174 
Causes of Fertility or Barrenness of Soils. 
the decomposition of peat bogs : they are easily recognised by 
their dark colour and small specific gravity. We shall proceed 
to consider these divisions in detail. 
Clays. — These soils are very tenacious, due to the alumina. 
Plastic when moist, they are readily moulded into any form, 
hence their value for the manufacture of earthenware, and tlie 
purer and least productive soils are best adapted for this purpose. 
When dry, clay falls down into an impalpable dust, but in 
nature clays generally bake, that is, dry on the surface, but 
remain moist underneatli, forming masses as hard as iron and 
very difficult to cultivate. The contracting influence of heat 
causes these soils to crack during dry weather ; the fissures thus 
formed are often of considerable size, due to tlie closing of the 
particles kept apart by the moisture, which clay so readily ab- 
sorbs. The presence of moisture, by excluding the atmosphere 
and the sun's rays, and by constant evaporation from the surface, 
causes clay soils to possess a lower temperature than any other ; 
they are eminently cold in a natural state. This property of 
absorbing and retaining moisture and gaseous matter, which 
may be tui'ned to the first advantage under judicious manage- 
ment, often renders clay soils unfit for cultivation in a natural 
state. The term heavy as applied to clay soils is not in reference 
to their specific gravity, which is less than sands or calcareous 
soils, but in consequence of their consistency making them diffi- 
cult to work. Their agricultural value varies extremely, depend- 
ing principally upon the proportions of the various ingredients 
as affecting the physical and chemical character, and we have 
examples of the poorest as well as most fertile soils. Pure clay 
does not enter into the composition of plants, nor as far as we 
know (except in very minute quantities) is it decomposed into its 
elements ; consequently a soil consisting of it only would be per- 
fectly barren although possessing great powers of absorption. 
Poor clays are the least desirable of all soils, on account of the 
heavy expense of cultivation, three and more often four horses 
being required for even the shallowest ploughing. Great judg- 
ment is required to know the proper time to work such land ; as 
in wet weather the pressure of the hoof will puddle the subsoil 
into pans of the most impervious character. Fortunately very few 
are so simple in their nature as to be unfit for cultivation. More 
frequently the poverty is due to the saturated condition (,f the 
mass preventing the atmosphere penetrating and effecting those 
clianges in the ingredients (thus rendered inert) which are indis- 
pensable before food can be provided for the plant. Before 
expending capital in reclaiming, it is highly important to ascer- 
tain whether the ingredients are of a nature to warrant it, and 
here we perceive an instance of the important assistance afforded 
