100 BUIXETIN 1051), U. S. DEPARTMENT OF AGRICULTURE. 



Under these circumstances fairly constant results may be expected 

 in i, coefficient determinations. In the absence of any 



other results reference is again made to the comparisons made by 

 Bi Shantz between hygroscopic and wilting coefficients. In 



17 itli soils varying from 0.9 to 16.5 per cent wilting coeffi- 



ru . nl j | miihI the ratio of hygroscopic to wilting coefficients to be 



average 0.680, with a probable error, or variation, in any 

 single determination of about 7.1 per cent of the wilting coefficient. 

 It is to be noted that the hygroscopic is so much lower than the 

 u ih in- coefficient that serious error would result from considering 

 them as interchangeable, though this proposal has sometimes been 



made. 



Calculation of the Available Moisture. 



&s 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 1<> 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 

 -.tnd there is 8 per cent available moisture, and in the clay 5 per 



•i! . or .1 =M—WO. The use of the last figures is certainly far more 

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

 the u-«' <»f the whole moisture figures, although, on account of vary- 

 ing concent rat ions of salts, even this figure for the available moist ore 

 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 

 linall\ into cubic centimeters per cubic meter, or any other measure 



\ \ ollline. 



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



i mined, as in the large capillary cans described, where the apparent 



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



>lume in cubic centimeters, which is approximately 1,030 cubic 



centimeters (usually less after centrifuging). 



I arrying the volume idea still farther, in studying any plant or 

 oup 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 



