Agriculture Reference
In-Depth Information
status of a soil also depends on its structure, location in the landscape, the nature of its
vegetative cover, the seasonal distribution of rainfall and evapotranspiration, and the
presence of impeding layers in the profile. Not all soils with high clay contents are
poorly drained. As illustrated below (oxisol), the microaggregation of clay and silt
particles into units (peds) equivalent to sand-sized particles may lead to certain clay-rich
soils having the drainage characteristics of more coarsely-textured soils.
Texture also influences the distribution of soil organisms and, conversely, may be
modified over time through the activities of soil animals. Finally, soil texture has a
substantial influence on the ease of cultivation. Cohesion between soil particles is usu-
ally lower in coarse- than in fine-textured soils and tillage generally requires a greater
expenditure of energy in the latter soils.
The proportions of the larger sized particles present in soils are conventionally deter-
mined by passing a dry and thoroughly-destructured sample of the soil through various
meshes. Estimation of the finer particles depends on their settling rates in water, or
under centrifugation. Recently, electronic counting devices have been developed and
allow the characterisation of particle size distributions at a much more detailed level of
resolution. The upper horizons of peat soils (histosols) are largely organic in nature
although variable amounts of mineral matter may also be present and different methods
of characterisation are required. Soil texture is normally determined in the laboratory
although a broad characterisation of textural class may usefully be made in the field by
wetting and manipulating a de-structured sample between the fingers (Birkeland, 1984).
The particles of less than 2.0 mm equivalent diameter comprise the
fine earth
fraction
of the soil which, in most soils, predominates in terms of mass and, more importantly,
surface area. It should be noted that the subdivisions of the fine earth fraction differ
between classification systems, although clays are defined in both the International and
United States systems as those particles less than in diameter. Table I.3 presents
the subdivisions of the fine earth and illustrates the very large increases in particle
numbers and surface areas in descending from the largest to the smallest of the four
particle diameter classes. As discussed below, the total surface areas per unit mass or
volume of soil (the specific surface area) is correlated with a range of other soil properties.
