iQH] FULLER— EVAPORATION AND SOIL MOISTURE 219 



That the water content of the soil at the time of wilting, or, in 

 other words, the magnitude of the wilting coefficient, is affected by 

 excessive evaporation rates, causing very high transpiration rates, 



has been shown by Brown (19) and by Caldwell (20), although 

 their work also goes to show that it holds perfectly within certain 

 limits differing but slightly from those obtaining in ordinary 

 mesophytic or semi-mesophytic habitats. Although the validity 

 of these objections to the wilting coefficient in extreme conditions 

 is recognized, it is nevertheless believed that it is, especially when 

 determined by the indirect method of Briggs and Shantz, an 

 efficient means of relating the range of soil moisture to the pro- 

 duction of vegetation and an important addition to the equipment 

 of ecological and agricultural investigators. 



METHODS 



moisture determinations 



the plant associations described in the previous part of this paper 



■ 



during the years 191 1-1912, and for the same months as the atmom- 

 eter records. In making the determinations, weekly samples, 

 each consisting of 200 to 250 grams of soil, were taken in each of the 

 associations at depths of 7 . 5 cm. and 25 cm. In order to provide 

 against unnecessary error, each sample consisted of two portions 

 of some 100 grams each, taken from spots several meters apart, 

 care also being taken that no soil was taken nearer than a meter to 



)les had been dug. The soil was placed 



sam 



wide-mou 



of / %^J - • ^~* 



weighed and dried at a temperature of ioo° to 104 C. until it 

 reached a constant weight (about 5-7 days). The percentage 

 of water to the dry weight of the soil was then calculated. 



The wilting coefficients of the same soils were determined by 

 both the direct and the indirect methods of Briggs and Shantz (17). 

 The results from the two methods agreed within the limits of experi- 

 mental error, which appeared to be much greater in the direct 

 method. This was particularly noticeable in the sandy soil of the 



coefficient 



All 



the wilting coefficients are the averages of at least 10 determinations 



