Particle size classes

There is a continuous range of particle sizes, but it is convenient to divide them into classes. Three major classification systems in use today are those of the International Society of Soil Science (ISSS), United States Department of Agriculture (USDA) and the Soil Survey of England and Wales (SSEW). These are illustrated in Figures 17.8 and 17.9. In this text the SSEW scale used by the Agricultural Development and Advisory Service of England and Wales (ADAS) is adopted. In each case, soil is considered to consist of

In natural conditions organic matter is more abundant at the surface and declines in concentration with depth (see p327), whereas under cultivation the organic matter is redistributed to create a distinct topsoil and subsoil with the boundary at 'plough depth'. The dark colour that the decomposed organic matter gives the soil makes the boundary obvious by the colour change.

International Society of Soil Science (ISSS)

0.02

International Society of Soil Science (ISSS)

Fine sand

0.002

Fine sand

Silt

United States

Dept. of Agriculture (USDA)

Gravel

V.Coarse sand

Soil Survey of England and Wales (SSEW)

Gravel

V.Coarse sand

Medium sand:

Medium sand:

Soil Survey of England and Wales (SSEW)

0.06

Fine sand

Silt

Silt

0.06

0.002

Clay i i Clay i i Clay i

Figure 17.8 Particle size classes (diameters in mm)

Root hair

Root hair

Sand Silt And Clay Comparison

Figure 17.9 The relative sizes of coarse sand, fine sand, silt and clay particles (based on SSEW classification) with root hairs drawn alongside for comparison. Note that even the smallest pore spaces between unaggregated spherical coarse sand grains still allow water to be drawn out by gravity and allow some air in at field capacity, whereas most pores between unaggregated fine sand grains remain water-filled (pores less than 0.05 mm diameter)

Figure 17.9 The relative sizes of coarse sand, fine sand, silt and clay particles (based on SSEW classification) with root hairs drawn alongside for comparison. Note that even the smallest pore spaces between unaggregated spherical coarse sand grains still allow water to be drawn out by gravity and allow some air in at field capacity, whereas most pores between unaggregated fine sand grains remain water-filled (pores less than 0.05 mm diameter)

those particles that are less than 2 mm in diameter. The silt and clay particles are sometimes referred to as 'fines'.

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