254 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 
salts or mineral manures. Different kinds of soil possess in 
varying degrees the power of extracting from their solutions and 
retaining certain inorganic substances, which are thus prevented 
from being washed down to the deeper layers and lost. Of all 
soils, sand possesses the least power of holding the various food 
materials ; that is, they are more liable to be washed down and 
lost. The power of retaining these substances increases in 
proportion to the amount of clay present. This is why sandy 
soils are less capable of holding a store of available food 
material and are more easily exhausted than the heavier classes 
of soils. Potash, lime, ammonia and phosphoric acid are 
among the substances which are capable of being absorbed and 
held in the soil, and the more concentrated the solution, and the 
more of it that is added, the greater is the amount retained. 
In virtue of this property, then, the soil tends to absorb and 
hold plant food material, but its capacity for doing so is limited 
by the dissolving action of the soil-water, which tends again to 
bring these substances into solution,,thus rendering them avail- 
able for absorption by the plant. So, in this way, the soil tends 
to store up food material produced by the natural weathering of 
the rock, or artificially added as manure; and, on the addition 
of these substances, others are liberated, which are either 
absorbed by plants or washed down to the subsoil. Chemical 
combinations and decompositions are constantly in progress— 
fixing up some substances and liberating others for absorption 
by the plant. These processes further tend to bring substances 
into solution which are otherwise insoluble in water and unavail- 
able for the plant. For example, lime and potash may replace 
each other in certain combinations, so that adding lime to the 
soil may mean more available potash for the plant, and, on 
the other hand, adding potash may ultimately lead to a 
greater absorption of lime. The power of absorption by the soil 
increases the more rapid and energetic these changes become. 
This simply means that a greater turnover is taking place in the 
soil; the material present in excess being stored up, and this is 
accompanied by a liberation of that which may be deficient. 
In other words, the greater the store of food material present 
in the soil, and the quicker these chemical decompositions take 
place, the greater will be the amount of food material at any 
time available for the plant, which is equivalent to saying the 
more fertile is the soil; and this desirable state of affairs can be 
