Geoscience Reference
In-Depth Information
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Clays
Whereas soil analysts can measure the proportions of sand, silt and clay fairly
precisely, and then read off the texture from the diagram, the farmer and soil surveyor
must often try to reach an opinion in the field by the feel of the soil. A key to the
identification of soil textures was revised in 1984 for this purpose by the Agricultur-
al Development and Advisory Service (ADAS) of the Ministry of Agriculture. The
procedure is easy enough for anyone to do, and is summarized here (
Table 2
). The
purchaser of a new home may like to know how easy the soil in his garden will be
to work, and how it is likely to respond to rain or drought. It needs practice and ex-
perience to become fully proficient, of course, but for most practical purposes it is
enough to differentiate soils into five classes - Sands, Light loams, Light silts, Clay
loams and Clays.
This classification takes no account of soils that have large amounts of organic
matter which can greatly modify the way soils behave, so ADAS has included a clas-
sification of humose and peaty soils. Organic-rich soils are prevalent in the Fenland
of East Anglia, and frequent in areas of high rainfall, mostly over 2000 ft (600 m) in
southern Britain but down to sea level in northwest Scotland.
Soil structure refers to the natural aggregates (structural units or 'peds') into
which the primary mineral particles and organic matter are formed. The size and
shape of these aggregates largely determine the distribution of living space within the
soil. Texture and structure in soil are often confused. As Stewart says “when assess-
ing the quality of cloth by its feel we should try to distinguish between characteristics
determined by the nature and thickness of the thread and characteristics determined
by the nature and closeness of the weave. Similarly, when feeling soil, we should try
to differentiate between characteristics determined by the nature and size of the fun-
damental particles and those determined by the manner of their arrangement.” Soil
texture is a primary characteristic while soil structure is a secondary and variable fea-
ture reflecting the way the soil has been modified by living agencies, including man.
Farmers and gardeners spend a great deal of effort to produce good soil structure,
or tilth, and this is much easier with a loam soil than with a heavy clay. Luckily, only
a few soils in Britain have more than 50 per cent clay - though this fact is little con-
solation for those who have to contend with soils on London and Oxford Clays which
do. Tilth in clay soils can be improved by digging in organic matter, while marling -
the spreading of calcareous loamy soil - was formerly practised very widely on light
soils in East Anglia. Where the fenland peats of Cambridgeshire are being eroded and
oxidized away, ploughing now brings up silts from below, and creates new soil tex-
tures with which to work.
