72 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 
In addition, for the greater refinement of this expression, it Trill 
be necessary to make a correction for the height to which the water 
must be lifted. In small plants this would be of no consequence, but 
with tall trees it is evidently the factor which brings the coefficient 
of availability to zero long before the soil moisture is exhausted. 
This correction may be taken as approximately 0.097 atmospheres 
per meter of height. Expressing this whole correction by G. 
P-(P>+G) 
It should be noted that the availability of the moisture is an ex- 
pression whose value will change only very gradually with the ex- 
haustion of the soil moisture, and is really based upon an assumed 
equilibrium between the osmotic pressure of the whole plant and that 
of the soil at the time of wilting, which it is possible to attain ap- 
proximately if the wilting of the plant is brought about very slowly. 
On the other hand, the coefficient of availability must be con- 
stantly fluctuating, being dependent both on supply and demand 
(loss). Thus the rapid loss of a considerable amount of water from 
the leaves must be almost immediately reflected in the osmotic pres- 
sure there, the gradient from leaves to roots, and the rate of intake 
at the roots and of transfer from cell to cell. The objects in the use 
of such an expression must be to show not only how the demands of 
the plant vary from time to time, but how nearly the demands created 
by certain atmospheric conditions may be satisfied. Thus some in- 
sight is obtained into the conditions governing growth-rate. 
6. There are various other conditions of the soil which have an 
effect upon plants and may or may not be fully indicated by the 
osmotic pressure of the soil solution at any time. Of these may be 
mentioned : 
(a) The hydrogen-ion concentration as an expression of the de- 
gree of alkalinity or acidity. 
(b) The make-up of the soil, and particularly its clay content as 
indicated by the mechanical analyses. 
(c) Humus content. 
(d) The capillary transporting power of the soil, by which water 
from distant regions may be brought to the roots. Obviously this 
may often be an extremely important factor in the economy of the 
plant. Its importance is somewhat minimized when moisture deter- 
minations are certainly made in the soil area which is reached by the 
roots. 
(e) Chemical content of all elements and compounds, with par- 
ticular reference to those which are necessary in nutrition. 
