THE MATERIAL INCOME OF PLANTS, 315 
of a root and may enter it if the conditions permit. But inasmuch as 
the mineral solutes in the soil waters are very similar, no matter what 
the character of the soil may be, this is probably of less importance to the 
plant than it would seem to be at first sight. 
Available water. By no means all of the water in the soil is free to 
migrate into the roots. There comes a time, as the films about the soil 
particles become thinner and thinner, when the adhesion of the water 
to the soil grains is equal to its diffusion tension. Leading up to that 
equilibrium, it grows increasingly difficult for the plant to balance its 
loss of water by that entering ; its cell sap has become more and more 
concentrated; and when the outgo surpasses permanently the income, 
permanent wilting usually ensues and often more or less extensive 
death of the foliage. 
The water content of a soil from which no more water can enter a 
plant manifestly depends upon the plants concerned, the nature of the 
soil, and other physical factors. It is no fixed quantity in any case, and 
at best can be determined only roughly. To say that it is in sand less 
than 0.5 per cent, in clay about to per cent, in loam about 12 per cent, 
in humus about 14 per cent, and in muck about 20 per cent, is merely 
to indicate the order of magnitude, not to state a fixed amount. These 
figures become more instructive when compared with the total capacity of 
such soils for water, which runs about as follows: sand, 15 per cent; clay, 
50 per cent; loam, 65 per cent; humus, 70 per cent; muck, 120 per cent. 
Effect of roots on soil. A considerable amount of carbon dioxid 
(CO 2 ) and less quantities of other substances diffuse from the root into 
the soil-water films. Solution of carbonates is increased by the pres- 
ence of CO 8 in water, as is shown by the readiness with which a polished 
marble plate may be etched by roots traversing its surface and giving 
off CO 2 . Reactions due to other solutes which diffuse from the root, or 
to excretions from it, may determine the solution of other sorts of soil 
particles, and the substances so dissolved may then enter the root. It 
is not known that these changes so produced in the soil are of any con- 
siderable importance in plant life. Whether by diffusion from the roots 
of live plants or by the decomposition of dead roots, or by both, it is 
certain that various complex organic compounds, not yet fully known, 
exist in soils, which may interfere seriously with the growing of plants 
thereon. In certain soils the character and quantity of these little 
known substances are so injurious that the soils are almost sterile. 
Even a watery extract from them proves harmful. In such cases the 
