Apr* X4.1923 Physiological Requirements 0f Rocky Mountain Trees 1 49 
or sands of granitic origin, while only i is found in the more loamy soils. 
This is at least suggestive that in the soils of freer capillary movement 
the very meager root system of spruce is not so great a disadvantage. 
From these facts we certainly can not draw the conclusion that in the 
osmotic sense spruce has any less control over soil moisture than fir. 
INDIRECT COMPARISONS OF THB SPECIES 
A very considerable amount of information has been secured in the 
somewhat routine process of determining the wilting coefficients for a 
large number of soils of almost every possible origin in connection with 
nearly every study in which soil quality or soil moisture has been an 
important factor. For the most part the wilting coefficients have been 
determined for each soil only with respect to one species, that one being 
the species which characterized the soil or site in the field. It is obviously 
necessary, before these results may be used for a comparison of the species, 
that each result should be related to some other measure of the moisture¬ 
holding properties of the soils, and the best measure at present available 
for any considerable number of the soils is the moisture equivalent at 100 
gravity. As we have seen in Table XX, however, even this does not bear 
a constant relation to wilting coefficients, when radically different types 
of soil are considered. Particularly does it seem that the coarse-grained 
granitic soils of the Pikes Peak region, which we have studied more than 
any other, have an unusually weak hold on the water until the amount 
is brought close to the wilting coefficient, so that the moisture equivalents 
of these soils are relatively low. 
Table XX .—Comparative 'wilting coefficients of spruce and Douglas fir in the 
same soils 
Kind of soil. 
Moisture 
equivalent. 
Wilting coefiBcient. 
Ratio of wilting 
coefficient to moisture 
equivalent. 
Eir. 
Spruce, 
Fir. 
Spruce. 
Per cent. 
Per cent. 
Per cent. 
Gi'anitic gravels over 50 per cent 
3 - 53 
I. 30 
I- 39 
0.368 
0-394 
rocks and coarse gravel, less than 
4*35 
I- 73 
I. 94 
• 398 
.446 
20 per cent silt and clay. 
4. 86 
2. 05 
2. 03 
. 422 
. 418 
5 - 03 
I- 77 
I. 80 
•352 
•358 
5-04 
2. 14 
2. 23 
.424 
.442 
5. 06 
2. 54 
2^34 
. 502 
. 462 
5 - 19 
2. 01 
1.87 
.387 
.360 
5 - 57 
I. 94 
I. 71 
•348 
•307 
5. 62 
2. 13 
2. 61 
•379 
. 464 
Granitic sandy loam. 
II. 05 
2. 94 
2. 91 
.266 
. 263 
Granitic spruce soils, varying from 
II. 68 
2. 72 
2. 88 
•233 
. 246 
rocky coarse sand to silt loam. 
II. 72 
2. 53 
2. 79 
. 216 
. 238 
14 - 45 
3 - 42 
4. 09 
•239 
. 283 
19 - 95 
6. 41 
6. 44 
. 321 
•323 
20.32 
5. 60 
5 - 78 
• 276 
. 284 
22. 02 
5-25 
5.61 
• 238 
•255 
26. 30 
8. 34 
8. 17 
•317 
•311 
29. 84 
7-44 
7. 98 
• 249 
.268 
42. 72 
17.09 
17. 40 
. 400 
.407 
73 - 50 
17- 53 
i8. 56 
. 238 
•253 
Quartz latite rocky sandy loams... 
II. 96 
5 - 13 
5. 20 
.429 
•435 
13. 98 
6. 96 
7. 14 
.498 
. sio 
14.85 
5 - 72 
6. 04 
•385 
.407 
Average of all. 
•3427 
• 3537 
. OIIO 
. 0029 
Mean difference. 
Probable error in mean difference. 
; 
