14 



It had long been thought that there could be no direct relation 

 established between the percentage of water in a soil and the amount 

 that is available for plant growth. Recently, however, a standard 

 has been discovered by Briggs and Shantz ('12) by which the water 

 content of the soil may be directly related to continuity of plant vital- 

 ity. They have termed this standard or constant the "wilting co- 

 efficient" of the soil, and defined it (p. 6) as "The moisture content 

 of the soil (expressed as a percentage of the dry weight) at the time 

 when the leaves of the plant growing in that soil first undergo a per- 

 manent reduction in their moisture content as the result of a de- 

 ficiency in the soil-moisture supply." Only the soil moisture over and 

 above the wilting coefficient is available for plant growth, and hence 

 this surplus has been termed "growth water." In this investigation 

 the wilting coefficients of the various soils were found by the "indi- 

 rect method" devised by Briggs and Shantz ('12:56-57) which is 

 based on the relationship of the wilting coefficient to the moisture 

 equivalent as determined by physical methods. These coefficients 

 have been represented (in figures 29, 30, and 31, plates XVI and 

 XVII) by transverse broken lines, hence the growth water, or that 

 portion of the soil moisture available for the growth of plants, is 

 denoted by the interval between the lines representing the total per- 

 centage of moisture present and those showing the amount of the 

 wilting coefficients. 



x ^ 



Interpretation and discussion of results 



1. The graphs in Figures 29 (PI. XVI) and 30 (PI. XVII) 

 show that the percentage of moisture at stations 2 and 3 at a depth 

 of 7.5 cm. was greater than at a depth of 25 cm. in each of the 

 weekly examinations that was made. (The results are graphically 

 represented, with the weekly intervals as abscissae, and the percent- 

 ages of soil moisture at the time of each examination as ordinates.) 

 No such regularity in the per cent, of moisture content at the differ- 

 ent depths was found at station 13 (PI. XVII, Fig. 31). 



2. The samples taken at a depth of 7.5 cm. show in a general 

 way that the per cent, of moisture content in the narrow ravine at 

 station 2 was less than in the broad ravine, station 3, and that the 

 percentage of moisture content at the foot of the gully slope at sta- 

 tion 13 was less than at either of the other two stations examined. 

 The samples from a depth of 25 cm. did not show any such varia- 

 tions. From this we must not hastily conclude that conditions were 

 more favorable for plant growth from the standpoint of soil mois- 

 ture in the broad ravine than in the narrow ravine, or more favorable 



