io8 
Journal of Agricultural Research 
Vol. XXIV, No. a 
cans; but, to encourage aeration of the soil, a 2-mch florist’s pot was 
inverted in the bottom of each can, and a glass tube one-eighth inch in 
diameter was so bent and placed that its lower end opened into the pot 
and its upper end just above the rim of the can. This tube served for 
supplying the necessary water and was at all times left open for aeration 
of the soil.^ It is believed that the amount of vaporization through the 
tubes was insignificant, though no control tests were made at the time. 
We are enabled to approximate the loss from this source by the observa¬ 
tions in 1920. However, in 1917, the soils were never allowed to become 
greatly heated, the potting cans being placed in similar cans having 
diameters of 6 inches, so that the sun never shone on the lower portions 
of the pots. 
Before weighing and potting, each tree was trimmed so that the longest 
roots would not be cramped in the can. The longer roots were spread 
around the porous pot in the bottom of the can, and the others were placed 
as the pots were filled, so as to be evenly distributed throughout the soil. 
When the cans were all filled to within a half inch of the tops, they were 
sealed with a 2 to i mixture of paraffin and vaseline, which held very well 
throughout the season in spite of occasional melting. 
The soil used was a specially prepared loamy sand of granitic origin, 
containing considerable humus derived mostly from leaves of limber 
pine and Arctostaphylos sp. Both sand and humus were sifted through 
one-eighth-inch screen. The resultant mixture was what would ordi¬ 
narily be considered a good potting soil. It was thought to be desirable 
to insure an abundance of nutrient material, and there is no reason for 
supposing that this was overdone. 
None of the soil placed in the pots was oven-dried, but a weighed 
amomt of air-dried soil was used in each, and during the process of 
potting several samples were taken for the purpose of determining the 
moisture content. The net oven-dried weight for each pot was then 
computed. 
The satiuration capacity of the soil used was originally determined to be 
about 40 per cent, and, in accordance with Kiesselbach’s (12) finding 
that transpiration occurs most freely when the soil is about half saturated 
and tie theory of Hilgard (ii) that half saturation permits the desired 
aeration, 20 per cent moisture was adopted as the standard at which the 
soil would be kept. Later it was found that with greater compactness 
this saturation might be much less, and after centrifuging, as low as 25.8 
per cent. However, the figure 31.9 probably applies most nearly to the 
condition of the soil in the pots. The corresponding capillarity was 
’ 28.2 per cent, and the moisture equivalent at 100 gravity was 11.05 
cent, using the term in the same sense as it is used by Briggs and Shantz 
(5) for the water-holding capacity under a force of 1000 gravity. 
The mean wilting coefficient was determined in 1920 to be 3.47 
per cent for Douglas fir and 3.91 per cent for spruce, or an average value 
of 3.69 per cent. On this basis, and assuming that the 20 per cent 
maximum moisture was evenly distributed, we should have as the avail- 
ability —^^^^^—^ = 0.816. It is more probable that the moisture within 
reach of the roots, at the bottom of each pot, was 25 per cent or more, 
making the availability at least 0.850. 
Table I shows all of the fundamental data regarding the trees, the 
amounts of soil used, and the gross weight of the pots as they were main¬ 
tained throughout the season. 
