DRAINAGE 229 



many soils that the crop is frequently damaged lie fore the water 

 table is lowered beyond the point of injury. The lines of tile 

 should be laid closer in somewhat impervious soils than in pervious 

 ones. In tight clay subsoils the tile drains should be not over four 

 rods apart, and no doubt two rods would be better. 



Coarse-textured soils generally drain better than fine ones. An 

 occasional tight stratum only a few inches thick may seriously in- 

 terfere with drainage. In general, limestone soils drain better 

 than strongly acid ones because of the granulation produced by the 

 limestone. Heavy soils are especially aided by shrinkage and the 

 formation of cracks to a depth of several feet which may not com- 

 pletely close. Earthworms, crayfish and other animals do much 

 to open up the soil for free movement of water, both laterally and 

 vertically. 



QUESTIONS 



1. What problems come up in the control of moisture? 



2. Define a waterlogged soil. What objections to it? 



3. Explain some of the results of lack of stability in a soil. 



4. Why are permanently saturated soils usually in poor tilth? 



5. How does drainage affect the available moisture? 

 0. Explain how aeration is affected by drainage. 



7. What is the effect of drainage on the specific heat of a soil ? Why? 



8. Why does drainage affect evaporation ? 



!). How may drainage affect crops and the length of growing siason? 



10. Why are decomposition and nitrification necessary? 



11. How does drainage prevent heaving? 



12. Why does a tight subsoil cause heaving? 

 1.'5. Why are open drains necessary? 



14. How are tight clay soils usually drained? 



15. What are some objections to open ditches? 



l(i. What precautions should be observed in tiling? Why? 



17. Upon what does the topography of the water table depend? 



18. How low should the water table be? 



10. Why is lateral movement of water through soils so slow? 



20. What are some of the soil conditions that aid drainage? 



REFERENCE 



1 Unpublished data, University of Illinois. 



General References. Whitson, A. K.. and Jones. E. R.. Bulletin 140, 

 Wisconsin Station, Drainage Conditions in Wisconsin, 1007. Kippin, E. ()., 

 Bulletin 2.14. Cornell Station. Drainage in New York. I'.tOS. .Fetfery. J. A., 

 Bulletin 50 (special), Michigan Station, Tile Drainage. 1011. Smith, A. 

 (}., Farmers' Bulletin 524. U. S. I). A.. The Drainage of the Farm. 1013. 

 Yarnell. D. T,., Farmers' Bulletin (>OS. I'. S. 1). A.. Trenching Machinery 

 for Tile Drains, 1015. Hills. .1. T,.. .Tones. C. II.. Williamson, C. (I., and 

 Burdick, K. T., Bulletin 17.'?. Vermont Station. Principles of Land Drain 

 age, 1013. Woodward. S. M.. Bulletin 30. T*. S. D. A.. Land Drainage by 

 "Means of rumps. 1015. Elliot. C. C... Farmers' Bulletin 1S7. T'. S. D. A'.. 

 Drainage of Farm Lands. 1004. Kincr. F. H.. Irrigation and Drainage. 

 Part II, Revised Ed., 1000. Jcffery, J. A., Land Drainage. lOlfi. 



