58 WILTING COEFFICIENT FOE DIFFEEENT PLANTS. 



The moisture-equivalent determinations given in the table repre- 

 sent in each case the mean of two determinations. The number of 

 wfitmg-co efficient determinations made upon each soil is shown in 

 column 4, and the mean of these determinations is given in column 

 5. The last column gives the ratio of the moisture equivalent to the 

 wilting coefficient for each soil. 



The table shows that the soils used in the comparison cover a wide 

 range in moisture retentiveness, the moisture equivalent increasing 

 from 1.6 per cent in sand to over 30 per cent in the clay loam, while 

 the wilting coefficient ranges from 0.9 per cent in sand to 16.5 per 

 cent in the clay loam. The mean ratio of the moisture equivalent 

 to the wilting coefficient for all the soils examined is 1.84. The prob- 

 able error of this mean is ±0.013; that is to say, considering the 

 series to be representative of soils as a whole, the chances are even 

 that if a similar series of determinations were made the mean of 

 the ratios would fall between 1.S27 and 1.S53. 



It will be noted that the greatest departures in the ratios are found 

 among the sandier soils. This is due to the fact that a slight experi- 

 mental error in determining either the moisture equivalent or the 

 wilting coefficient affects the ratio markedly, owing to the small 

 percentages of moisture retained by these soils. 



The significant feature of the results here presented is the fact that 

 through the wide range of moisture retentiveness exhibited by the 

 soils employed, the ratio of the moisture equivalent to the wilting 

 coefficient appears to be constant within the limits of experimental 

 error. In other words, two determinations of the moisture reten- 

 tiveness of these soils, one physical and the other physiological, show 

 a linear relationship which is independent of the texture of the soil. 

 This relationship is expressed by the following formula: 



Moisture equivalent ., . „» . 

 184±0013 ■ = wilting coefficient. 



In order to compare the moisture available for growth in one soil 

 with that in another, we must know or be able to estimate accurately 

 the wilting coefficient of each soil. The mi ni mum limit of moisture 

 available for growth is the datum line from which all comparisons 

 should be niade. This datum can be established directly by wilting- 

 coefficient measurements, or it may be calculated by means of the 

 ratio just established. The latter method is by far the simpler and 

 more expedient for field work. The soil sample taken in the field 

 for soil-moisture determination, although ample for duplicate meas- 

 urements of the moisture equivalent, is usually not large enough for 

 a single wilting-coefficient determination. Moreover, the period of 

 time required for wilting-coefficient determinations, combined with 



230 



