THE SOIL 85 
(a) The amount of water entering the soil ; 
(6) The character of the water ; and 
(c) The power of the soil to retain it. 
(a) The Amount of Water entering the Soil depends 
upon— 
ab The amount of water received from above—i.e., 
upon the rainfall, and upon the percolation of water from 
rivers and lakes, springs, and adjacent sheets of water. 
2. The amount of water sucked up from below. This 
depends upon the size of soil-particles, the presence or 
absence of ground-water, and its depth below the surface 
of the soil during the vegetative season. 
(>) The Availability of this Water to Plants depends, as 
we have already shown, upon its chemical nature. The 
roots of plants can only absorb very weak alkaline solu- 
tions. For this reason, if the proportion of material in 
solution in the water is greater than a certain small amount 
(1 to 3 per cent.), or if souring acids be present, the soil, 
though it may be physically wet, is physiologically dry, 
and little water is absorbed. The soil also must be well 
aerated, otherwise the water present is useless. 
(c) The Amount of Water retained by the Soil depends 
upon its physical properties, and the physical properties 
are all directly or indirectly connected with the size of 
the particles forming the soil. 
The Physical Properties of the Soil, and their Relations 
to Water.—The rock-fragments forming the soil vary in 
size from large stones to an impalpable dust, which, when 
dry, remains for a long time suspended in water, and 
which, when wet, becomes compacted into a greasy, heavy 
clay. The particles, however, touch one another at a 
few points only, leaving between them pores, or spaces, 
which are normally filled with water and air. In ordinary 
soils the water forms a film of varying thickness round 
the particles, air occupying the rest of the pore-space. 
As the particles are in contact, the water-films surround- 
ing them are everywhere in continuity with one another. 
These films are elastic, and the withdrawal of water at 
any point sets up a flow of water along the adjacent 
films until equilibrium is restored. If a section of some 
coarse-grained soil containing water be examined, it will 
be seen that the water-films increase in thickness as we 
