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Soil Particles

Soil Colour | Soil Particles | Bonding and Aggregation | Porosity | Changing Soil Structure | Soil Strength

Mineral soil particles come from rock and sediments. The coarser materials are usually fragments of the original materials, while the finer materials (commonly clay) are usually the product of considerable weathering.


The mineral particles are primary determinants of soil structure.

Size distribution
To deal with this range we use four groups:


Gravel

>2 mm

Sand

0.02 – 2 mm

Silt

0.002 – 0.02 mm

Clay

<0.002 mm

The smaller the particle, the greater the surface area per unit volume. Bonding and aggregation is much more significant with the availability of large surface areas. Clay sized materials (when moist) impart a "plastic" feel to kneaded soil samples. Silts adhere well but are not plastic. Sands will not knead and fall apart in the hand.

These responses to moist kneading are grouped as
soil texture and can be used to make estimates of particle size distribution in soil samples.

The special case of clay minerals
Clay minerals are not only small - they are flat and "plate" like. Further, the flat surfaces commonly have a net negative charge. These two extra characteristics make them especially surface active.


The particular level of surface activity will thus vary amongst clay particles depending their particular mineral composition, and what cations (positive charged ions free within the soil environment) are available. These variations in surface activity mean that different clays can range from strongly aggregating (electrostatic bonding) to strongly disaggregating (electrostatic repulsion). It is important to know such behaviour when attempting to manage soil health, refer
bonding and aggregation and dispersion.

Related links
Mineral Clay information on wikipedia (external link)
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