SUMMARY. 73 



DETERMINATION OF THE HYGROSCOPIC COEFFICIENT. 



Hygroscopic coefficient=wilting coefficient X 0.68. 

 Hygroscopic coefficient=moisture equivalent X 0.37. 

 Hygroscopic coefficient= (moisture-holding capacity— 21) X0. 234. 

 Hygroscopic coemcient=0.007 sand+0.082 silt+0.39 clay. 



DETERMINATION OF THE MOISTURE-HOLDING CAPACITY. 



Moisture-holding capacity = (wilting coefhcientX2.9)-f 21. 

 Moisture-holding capacity = (moisture equivalentX1.57) + 21. 

 Moisture-holding capacity = (hygroscopic coefficient X 4. 26) +21. 

 Moisture-holding capacity=(0.03 sand+0.35 silt+1.65 clay)+21. 



These formulas establish for the first time a relationship between 

 the various physical and physiological measurements of moisture 

 retentivity, and while the coefficients may be modified as the result 

 of further investigation, it is believed that the equations will prove 

 of practical value in the study of the relationship of the plant to soil 

 moisture, both in the field and in the laborator} 7 . 



DETERMINATION OF THE MAXIMUM AVAILABLE MOISTURE. 



The maximum available moisture in any soil is represented by the 

 difference between the moisture-holding capacity and the wilting 

 coefficient. It is therefore possible to express the maximum amount 

 of available moisture that a soil is capable of holding in terms of the 

 relationships given above. It should be recalled that the determina- 

 tions of moisture-holding capacity, upon which the relationships are 

 based, were made with a soil column 1 centimeter in height. The 

 amount is therefore far in excess of that found in drained soils under 

 field conditions. The relationships are expressed in the following 

 formulas : 



Maximum available moisture = (wilting coefficient XI- 9) +21. 

 Maximum available moisture=moisture equivalent+21. 

 Maximum available moisture = (hygroscopic coefficientX2. 8)4-21. 

 Maximum available moisture =(0.02 sand+0.23 silt+1.08 clay) +21. 

 Maximum available moisture = (moisture-holding capacity X0. 65) +7. 



The formulas show that the difference in the maximum amount of 

 available moisture that two soils are capable of holding is equal to 

 the difference of their moisture equivalents; to 1.9 times the difference 

 of their wilting coefficients; and to 2.8 times the difference of their 

 hygroscopic coefficients. 



SUMMAKY. 



The object of this investigation was to determine the extent of the 

 variation exhibited by different plants with respect to the minimum 

 point to which they can reduce the moisture content of the soil before 

 permanent wilting occurs. It has heretofore been believed that 



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