314 SOIL PHYSICS AND MANAGEMENT 



the soil. If a heavy soil or a soil rich in organic matter should 

 become puddled in this way by a shower, upon drying, shrinkage 

 cracks will be formed through which air may enter. Tillage would 

 not be so necessary in that case. In light sandy soils this puddling 

 will not take place. 



QUESTIONS 



1. Give uses of air in soils. 



2. What are some indications by the plant that oxygen is deficient? 



3. Upon what does the amount of air that the soil will contain depend? 



4. What causes variations in the composition of soil air? 



5. In table on page 310 what is the average amount of nitrogen? 

 G. How does this compare with the normal amount in air? 



7. Define aeration. 



8. Why is it necessary? 



9. What is diffusion ? Illustrate. 



10. What effect does porosity have on diffusion ? 



11. How does temperature affect it? 



12. How does removal of water aid diffusion? 



13. How much change in atmospheric pressure in a week? 



14. What is Boyle's law? 



15. What part of an atmosphere is represented by a change of 0.45 inch 



of pressure? 

 1(>. How does atmospheric change effect aeration? 



17. Explain the effect of temperature changes on aeration? 



18. Give average change for the four months for each depth and for each 



treatment. 



19. How much of a change in volume would occur for each if the soil 



volume were one-fourth air? 



20. Explain the effect of tillage on aeration. 



21. Give effect of wind movement. 



22. What is a water-logged soil ? 



23. How may it be avoided? 



24. Why is it detrimental to the crop? 



25. What effect does running together have on aeration ? 



REFERENCES 



', Johnson, S. W., How Crops Feed, 1891, p. 219. 

 * Unpublished Data, University of Illinois. 



