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Physical Properties: Soil Texture
Soil texture describes the size (diameter) of the soil particles.
Where larger mineral particles predominate, the soil is gravelly
(d > 2mm), or sandy (0.05 < d < 2); where smaller, colloidal
mineral particles are dominant, the soil is claylike (d < 0.002).
Soils can have any combination of gravel, sand, and clay and a textural
chart is used to describe the different soil textures as shown below:
Any soil composed of all three particle sizes and not exhibiting
the physical properties of any one of them is referred to as a loam.
Texture influences plant growth by its direct effect on soil aeration,
water infiltration, cation
exchange capacity (CEC), and erodibility. Infiltration and permeability
are rapid in sandy soils, very slow in clay soils, and intermediate
in loam soils. Soils that are granular, with a large diversity in
particle size, have many large and small pores - a desirable characteristic
for plant growth.
Soil texture can be estimated in the field by experienced soil
scientists. Although, accurate determination of soil texture is
measured in the laboratory. The procedure commonly used involves
determining particle sizes by the rates that they drop out of solution.
The soil particles are first suspended in a solution. This is usually
accomplished using chemical treatments along with a high-speed blender
or sonicator. While a set of sieves can be used to separate out
the sand fraction, a sedimentation
procedure is usually used to determine the amounts of silt and clay.
Soil particles are denser than water and they tend to sink. Depending
on their size, they will settle at different velocities with larger
particles settling faster. By measuring the amount of soil still
in suspension after various amounts of settling time (using a pipette
or hydrometer), the percentages of each size fraction can be determined,
ultimately identifying the soil textural class. The size of the
particles can be determined because settling time can be related
back to the diameter of a particle by Stoke's Law (Brady
and Weil, 1999).
Soil Structure
Soil structure is another description of the soil that describes
the soil peds
(aggregates)
in terms of shape. They are classified in four principal groups
of shapes: spheroidal, platy, prismlike, and blocklike. The different
shapes of aggregates in the soil define the pattern of pores and
peds. This pattern defines the soil structure which greatly influences
water movement, heat transfer, aeration, and porosity in soils.
Common activities on disturbed lands such as tillage and liming
impact soils largely through their effect on soil structure, especially
in the surface horizons. Soil structure is usually determined in
the field by a trained individual (Brady
and Weil, 1999).
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