198 



Hilgard 8 gives the following uses of hygroscopic moisture in 

 plant growth: " (1) Soils of high hygroscopic power can with- 

 draw from moist air enough moisture to be of material help in 

 sustaining the life of vegetation in rainless summers or in time 

 of drouth. It cannot, however, maintain normal growth save in 

 the case of some desert plants. (2) High moisture absorption pre- 

 vents the rapid and undue heating of the surface soil to the danger 

 point, and thus often saves crops that are lost in soils of low hygro- 

 scopic power." 



QUESTIONS 



1. What forces act upon water in soils? 



2. What forms of moisture are found in soils as a result of these forces? 



3. Define hygroscopic moisture. 



4. How does size of particles affect the amount of hygroscopic moisture? 



5. What effect do colloids have? 



6. What effect does temperature have on "hygroscopic moisture in com- 



paratively dry air? 



7. What effect if the air is saturated? 



8. What effect does organic matter have on hygroscopic moisture? 



9. What relation exists between the adsorption of soils from different de- 



grees of saturation? 



10. How do the ratios of humidity and adsorption compare in the table on 



page 196? 



11. Define hygroscopic coefficient. 



12. How is it determined? 



13. Can plants use hygroscopic moisture? 



14. Which soil in the table on page 197 has the highest hygroscopic co- 



efficient? Why? 



15. Which contains the highest amount of capillary moisture after the wilt- 



ing coefficient is reached ? 



10. If a clay loam soil weighs 80 pounds per cubic foot, how many tons 

 of unavailable moisture is in the soil to a depth of two feet per 

 acre? (See table on page 197.) 



17. What is the use of hygroscopic moisture? 



18. The wilting coefficient of a clay loam is 10.2 per cent, what is the 



hygroscopic coefficient? 



19. If the moisture holding capacity of a soil is 23.2 per cent, what is the 



hygroscopic coefficient? 



20. If a soil contains 83.1 per cent of sand, 8.0 of silt and 7.5 of clay, what 



is the hygroscopic coefficient? 



REFERENCES 



^riggs, L. J., Journal of Physical Chemistry, Vol. 9, 1905, pp. 017-641. 



* Hilgard, E. W., Report of the California Station, 1892-3-4, p. 70. 



* Hilgard, Soils, 1906, p. 196. 

 4 Op. Cit., p. 198. 



* Dobeneck, A. F., Von Untersuchungen (iber das Absorptionsvermogen und 



die Hvgroskopizitiit der Bodeukonstituenten. ForscJi. a. d. Gebiete 



d. Agri.-Physik., Band XV, 1892, Seite 103-228. 

 Briggs, L. J., and Shantz, H. L., Bulletin 230, Bureau of Plant Industry. 



U. S. D. A., The Wilting Coefficient for Different Plants and its Indi- 



rect Determination, 1912, p. 73. 

 7 Op. Cit., p. 65. 

 Hilgard, Soils, 1906, p. 200. 



