120 BULLETIX 1059, IT. S. DEPARTMENT OF AGRICULTURE. 
Computing the coefficient of availability. 
In computing the coefficient of availability of the moisture at a 
given part of a tree or other plant, allowance must be made, as has 
been said, not only for the osmotic pressures at work in the plant 
and soil, but for the distance through which these must operate, 
and the effect of gravity on the balance between the two forces. As 
previously suggested, let 
P = osmotic pressure in the plant. 
P'=opposing pressure in the soil water, 
Z=A=height of plant in centimeters at point where P is determined. 
G = weight of the column of water to be lifted, in atmospheres, or 
equal to h X 0.00097 atmosphere. 
AA, representing the coefficient of availability, is equal to 
P-P'-G 
h 
It may then be assumed that the foliage of the tree, at a height 
of 30 meters above the ground, has been determined by its freezing- 
point depression to possess an osmotic pressure (P) of 25 atmos- 
pheres. 
A more complicated case may also be considered. A soil, previ- 
ously tested, is found to possess an osmotic pressure of 25 atmos- 
pheres at -I per cent moisture content and of 5 atmospheres at 20 per 
cent moisture content, the former being appreciably above the wilt- 
ing coefficient. This soil is found to be currently at 6 per cent 
moisture content. Its osmotic pressure P' may then be computed 
as 
25 - 2( on _ 4 ) = 22 - 5 atmospheres. 
The formula for this case then reads 
a a 25 - 22,5 - 3000(0.00097 ) _ - 0.41 _ 
^=— 3000 "■" 3000"" - 00013 ' 
The coefficient of availability being a negative quantity of any 
magnitude, it is evident that the part of the tree which has been 
examined can not obtain water from the soil unless (1) the moisture 
content of the soil is increased, or (2) the foliage may withstand 
further drying and the creation of a higher pressure, without injury. 
Under the conditions stated as to the wilting coefficient of this soil. 
it is still probable that the part of the tree examined may obtain 
water when it attains a drier state. 
In the examination of a tree branch of appreciable length, it may 
be necessary and desirable to make an additional allowance in h for 
the horizontal distance, as well as the distance from the ground. 
This addition, however, would not apply to the calculation of G. 
