82 THE WATER-SUPPLYING POWER OF THE SOIL 



complex property as such. Atmometry made little or no progress, as 

 far as its use in physiology is concerned, so long as interest therein 

 centered mainly in the analysis of evaporation into its component 

 elements, but atmometric measurements became at once valuable in 

 physiological work as soon as this almost futile attempt at analysis was 

 largely replaced by suitable measurements of the evaporating power of 

 the air as such. In a parallel way, it is to be hoped that studies of the 

 water-supplying power of the soil may eventually place the subter- 

 ranean environment (as far as water is concerned) in as satisfactory a 

 condition for quantitative study as are now the water-relations of the 

 aerial environment. 



The influence of temperature upon the water-supplying power of 

 the soil, or at least upon its measurement, appears to be of great im- 

 portance. This is not seriously considered in the present paper, but 

 should receive attention in later studies along these lines. 



Turning now to the actual water-supplying powers which have been 

 dealt with in the preceding pages, it appears that our osmotic solution 

 (5-weight-molecular cane-sugar) was too concentrated to enable us to 

 measure the water-supplying power of our soil mixtures with water 

 content much above their critical optimum. The effect of such high 

 water-absorbing power in the instrument is profoundly to alter the 

 soil condition in the vicinity of the membrane, and thus to introduce 

 a new and disconcerting factor into a problem already very complex. 

 Future work will need to deal with lower osmotic concentrations of the 

 absorbing medium, if apparatus may be devised for their use. 



Below the critical moisture content our tests gave quite satisfactory 

 results for such a preliminary study as the present. The osmometers 

 exhibited rates of water absorption that were maintained nearly con- 

 stant for many hours. Among other results, it may be here recalled 

 that a soil in which a Phaseolus plant had become strongly wilted, 

 with very low evaporating power of the air, exhibited a measured 

 water-supplying power expressed as 0.03 c.c. per hour through 10 

 sq. cm. of cross-sectional area, at a temperature ranging from about 

 28 to about 31. This power, measured thus as a rate, may be com- 

 pared with the water-supplying power of pure water in regard to the 

 same osmometer. It will be remembered that this latter power was 

 found to be about 1.36 to 1.39 c.c. per hour, for approximately the 

 same absorbing area and temperature range. It is therefore possible 

 to define the soil sample just considered as possessing a water-supplying 

 power of about 0.02, when the supplying power of water is considered 

 as unity This may be taken as an example of the sort of quantitative 

 measurement toward which our studies have been looking, as a goal 

 distant but nevertheless apparently attainable. 



For soils with moisture contents higher than the apparent critical 

 optimum, our method, as has been stated, appears generally to fall short 



