Apr. 14, 1923 
Physiological Requirements of Rocky Mountain Trees 121 
soil. 
The soil in which the trees were potted was nearly the same as that 
used in 1917. There had been added to the original soil a somewhat 
more clayey sand of granitic origin, so that in 1920 we find a greater 
water-holding capacity, a slightly lower moisture equivalent, and a third 
higher wilting coefficient. The data on the 1920 soil are: 
Percents 
Saturation. 36. 46 
Capillarity. 31.02 
Moisture equivalent. 10. 42 
Wilting coefficient (average for spruce and yellow pine). 4. 88 
It was planned to carry about 12 per cent moisture, and to prevent the 
saturation of the lowest stratum of the soil by injecting the daily supply 
of water near the surface. The feed tube, then, instead of opening inta 
the inverted clay pot at the bottom of the can, was bent about 2 inches 
below the surface and opened into the soil near the center of the can.. 
Aside from this the potting arrangement was the same as in 1917. 
The 12 per cent moisture would constitute, in the average of the 22 
cases, 284.4 8^* water. At the outset, however, the average pot was 
given 528.8 gm. of water, in order to create a very favorable condition. 
In the average case this supply lasted much longer than had been antici¬ 
pated and was not brought down to standard until after July i, so that 
during the first two months the moisture conditions were by no means, 
uniform. 
Finally, beginning September 3, the water content was gradually 
reduced, until on September 27 it was 70 gm. below the standard, or in 
the average case amounted to 9.05 per cent. This change was designed 
to simulate the usual autumn drying of the soil. 
We thus have the following average conditions in the several pots, 
comparing with a mean availability of about 0.850 through the entire 
season of 1917: 
May II, 528.8 gm. =22.31 per cent = availability 0.780; decreasing to 
284.4 gm., or 12 per. cent, about July i = availability 0.594; reaching 
September 27, 214.4 gm., or 9.05 per cent = availability 0.461. 
It is, therefore, seen that in the osmotic sense the conditions for ready- 
absorption of the water were far less favorable in 1920 than in 1917. 
PROCEDURE 
The procedure in handling the pots was in minor details almost the 
same as in 1917. On the other hand, as has been mentioned, they were 
not under glass but under a canvas cover which was raised during the 
entire day except when showers occurred, permitting unmodified sunlight 
to reach the trees and also allowing much freer air circulation, the 
primary effect of which was, undoubtedly, to prevent the occurrence of 
excessive temperatures. While the extreme sun temperatures recorded 
were not materially lower than in 1917, the air temperatures were scarcely 
above those outside the shelter and were very much lower than in 1917, 
when they averaged 6° F. higher than the outside air. 
The revolving table on which the pots were exposed was nearly 5 feet 
in diameter and made a revolution every two hours. 
