Medical Officers of the Army of India.
19
3. Clay-soils, containing above 50 per cent. of clay.
4. Vegetable moulds (humus soils), containing more than 6 per cent. of organic
matters or humus.
5. Marly soils, or soils in which the proportion of lime is more than 5, but does not
exceed 20 per cent. of the whole weight of the dry soil, and that of clay is
more than 20, but less than 5o per cent.
6. Loamy soils, or soils in which the proportion of clay likewise varies from 20 to
50 per cent., but which at the same time contain less than 5 per cent, of lime.
* * * * * * * *
3. Clay-soils.—The properties of clay-soils are diametrically opposed to those of
sandy soils. Stiffness, impenetrability, great power of absorbing and retaining moisture,
and great adhesiveness characterise this class of soils. They are consequently cold, stiff
soils, which are expensive and difficult to cultivate. When properly cultivated, some
are turned into highly fertile soils. Their faults arise out of their mechanical structure,
not out of their chemical composition, which latter alone is represented in the following
analyses. The former may be corrected by drainage, burning, the use of bulky manures,
and the addition of lime, ashes, &c.
1
According to this, the subsoil of Calcutta has no claim to be regarded as a
clay one either in respect of its chemical constitution or its physical properties,
for in the first place, even on the most liberal calculation, it does not contain
anything nearly attaining the standard of clay content, and in the second it is
neither retentive nor adhesive. The idea that it is retentive is probably founded
on the fact that at the level at which it occurs it is in a constant state of satura
-tion, but this saturation is not due to any special retentive property in it, but is
simply the result of the very low level of the country and the great masses of
water constantly entering it and serving to bring the water level even when at its
lowest close to the surface. The most casual observation, however, shows that
on removal from its native site the material, in place of being retentive, parts with
its moisture with extreme readiness.
The following experiments illustrate the degree of impermeability which it
possesses. It has been already mentioned that on drying the material becomes
very hard. This hardness does not indicate any coherence under the influence
of renewed access of moisture. This must be familiar to any one who has ever
had experience in the excavation of deep tanks in and around Calcutta, where it
is a matter patent to the most superficial observation that there is no necessity
for expending labour in breaking down the hard, rough masses of the subsoil if
spread out over the surface of the surrounding ground, as they rapidly melt
down and disappear under the influence of the first heavy rainfall. In this
respect they differ greatly from masses of peat from the beds of this material,
which are frequently intercalated in the so-called blue clay. The incoherence
of the dry material appears very conspicuously on immersing portions of it in
water. If a fragment be dropped into a beaker full of water, an immediate and
violent discharge of bubbles of air takes place and is accompanied by a rapid
1A Cyclopædia of Agriculture, Practical, and Scientific, edited by J. C. Morton, London, 1871, Vo
l. II, pp. 869—870.
D 2