18 



WILTING COEFFICIENT FOR DIFFERENT PLANTS. 



that a limit is reached only when the soil moisture comes into approxi- 

 mate equilibrium with the moisture content of the air. The soil- 

 moisture content at the wilting point can not then be considered as 

 nonavailable to the plant, and for this reason has been designated in 

 the present paper as the wilting coefficient. 



In a recent paper 1 the writers referred to a proposed investigation 

 of the relative advantages of the wilting point and the death point of 

 plants as criteria in comparing plants in their relation to soil mois- 

 ture. The data given in Tables II and III provide a basis for com- 

 paring these criteria. A comparison of the probable errors for single 

 determinations is the most satisfactory way to judge of the relative 

 accuracy. These errors are given in Table IV, expressed as a per- 

 centage of the numbers which they affect. That part of Table III 

 referring to water cress does not contain sufficient observations to 

 give a probable error of any significance. 



Table IV. — Relative magnitude of the probable errors of a single observation using the 

 wilting point and the death point as criteria. 



Designation of data. 



Number 

 of obser- 

 vations. 



For 



wilting 

 point. 



For 

 death 

 point. 



Wheat, control tank, Table II 



19 

 9 

 10 



Per cent. 

 2.5 

 2.0 

 4.5 



Per cent. 

 2.4 





3.9 



Wheat, Table III 



6.3 











3.0 



4.2 









It will be seen from Table IV that the average error in determining 

 the wilting coefficient is considerably less than that accompanying 

 the determination of the moisture content corresponding to the 

 death point. Death-point determinations showing a very satisfac- 

 tory agreement can, however, be obtained with seedlings by the use 

 of the temperature-control tank, as shown by the results given in 

 the first part of Table II. But other and more important considera- 

 tions combine to make the wilting point decidedly superior to the 

 death point as a physiological criterion of soil-moisture conditions. 

 The wilting coefficient is practically independent of the kind of plant 

 used as an indicator, as will be shown later, while the death point 

 varies with the plant used, some plants dying much more quickly 

 after wilting than others. The time required for a determination 

 is also shortened by using the wilting point. But the great advan- 

 tage of using the wilting point is that it practically marks the cessa- 

 tion of growth and so constitutes a datum from which the water 

 content available for growth in a particular soil may be determined 



1 Briggs, L. J., and Shantz, H. L. A Wax Seal Method for Determining the Lower Limit of Available 

 Soil Moisture. Botanical Gazette, vol. 51, 1911, pp. 210-219. 

 230 



