A WAX SEAL METHOD FOR DETERMINING THE LOWER 
LIMIT OF AVAILABLE SOIL MOISTURE* 
LYMAN J. Brices anno H. L. SHANTZ 
(WITH TWO FIGURES) 
All soils upon which the plant cover has wilted through lack 
of water will still be found to contain moisture, varying in amount 
from less than 1 per cent in a coarse sand to 25 per cent or more 
in the heaviest clay soils. It is therefore highly important in all 
studies of the relationship of soil moisture to plant growth to recog- 
nize clearly that a simple statement of the moisture content of the 
soil gives no indication whatever of the amount of water actually 
available to the plant. To ascertain the amount of available soil 
moisture, it is necessary to determine the actual water content of 
the soil, and in addition the minimum to which the plant can 
reduce the soil moisture content. The difference between these two 
determinations represents the soil moisture that is actually avail- 
able to the plant. The minimum to which a plant can reduce the 
water content of a soil is dependent upon a number of variable 
factors, and is somewhat lower than the moisture content cofte- 
sponding to the wilting point. Practically speaking, the permanent 
wilting of the plant marks the cessation of growth, and in accordance 
with previous usage has been considered in the present paper as the 
criterion of non-availability. In a subsequent paper, we PrOPos* 
to compare the minimum of available moisture, as determined by 
the wilting point and by the death point, and to show to what 
extent this determination is affected by varying conditions of 
temperature, humidity, and light. ; 
The method which we have employed consists essentially (x) in 
the use of an impervious pot, (2) in sealing over the soil surface 
in the pot with wax so as to prevent all evaporation from the soll, 
and (3) in keeping the soil mass at approximately constant tempera- 
ture. Under these conditions the water remaining in the 50! 
at the time of wilting is non-available to the plant. 
* Published with the permission of the Secretary of Agriculture. 
Botanical Gazette, vol. 51] i 
