Soil permeability is the property of the soil to transmit water and air and is one of the most important qualities to consider for fish culture.

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The more permeable the soil, the greater the seepage. Some soil is so permeable and seepage so great that it is not possible to build a pond without special construction techniques. You will learn about these techniques in a later volume in this series.

Soils are generally made up of layers and soil quality often varies greatly from one layer to another. Before pond construction, it is important to determine the relative position of the permeable and impermeable layers. The design of a pond should be planned to avoid having a permeable layer at the bottom to prevent excessive water loss into the subsoil by seepage.

The dikes of the pond should be built with soil which will ensure a good water retention. Again, soil quality will have to be checked with this in mind.

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9.1 Which factors affect soil permeability?

Many factors affect soil permeability. Sometimes they are extremely localized, such as cracks and holes, and it is difficult to calculate representative values of permeability from actual measurements. A good study of soil profiles provides an essential check on such measurements. Observations on soil texture, structure, consistency, colour/mottling, layering, visible pores and depth to impermeable layers such as bedrock and claypan* form the basis for deciding if permeability measurements are likely to be representative.

Note: you have already learned that soil is made up of a number of horizons, each of them usually having different physical and chemical properties. To determine the permeability of soil as a whole, each horizon should be studied separately.

9.2 Soil permeability relates to soil texture and structure

The size of the soil pores is of great importance with regard to the rate of infiltration (movement of water into the soil) and to the rate of percolation (movement of water through the soil). Pore size and the number of pores closely relate to soil texture and structure, and also influence soil permeability.

Permeability variation according to soil texture

Usually, the finer the soil texture, the slower the permeability, as shown below: