BIOLOGICAL PAPERS 1 33 



II. SOIL MOISTURE. 



While ecologists have usually been agreed that the water of the 

 soil is the most important single factor limiting the development 

 of vegetation, very few quantitative determinations of this factor 

 have been made. This has been due largely to the difficulty in 

 obtaining a standard by which the water content could be related 

 to plant growth. The percentage of water in one soil which would 

 produce a luxuriant vegetation, in another of difterent texture, 

 would not support any plant life whatever. Livingston'^ recog- 

 nized that the water-holding capacity of soils varied and had a 

 fairly constant relation to the relative soil moisture conditions, 

 but only with the very recent work of Briggs and Shantz^ has a 

 satisfactory function of soil moisture been recognized by which 

 to relate vegetation to the actual amount of water present in the 

 soil. They determined the amount of water present in a particu- 

 lar soil when permanent wilting occurred in Kubanka wheat, the 

 plant adopted for the standard. This amount expressed in per- 

 centage of the dry weight of the soil they have termed the zvilt- 

 ing coefficient, and it represents the water content above which 

 all growth must occur : life, however, is maintained for very con- 

 siderable periods with much less water. The same workers also 

 gave data which show that many ordinary mesophytic plants vary 

 very little in their wilting coefficients from the standard wheat 

 employed. 



In order to determine the soil moisture conditions in the oak- 

 hickory forest at Palos Park, weekly samples, each consisting of 

 about 300 grams of soil, were taken at atmometer stations one, 

 two and four, from depths of 7.5 cm. and 25 cm. below the sur- 

 face. The soil was placed in tightly closed wide-mouthed jars, 

 brought to the laboratory, weighed and dried at a temperature 

 of 100° to 104° C. until it ceased to lose in weight. The per- 

 centage of water to the dry weight of the soil w^as thus obtained. 

 Using the same soils, the wilting coefficients were obtained accord- 

 ing to the wax seal method described by Briggs and Shantz (loc. 

 cit.). Graphs representing the range of soil moisture have been 

 plotted with weekly intervals as abscissae, while the ordinates 

 represent the percentage of the soil moisture present. The wilt- 

 ing coefficients are represented upon the same diagrams in broken . 



'Livingston, B. E. Relation of soil moisture to desert vegetation. Bot. Gaz., 50: 

 241-2^6. 1910. 



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

 its indirect determination. U. S. Dept. of Agr., Bur. of Plant Industry, Bull. No. 

 230, 1912. 



