146 
Journal of Agricultural Research 
Vol. XXIV, No. » 
gers, and it is their performance, rather than that of older trees, which 
determines the composition of forest types. 
Although only a few wilting tests have been made in which the several 
species have been observed growing in the same soil, a considerable amount 
of information has been obtained on each species in a variety of soils; 
and by reference to the physical properties of these soils we may obtain 
fairly satisfactory comparisons. In each case, the species used was that 
one which occupied the given soil or predominated in the type in the 
field. 
PROCEDURE 
In general the intention has been to secure the wilting coefficient for 
the soil as found in the field—^that is, with the normal admixture of 
rocks and gravel, since the saturation and capillary capacities and other 
physical measurements were on this basis. In this respect mountain 
soils present difficulties ordinarily not met with in agricultural work. 
To attain this end it is not sufficient to sample ffie soil for moisture 
content after the seedlings have wilted. The mositure content must be 
determined for a mass of soil large enough to represent normal pro¬ 
portions of rock and finer material. 
Pans about 10 inches square were used in the earliest tests, the soil 
being in a layer from i to inches deep. These were sometimes 
found to be too shallow to accommodate the rocks which should be in¬ 
cluded; consequently a standard round pan was specially made, having 
a diameter of 7 inches, a depth of 3 inches, a soil depth oi 2^4 inches, and 
an ordinary soil weight of about 4 pounds. A few holes were punched 
in the bottom of each pan to prevent excessive wetness and to aid aeration, 
it being the belief that with the pans on a bench the evaporation rate 
through these small holes could never be an important factor. In fact, 
though the soil surfaces have usually been paraffined, it has never been 
attempted to make the coatings air-tight, since the object is not to 
prevent water loss from the soils but to insure that when wilting occurs 
the moisture distribution throughout the soil shall be fairly uniform. 
The vegetation has been secured by sowing seeds of the desired species 
in the pan of soil, watering these moderately, and permitting the seed¬ 
lings to develop for about a month before coating iJie pans and cutting 
off the moisture supply. Beyond the age of a month the seedlings may 
rapidly lignify, so that the wilting does not occur promptly or is very 
difficult to detect. This is true of Douglas fir seedlings at any age. It 
is, of course, realized that seedlings of this age may not exercise the same 
control over moisture as would older trees. 
The soil sample is placed in the pan in an air-dry condition and is 
oven-dried to determine its net weight. This practice may have had 
some effect on the colloids but is fully justified by the assurance it gives 
that micro-organisms will be eliminated and will not cause the untimely 
death of the seedlings. However, as mountain soils are rarely strong 
in clay, as the samples have always been air-dried first, and as the oven 
temperature has been only 92° C., it is thought any change in soil quali¬ 
ties may virtually be ignored. 
After drying, a cupful of soil is taken from the pan, a weighed lot of 
seeds is strewn over the smooth surface of the remaining soil, and the 
cupful is then used to cover the seeds. 
The moisture applied to induce germination and development of the 
seedlings has usually been left wholly to judgment, the intention being 
to give all that can be used and never to permit the surface to become dry. 
