INDIRECT DETERMINATION OF THE WILTING COEFFICIENT. 59 



the uncertainty which accompanies all physiological work when du- 

 plication is impossible, makes this determination less expedient, and 

 the results in such cases less reliable than those derived from the 

 moisture equivalent by the use of the ratio here established. 



ADDITIONAL DATA ON THE RELATION OF THE WILTING COEFFICIENT 

 TO THE MOISTURE EQUIVALENT. 



At the time of making the experiments the results of which are 

 given in Table XVII, a clay loam having a moisture equivalent of about 

 30 per cent was the heaviest type of soil that was available. After 

 developing the relationship between the moisture equivalent and the 

 wilting coefficient, it was desired to determine whether this relation- 

 ship held also for very heavy soils. Accordingly, a similar series of 

 determinations was made with clay soils from near Bellefourche, 

 S. Dak., which are derived from the Pierre shale and include the 

 heaviest soils which we have seen. The results of these determina- 

 tions are given in Table XVIII. The moisture equivalents of these 

 samples range from 32 to 57 per cent, while the wilting coefficients 

 range from about 17 to nearly 31 per cent. In other words, plants 

 wilt in the heaviest types when the soil-moisture content is over 

 30 per cent, showing the remarkable moisture retentiveness of these 

 soils as compared with dune sand, in which the wilting coefficient 

 is less than 1 per cent. 



Table XVIII. — Relation of the wilting coefficient to the moisture equivalent of heavy 

 clay soils from Bellefourche, S. Dak. 





Soil. 



Moisture 

 equivalent. 



Coefficient. 



Ratio of 

 moisture 



equivalent 

 to 



coefficient. 





Number 

 of observa- 

 tions. 



Mean. 



p 



31.9 

 40.2 

 43.5 

 44.9 

 45.7 

 48.8 

 52.4 

 56.2 

 57.0 



3 

 2 

 3 

 2 

 3 

 3 

 3 

 3 

 3 



16.8 

 22.2 

 23.3 

 24.2 

 25.9 

 25.7 

 27.4 

 30.4 

 30.9 



1.89 



0, 



1.82 



R 



1.86 



s 



1.85 



T 



1.78 



u 



1.90 



V 



1.91 



W 



1.84 



X 



1.84 

















1.85 









! 



± .02 









| 





The mean ratio of the moisture equivalent to the wilting coefficient 

 for these heavy types of soil was 1.85, with a probable error of ±0.02. 

 The mean ratio agrees, then, within its probable error, with the result 

 given in Table XVII. In other words, the relationship established 

 from the determinations given in that table holds also for the heaviest 

 soil types. 



230 . ., . 



