BoviE: NON-AVAILABLE WATER IN SOILS 276 
spreading the soils on a watch glass, and exposing them for one 
week to an atmosphere saturated with water vapor. The increase 
in weight was used as a measure of the hygroscopic water. His 
results show that, while the plant can live in a relatively dry soil, 
it can not use the hygroscopic water. 
Edmond Gain® repeated the experiments of Sachs ‘with many 
plants in order to settle the following questions: (1) Is the power 
of resistance to drought of a given plant the same in different soils? 
This question must be answered in the affirmative. (2) Is the 
water content of the soil, at the time the plants wilt, the same for 
all stages of plant growth? No, it fluctuates in such a way as to 
produce a curve.’’ He did not report the methods employed, nor 
give an analysis of the soils used. 
A number of determinations of the amount of non-available 
water in various soils were made by King."' He concluded that 
one of the reasons why the clayey soils retained more water than 
the sands is because the small grains of the clay present more 
surface for retaining the water, the thickness of the film being the 
same inall cases. He gave a formula for computing the percentage 
of moisture in a soil which a given thickness of film will produce. 
By the use of this formula, a set of theoretical values was obtained, 
for the soils studied, which agree very closely with the percentages 
of non-available water found. 
In 1902, Hedgcock® considered the relation of heat, light, and 
humidity to the amount of non-available water. His methods 
were such that he could not vary one factor while the others re- 
mained constant. The results as a whole would seem to show that 
any condition which lowers the vigor of the plant, raises the 
amount of non-available water. When he compared the various 
soils, he found that the amounts of non-available water retained 
by the soils increased in the following order: sand, loess, clay, 
loam, humus, saline soil. Judging from the sizes of the soil grains, 
the clay should have come after the saline, with the highest per- 
centage. Hedgcock concluded that some factor other than the 
extent of soil surface is involved. ~The percentage of dissolved 
substances in the soil solution was the only condition noted that - 
varied in the right direction. He, therefore, concluded that as 
the soil water diminishes, its concentration increases until finally 
