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Tuesday, 6 February 2018

SAND, SILT and CLAY

Any sample of soil is made up of a continuous array of particles
which may range from the very smallest clay particles to large
gravel. In order to describe soils scientist have established a classification
system for ranges of particle sizes; size ranges which reflect their role in many of the soil properties we are familiar with. This classification system divides the particles into four classes - gravel and stone, sand, silt,and clay. Table 1 lists the Canadian classification system for the size range of particles which may be found in a soil
sample.


CLAY
The scientists have assigned the name clay to the finest particles and
not without reason. Clay size particles are the source of most of the
chemical properties of soil.They are responsible for the retention of
many of the plant nutrients in the soil, such as calcium, magnesium,
potassium, trace elements and some of the phosphorus. Clays react with
the breakdown products of organicmatter to stabilize the humus in the
soil.A soil without clay particles can be a very infertile soil.
Clays, because of their very small size and very large surface area, are
able to retain greater amounts of water than sandy soils.On the other

hand, as will be discussed in a latter article, clays hold the water more

closely and do not release the water.

as readily to grass roots as sands. Clay particles have a vastly greater
tendency to stick together than sand, thus it is common farmer
knowledge that soils high in clay are difficult to till.When a small sample
of a clay soil is wetted and rubbed between the fingers it will feel very
sticky and is easily formed into a string.

SILT
The particles classified as silt are intermediate in size and chemical and
physical properties between clay and sand. The silt particles have
limited ability to retain plant nutrients, or to release them to the
soil solution for plant uptake. Silt tends to have a spherical shape,
giving a high silt soil a soapy or slippery feeling when rubbed between
the fingers when wet and is more difficult to form into a string
than clays. Because of the spherical shape, silt also retains a large amount of water,
but it releases the water readily to plants. While silt soils are generally
considered very fertile for the growth of plants, largely due to
their water characteristics and ease of cultivation, engineers dread
working with them due to their relatively easy release of water and lack
of ability for the particles to stick together.



SAND
Sand particles are essentially small rock fragments, and as such, have
little or no ability to supply grass with nutrients or to retain them
against leaching.As rock fragments, sandy soils feel gritty between the
fingers. The sand grains have little ability to stick together; thus sandy
soils can not be rolled into a string when wetted.
It is well known that sandy soils are droughty soils because they retain
little water when wetted. Never the less what water is retained is
released to plants easily. When rain or irrigation occurs the water readily
penetrates the soil surface, the excess moves through rapidly and
the soil remains well aerated. These properties make sands a desirable
medium for growing sports turf where there is no limitation in appling
water and nutrition, as needed, throughout the season.

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