Causes of Fertility or Barrenness of Soils. 187 
nations of which determine the fertility of each particular soil. 
Clay forms a matrix in which all other substances act ; it is a 
storehouse, in which are collected and brought into contact those 
bodies which possess an affinity for each other's society: having 
united together, they are taken care of until required, when they 
are slowly produced. Silicate of alumina unites with silicates 
of other earths and alkalies, forming what are called double 
silicates — substances which appear to play a most important part 
in the nutrition of plants. 
Lime is also very abundant in nature, being found in all 
fertile soils ; indeed, as it enters into the composition of every 
kind of plant, we may safely conclude that it is necessary to 
vegetation. It is an oxide of the metal calcium, possessing basic 
properties ; having a great affinity for moisture and carbonic 
acid, on exposure to the atmosphere it rapidly becomes a 
hydrate, and finally carbonate of lime, in which state it princi- 
pally exists in soils, though it is also found as sulphate and 
phosphate. From the earliest times lime either as carbonate or 
oxide has formed an important dressing for all kinds of land : 
whenever new land is brought into cultivation, or old pasture 
broken up, quicklime should be applied, whether the soil be stiff 
clay or light sand. We are better acquainted with the action of 
quicklime than of the carbonate, owing to its having engaged 
more attention from the chemist ; but it is reasonable to suppose 
that the action is similar in both cases, only much more rapid 
and effective in the former, and therefore its application is to be 
preferred. As much less is required, the expense of burning is 
compensated by the saving in labour. Much difference of opinion 
still exists as to the action of lime ; some chemists would limit 
its effects to vegetable matter only, others confine its action to 
the decoitiposltlon of mineral matters, while a third class look 
upon it principally as a manuring substance. We believe its 
value is due to all tluee causes. That lime has a most beneficial 
effect on inert vegetable matter is clear, from the advantages 
which follow its application to peaty soils ; that inert vegetable 
matter exists in soils that have been long in cultivation and fre- 
quently manured is most certain ; and that lime would in such 
cases prove as lertlllsing as a dressing of manure seems reasonable 
to conclude : but of course, as its effect is destructive, and in this 
sense dependent upon the presence of vegetable matter, it can 
never be substituted for manure. Its action consists in reducing 
to an available form those substances which have not been already 
absorbed by plants on account of their insoluble condition. This, 
it is now generally believed, is effected by the gradual reduction 
of the humus into its ultimate products, carbonic acid and water, 
and possibly ammonia or nitric acid; the nitrogen of the atmos- 
