I40 



ILLINOIS ACADEMY OF SCIENCE. 



in similar strata of the mesophytic beech-maple forest, which was 

 found to average but 8 cc. per day for the same period. 



SOIL MOISTURE. 



In determining the range of soil moisture, samples of about 200 

 grms. of soil were taken weekly from May i to October 31, 1911, 

 at 7.5 cm. and 25 cm. below the surface. The soil was taken to 

 the laboratory in tightly closed containers and dried at 104° C. 

 The water content thus determined is expressed in percentages 

 of the dry weight of the soil. It is found to range from 2 to 8 

 per cent, an apparently small amount at all times, but here the 

 important question is not the absolute amount of water present 

 in the soil, but how much is there that is available for the use of 

 plants? Tests made by the methods devised by Briggs and Shantz 

 of- the Bureau of Soils, United States Department of Agricul- 

 ture,^ showed that young wheat plants wilted in this soil only 

 when its water content fell as low as .75 per cent ; in other words, 

 the wilting coeMcient of this dune sand for wheat is .75 per cent. 

 Other investigations by the same workers indicate that a very 

 similar zvilting coefficient obtains for many herbaceous plants ; 

 hence we are safe in saying that so long as there is i per cent of 



Figure 2. Graphs showing the range of soil moisture in the cotton- 

 wood dune; the heavy lines at 7.5 cm. and the light line at 25 cm. depth; 

 wilting coefficient represented by a broken line. 



water in this sand there is a sufficient supply for the growth of 

 such a plant as wheat. 



Plotting the soil moisture determinations as graphs having the 



^ Briggs, L. J., and Shantz, H. L. The wilting coefficient for different plants and 

 its indirect determination. U. S. Dept. Agric. Bur. Plant Ind., Bull. 2io: 1912. 



