100 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



results reference is again made to the comparisons made by Briggs 

 and Shantz between hygroscopic and wilting coefficients. In 17 tests, 

 with soils varying from 0.9 to 16.5 per cent wilting coefficient, they 

 found the ratio of hygroscopic to wilting coefficients to be on the 

 average 0.680, with a probable error, or variation, in any single deter- 

 mination of about 7.1 per cent of the wilting coefficient. It is to be 

 noted that the hygroscopic is so much lower than the wilting co- 

 efficient that serious error would result from considering them as inter- 

 changeable, though this proposal has sometimes been made. 



Calculation of the Available Moistuke. 



As has been stated, when the current moisture of the soil has been 

 measured, and the nonavailable has been measured in the laboratory 

 by the direct method of wilting tests, or indirectly through the 

 capillary moisture, moisture equivalent, or hygroscopic coefficient, it 

 is then only necessary to subtract the wilting coefficient from the 

 whole moisture to have a measure of the amount of water which, 

 under the most favorable circumstances, will be available for growth. 

 For example, if in sand and clay, respectively, the whole moistures 

 are 10 and 20 per cent, and the wilting coefficients of these soils 

 are respectively 2 and 15 per cent, then it is evident that in the 

 sand there is 8 per cent available moisture, and in the clay 5 per 

 cent, or A=M — WC. The use of the last figures is certainly far mor? 

 expressive of the relative conditions in the two soils than would be 

 the use of the whole moisture figures, although, on account of vary- 

 ing concentrations of salts, even this figure for the available moisture 

 does not give a direct means of comparing the moisture conditions 

 of radically different soils. 



Of course, if the measure of available moisture is to be used most 

 fully as an index to supply, the percentage should be transposed 

 finally into cubic centimeters per cubic meter, or any other measure 

 of soil volume. 



This is very readily done if the apparent density has been de- 

 termined, as in the large capillary cans described, where the apparent 

 density is obtained by dividing the dry-soil weight, in grams, by the 

 volume in cubic centimeters, which is approximately 1,030 cubic centi- 

 meters (usually less after centrif uging) . 



Carrying the volume idea still farther, in studying any plant or 

 group of plants it is obviously desirable to know how much soil sur- 

 face can be drawn upon. Thus a yellow pine on a dry site may 

 actually have a much greater supply of moisture than a crowded 

 spruce on a moist site. Consideration of this point of view will lead 

 to the conclusion that soil moisture figures, as ordinarily given in 

 percentages of the dry-soil weight, have almost no significance 



