28 SOILS AND FERTILIZERS 



These soils show a remarkably high content of phosphoric 

 acid and an abundant supply of the other substances needed 

 by plants. 



Sand dunes and volcanic dust are two other forms of 

 seolian soils but nowhere are these soils of much agricultural 

 importance. 



QUESTIONS 



1. How may soils be divided with respect to the localities in 

 which they have been formed ? 



2. What common plant-food materials have been lost in great- 

 est quantities by residual soils ? Why are these soils likely to have 

 a large proportion of clay ? 



3. In what four regions of the United States are residual soils 

 found to be predominant ? 



4. What is the characteristic constituent of cumulose soils ? 

 For what agricultural purposes are muck soils largely used ? In 

 what important plant nutrient are they likely to be deficient ? 



5. How is the velocity of a stream likely to affect the nature 

 of a soil with respect to its proportion of sand and clay ? What 

 kinds of streams form little alluvial soil ? 



6. Why are marine soils characteristically sandy ? For what 

 agricultural industry are they frequently used ? 



7. Are marine soils usually rich or poor in plant-food materials ? 

 Why? 



8. State over what areas in the United States lacustrine soils 

 are found. 



9. Why do glacial soils resemble chemically the rocks from which 

 they were formed ? What is a characteristic difference between 

 residual soil and glacial soil when both are formed from rocks rich 

 in plant-food materials ? 



10. Describe the mode of formation of the two principal kinds 

 of seolian soils in the United States. Are they characteristically 

 rich or poor in plant-food materials, and in what one particularly ? 



11. Using any map of the United States as a base (preferably a 

 colorless map showing the state boundaries and river courses), draw 

 lines tracing roughly the regions occupied by residual, alluvial, marine, 

 glacial, and seolian soils. These areas may then be shaded or colored 

 differently and a soil map of the United States thus be made. 



