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



Or, in other words, there is a probable error of less than 1 per cent 

 in this average relationship. Single determinations, however, show 

 a probable variation of 2.9 per cent of the wilting coefficient, as 

 measured by this direct means. 



This work, though extremely thorough, was confined wholly to 

 soils encounted in agricultural regions, and while these varied be- 

 tween 1.6 to 57 per cent moisture equivalent, they were undoubtedly 

 more homogenous than forest soils in general, and lacked the compli- 

 cating features of both rocks and large quantities of organic matter. 

 It is not desired to suggest that, if this method were readily applicable 

 to forest soils, and if experimental error both in wilting coefficient 

 and moisture equivalent determination could be largely eliminated, 

 the general relationship would be found different in the case of 

 forest soils. Unf ortunatery, no one has made sufficient use of the 

 moisture equivalent, in connection with wilting tests on forest species 

 and forest soils, to determine whether the formula of Briggs and 

 Shantz holds good. It is hardly to be doubted, however, that a for- 

 mula must be worked out for each species, or the species of each 

 general climatic region. Also, there is little doubt that occasional 

 soils will be found in which, owing to exceptional alkalinity or 

 acidity, the wilting coefficient is extremely high, and hence the 

 formula breaks down. 



In connection with the capillary moisture determinations by 

 Bates (105), data on corresponding moisture equivalents have also 

 been given in Tables 2 and 3. These, as pointed out, were deter- 

 mined on samples which had just passed through the capillarity 

 tests. The 4 by 5-J inch soil cans were placed in a machine of such 

 speed and radius as to develop a centrifugal force of 100-gravity, 

 the radius being computed to the center of the 5-inch column of 

 soil. Ordinarily, 30 minutes of revolution suffices to extract the free 

 water susceptible to this force, but with a heavy clay an hour may 

 be required. 11 



11 In order to show the importance of the time element, where such large masses of 

 soil are being treated, and also to illustrate the very great difference between the water- 

 holding powers of sand and clay, two samples were weighed repeatedly after short 

 periods on the centrifugal machine The one sample consisted of very fine, thoroughly 

 washed sand from granitic soils, the other entirely of silt and clay from innumerable 

 sources, the clay probably not constituting over one-fourth of the whole mass. Both 

 samples had previously been compacted by centrifuging, so that the rapid loss of mois- 

 ture in the first period can not be ascribed to loose structure. The test was somewhat 

 complicated by a freezing atmosphere which, in fact, necessitated cessation before an 

 end point for either soil was plainly reached. From a mass of soil of about 1,070 

 grams in either case, the sand gave up in 80 minutes 276.3 grams of water, of which 

 230.7 grams (S4 per cent) was released in the first 2* minutes of centrifuging. The 

 corresponding figures for the silt and clay were 76.2 grams, and 12.7 grams or 17 per 

 cent. In the last 20 minutes of the 80-minute period the loss for the sand was 3.1 

 grams and for the clay 12.8 grams. 



