Causes of Fertility or Barrenness of Soils. 
185 
in many cases would in time disintegrate and deepen the soil. 
Of course they must be removed where so larj2:e as to interfere 
with cultivation ; but the benefit is very doubtful in other cases. 
The presence of stones scours the plough, as it is termed, that is, 
keeps the share and turnfurrow clean. 
Colour may be regarded as a physical character of soils, and 
may be noticed as appearing to exercise a marked influence upon 
fertility, due in a great measure to its affecting the temperature. 
Pedestrians are well aware of the difference felt in a very hot 
sunshine in walking over a white or dark soil. In the first case 
the rays are reflected and strike upon the body with uncomfortable 
force ; in the second they are absorbed, and the ground, or rather 
air, appears cool and refreshing. Tlie darker a soil the greater 
its absorbing power ; but to compensate in some measure for this, 
the lisjht- coloured soil retains heat longest. 
II. — Chemical Chakacters, with General Account of the 
Results of Analyses hitherto made. 
By the term chemical character of a soil, we refer to the 
presence or absence of those ingredients with which the science 
of chemistry has made us acquainted. The ashes of plants are 
made up of a number of mineral substances, varying in different 
kinds, but always identical in the same species ; and as these 
matters must be derived from the soil, we should expect to find 
fertile soils abounding in, and barren soils destitute of them. This, 
however, is not always the case : of course fertile soils must 
possess them, but infertile also often exhibit abundance of such 
food, and therefore we believe their value may depend more 
upon the particular state of combination in which they exist, 
than merely their presence or absence in a soil. Chemical 
analysis often fails to detect sul)stances, which may yet exist in 
sufficient quantity for vegetable life ; or again, from some slight 
impurity in the re-agents, it may indicate bodies, that are absent 
from the soil. Owing to these difEculties, the science of 
chemistry has not produced those results which were naturally 
expected. Little reliance can therefore be placed upon the 
mere tabular results of an analysis, the object of which is to 
point out the relative quantities of the different mineral matters 
in a soil without reference to their state of combination. We 
do not wish to infer that no value is to be attached to the ordi- 
nary analyses of soils, but would only point out their liability to 
error. Fertile soils contain the following substances : silica, 
alumina, peroxide of iron, lime, magnesia, potash, soda, sulphuric, 
phosphoric, and carbonic acids, and chlorine. With the excep- 
tion of alumina, all these exist in the ashes of plants, being built 
