two things, organic 

 matter and compounds of iron. Color is a fair indication of the 

 value of a soil. If a large amount of organic matter is present the 

 soil will be black or brown, while a smaller amount may be obscured 

 by the more highly colored iron compounds. Dark brown or black 

 soils are usually fertile. The color imparted by iron compounds 

 varies with the degree of oxidation. 



7. Texture. The size of the soil particles and the proportion 

 of each grade are the most important factors in grouping soils into 

 classes and types. 



QUESTIONS 



1. Why are soils so complex? 



2. Why should the geological formation be used as the basis of classifi- 



cation ? 



3. What is the value of a soil map based upon the geological formation? 



4. Are all soils, derived from the same class of rocks, the same? Why? 



5. What part does heat plav in forming soils? 



6. What is the significance of moisture in soil formation? 



7. What are the divisions of soils based on moisture? 



8. Why may Texas with a 30-inch rainfall be more arid than North Dakota 



with 20 inches? 

 J). What effect does heavy rainfall have on organic matter? 



10. What agency predominates in the formation of soils of arid regions? 



11. Why do humid soils have a larger percentage of quart/, than arid soils? 



12. Why are arid soils gray or light in color? 



13. What agency is most active in the formation of humid soils? 



14. Why do arid soils have large amounts of earlxinates? 



15. Why are humid soils more highly colored than arid ones? 



16. Explain the source of organic matter of soils. 



17. (live differences Itctween timl>er and prairie soils. 



18. Locate the large prairie region of the United States. 



19. Why do the heavy forests add so little organic matter to the soils? 



20. What is the significance of color in soil classification? 



21. What is meant by texture of a soil? 



REFERENCES 

 1 Coffey. T,. X.. A Study of the Soils of the I'nited States. Bulletin 85, 



Bureau of Soils. I". S. D. A., p. 15. 

 If 0p. Cit. p. 15. 

 Op. Cit. p. 14. 



