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Physical Properties Of Soils

SOIL TEXTURE

The particles that make up soil are categorized into three groups by size – sand, silt, and clay. Sand particles are the largest and clay particles the smallest. Most soils are a combination of the three. The relative percentages of sand, silt, and clay are what give soil its texture. A clay loam texture soil, for example, has nearly equal parts of sand, slit, and clay. These textural seperates result from the weathering process.

There are 12 soil textural classes represented on the soil texture triangle. This triangle is used so that terms like “clay” or “loam” always have the same meaning. Each texture corresponds to specific percentages of sand, silt, or clay. Knowing the texture helps us manage the soil.

SOIL STRUCTURE

Soil structure is the arrangement of soil particles into small clumps, called peds or aggregates. Soil particles (sand, silt, clay and even organic matter) bind together to form peds. Depending on the composition and on the conditions in which the peds formed (getting wet and drying out, or freezing and thawing, foot traffic, farming, etc.), the ped has a specific shape. They could be granular (like gardening soil), blocky, columnar, platy, massive (like modeling clay) or single-grained (like beach sand). Structure correlates to the pore space in the soil which influences root growth and air and water movement.

Bulk density

Bulk density of soil is the mass of soil per unit volume of soil (volume includes both soil and pores) – expressed in g/c.c.

Pore Space

Bulk density

Particle density can be determined using specific gravity bottle technique and bulk density by taking soil core samples of known volume in the field and determining the even dry weight (Black et al., 1965a). Water and air movements through soil depends on the pore space and the size distribution of the pores (microPores and macropores). Lower the pore space or higher the bulk density of the soil, the higher the suitability of the soil for aquaculture.

Soil Colour

Soil color is influenced primarily by soil mineralogy – telling us what is in a specific soil. Soils high in iron are deep orange-brown to yellowish-brown. Those soils that are high in organic matter are dark brown or black. Color can also tell us how a soil “behaves” – a soil that drains well is brightly colored and one that is often wet and soggy will have a mottled pattern of grays, reds, and yellows.

Consistence

is the resistance of a soil to deformation or rupture and is determined by the cohesive and adhesive properties of the soil mass. This is a term used to designate the manifestation of the cohesive and adhesive properties of soil at various moisture contents. A knowledge of the consistence of the soil is important in tillage operations, traffic and pond constructions. Consistence gives also an indication of the soil texture.

Consistence is described for three moisture levels:

  1. Wet soil – non sticky, slightly sticky, sticky, very sticky; non plastic, slightly plastic, plastic and very plastic.
  2. Moist soil – loose, very friable, friable, firm, very firm, extremely firm.
  3. Dry soil – loose, soft, slightly hard, hard, very hard, extremely hard.

Description of the consistence terms mentioned above can be obtained from “Guidelines for Soil Profile Description” by FAO (FAO, 1974).

Particle density

of soil is the mass per unit volume of soil particles (soil solid phase) – expressed in g/c.c. Most soils have particle density of about 2.6 g/cc. Presence of organic matter decrease the density and iron compounds increase the density.

Soil permeability

Soil permeability is the ability of the soil to transmit water and air. An impermeable soil is good for aquaculture as the water loss through seepage or infiltration is low. As the soil layers or horizons vary in their characteristics, the permeability also differs from one layer to another. Pore size, texture, structure and the presence of impervious layers such as clay pan determines the permeability of a soil. Clayey soils with platy structures have very low permeability.

Permeability is measured in terms of permeability rate or coefficient of permeability (cm per hour, cm per day, cm per sec.).

Permeability rate or coefficient of permeability is determined in the laboratory by measuring the rate of flow of water from a constant head of water through a colomn of soil at specific moisture content and other conditions. It is determined in the field by digging a hole of approximately 30 cm diameter, smearing the sides of the hole with heavy wet clay or lining with plastic sheet and measuring the rate of infiltration of water by filling the hole repeatedly with water and noting the time it takes for the water level to go down by a specific depth.


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