120 BULLETIN" 1059, U. 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 betAveen 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 AX 0.00097 atmosphere. 

 A A, representing the coefficient of availability, is equal to 



P-P'-G 



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 4 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 — 5\ 

 ll r>n_4 ) = 22.5 atmospheres. 



25 -2( 



The formula for this case then reads 



25-22.5-3000(0.00097 ) = - 0A1 = 

 AA ~ 3000 3000 V-V"" 167 



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. 



