WATER OF SOILS 193 



11. How much for a 50-bushel corn crop? A GO-bushel oat crop? 



12. Upon what does the amount of water used by plants depend? 



13. How much of the earth's land surface is adapted to ordinary humid 



agriculture? 



14. What part of the United States is humid? (See Rainfall Map, Fig. 86.) 



15. How does the rainfall vary during different months? 

 1(5. How does soil affect the moisture supply? 



17. What part does evaporation play in the storage of water? 



18. What are the advantages and disadvantages of expressing moisture 



content in per cent of weight of soil? 



19. What is the best base to use? Why? 



20. Give advantages of expressing moisture content in cubic inches or per 



cent of volume. 



21. Why is it well to express the moisture content in acre-inches? 



REFERENCES 



1 Lawes and Gilbert, Jour. Hort. Soc., 18f>0. 



*Hellriegel, Kxperiment Station Record. IV, p. f>32. 



3 Wollny, Experiment Station Record, IV, p. 532. 



King,*F. 11., Physics of Agriculture, 1007, p. 130, also llth and 14th An- 

 nual Reports Wisconsin Station. 



Widtsoe, J. A., Dry Farming. 1911, p. 33. 



' Mosier, J. G., Bulletin 208, Illinois Experiment Station, Climate of 

 Illinois, 1917. 



13 



