SOIL WATER 85 



expensive in first cost, is economical in operation in many 

 soils. After the ditch has been opened to its full depth, 

 it is necessary to go over the entire bottom to remove loose 

 dirt and to give it the necessary grade. This must be done 

 by hand. Either a ditching spade or a drain scoop is the 

 best implement to use. A fall of at least four inches in a 

 hundred feet is necessary under most conditions, but in 

 clay soils less fall is permissible, as there is less danger of 

 silt entering the drains. 



QUESTIONS 



1. Name the three forms in which water is present in soils. 



2. Explain what is meant by hygroscopic water. Capillary 

 water. Gravitational water. 



3. On what does the content of hygroscopic water depend ? 



4. Name six conditions that tend to increase the capillary 

 water capacity of soil. 



5. Explain the relation of soil texture to the movement of 

 capillary water. 



6. How does soil texture affect the rate of movement of 

 capillary water ? 



7. What are the conditions that affect the rate of flow of grav- 

 itational water ? 



8. Explain what is meant by the water table. 



9. Describe three ways in which water contributes directly to 

 plant growth. 



10. What is the transpiration ratio ? 



11. Name three factors that influence it. 



12. Calculate the number of inches of rainfall transpired by a 

 three-ton crop having a transpiration ratio of 250. 



13. Name four ways in which water leaves soil. 



14. What is the principle of the soil mulch ? 



15. State four ways in which drainage benefits soils. 



LABORATORY EXERCISES 



Exercise I. — Determination of the percentage of water in a soil. 

 Materials. — Samples of moist soil, torsion balance, evaporating 

 dishes, air oven and flame, desiccator. See Plate IX. 



