2(1 
Research Bullet in No. 3 
They 11ms really divide the total water content of any par- 
ticular sample of soil into three parts: 
1. Water available for growth — the difference between the total 
water content and the wilting coefficient. 
2. Portion of the available water not available for growth — the 
difference between the wilting coefficient and the moisture 
content of the soil when in equilibrium with the air. 
3. Nonavailable water — the moisture content of the soil when in 
equilibrium with the air, or, in other words, the water con- 
tent of the air-drv soil. 
They make no distinction between the physiological impor- 
tance of the portion of the water between the wilting coefficient 
and the hygroscopic coefficient and that of the portion between 
the hygroscopic coefficient and the air-dry condition. 
Tn their latest publication 1 they modify their earlier state- 
ment in regard to the significance of the wilting coefficient, de- 
duced from their studies using drinking glasses, when applied to 
field conditions or to experiments in which large pots are used. 
"The results of some of the soil moisture determinations made 
on the Great Plains during the summer of 1911, which was ex- 
tremely dry, indicate that a crop is capable, under certain con- 
ditions, of reducing the moisture content of a part of the soil be- 
low the wilting coefficient, before the crop actually wilts. * * * 
It follows that the amount of moisture available for growth may 
be somewhat greater than that calculated by the use of the 
wilting coefficient as a datum. 
"It often happens under field conditions and also when plants 
are grown in large pots that at the approach of a drouth the 
lower leaves die very gradually. This self-pruning continues in 
extreme cases until the entire leaf area is dead. Often the gen- 
eral wilting of the plant can not be observed during the whole 
process. Under these conditions the wilting point is at best in- 
definite, and the soil moisture content is usually reduced far be- 
low the wilting coefficient before the death of the plant. The 
water which is removed from the soil after the moisture content 
has been reduced to the wilting coefficient may result in some 
growth but this is so small in proportion to the water consump- 
tion that it is of little practical value from the standpoint of crop 
production." 
Their use 2 of the data from the single pot experiment with 
1 Application of Wilting Coefficient Determinations in Agronomic In- 
vestigations, American Society of Agronomy, Nov., 1911, vol. 3, p. 257. 
2 Bulletin 230, Bureau of Plant Industry, p. 66. 
