74 REPORTS ON THE STATE OF SCIENCE.——1918. 
probably connected with the relation between vapour pressure 
and moisture content. But there is clearly something else at 
work, for the curve is not of a simple exponential type. It is 
necessary to allow for another factor: the effect on the rate of 
evaporation of the water surface in the soil, which obviously 
diminishes in area as evaporation continues. 
The equation finally developed by Keen is:— 
d 3 7 WSs \ 
fg GW 2: ( / 00+ 1) (og. (w + K) — log, K), 
=rate of evaporation. 
where 
w=percentage of water present by weight. 
s=specific gravity of the soil. 
A and K=constants. 
This relationship holds without any break, proving that all the 
water in a normally moist soil is held in the same way without 
change in physical state. At one end of the curve the water is 
more easily given up than at the other, and in the competition for 
water between soil colloids and plants or micro-organisms some 
kind of equilibrium may be attained under definite conditions: 
this equilibrium is the ‘‘ wilting point’ of the physiologist. On 
this view the other constants and critical points that have been 
indicated by various investigators are all equilibrium points and 
do not represent breaks in the condition of water in the soil. 
The retention of liquid water by soil, or, in other words, the 
resistance to drainage, is no doubt influenced by soil colloids. 
Attempts have been made by Alway and McDole to trace some 
connection between the amount of water absorbed by dry soil 
from a moist atmosphere and its water-holding capacity: in so 
far as these phenomena are related it is probably through their 
relationship to the soil colloids. The influence of salts on the 
permeability of soil to water, which is probably as much physieal 
as chemical, has been studied by Hissink, 
Soil Acidity. 
It has long been known that some soils are acid to litmus paper, 
but become neutral on the addition of calcium carbonate. Acidity 
is not tolerated by most cultivated plants, and the agriculturist 
has therefore to guard carefully against it: the problem is one of 
high technical importance. 
The older chemists took the obvious view that acidity was due 
to some special soil acid or acids, to which various names were 
given: humic acid, ulmic acid, creniec acid, apocrenic acid, ete. 
But no acid satisfying the modern chemist could be isolated, and 
when the colloid conception was introduced Cameron pointed out 
that the phenomena could all be explained as simple colloidal 
manifestations and did not require the assumption of soil acids 
at all. It was only necessary to suppose that the soil colloids 
absorbed the base more readily than the acid from blue litmus 
