Physical properties of soil - Principles of Horticulture - page 145

Table 12.2 The main differences between topsoil and subsoils in typical garden soil in Britain and Ireland

Characteristic Topsoil Subsoil Further details

Colour *** Dark brown/black; lighter if high

chalk content

Light browns or grey see p. 144

Texture Higher in the finer particles especially

clay

see p. 146

12

Organic matter content (%): 2-5 <1 see p. 158

Living Enormous numbers especially

near the surface
Many roots

Comparatively low numbers see p. 158

Few roots

Dead organisms Large quantity Very little see p. 158

Humus Present None see p. 158

Pore space Naturally more 'open' More compressed see p. 144

Can be maintained near 50% +
Can be improved by cultivating Too deep for cultivation** see p. 144

Aeration High proportion of large pores Limited large pores see p. 144

so good aeration so poorer aeration*

Water content Depends mainly on soil texture Depends on soil texture Table 12.3

but improved by the OM content see p. 152

Nutrient content In nature, the site of nutrients Low nutrient content * see p. 168

Enriched by addition of fertilizers Below main feeding roots see p. 172

Suitability for plants Can be ideal, primary source of

nutrients and water

Poor*, but important water reserve

Effects of weather Exposed to extremes of freezing

and thawing; wetting and drying

Protected from the extremes of
weathering by topsoil

Note:
* Avoid mixing with topsoil.
** Cultivation depth normally considered to be a spade depth ('a spit') or plough depth.
*** Colours depend on the coating on the particles; most commonly iron oxide which exists in two forms: when plenty of oxygen
present, it is in the ferric oxide form (rust); but if waterlogged, has limited oxygen supplies during soil development so the
ferrous oxide form prevails (grey, bluish). The degree of whiteness because of the chalk in the soil depends on how much is
present. Humus on the particles darkens the soil.

Composition of soils

according to the iron oxide coating from very pale
yellow to rich, rusty reddish brown. Silver sand has
no such coating. Sand grains are inert; they neither
release nor hold on to plant nutrients. They are not
sticky.
Soils dominated by coarse sand are usually free
draining but have poor water retention, whereas those
composed mainly of fine sand can hold much larger
quantities of water against gravity. All the water held
on all sand particles is readily removed by roots.

Although we tend to think of soil particles as 'too
small to see', there are huge size differences between
coarse sand, fine sand, silt and clay. This has a
significant effect on root penetration, water-holding
properties and aeration (Figure 12.7). If a basketball is
used to represent the size of fine sand, then coarse
sand would be size of a room in a house, whereas
marbles would represent silt particles and sugar grains
the largest of the clay particles.

Clay

Sand

Clay particles are those less than 0.002 mm in
diameter.

Sand grains are soil particles between 0.06 and
2.0mm in diameter.

Clay particles tend to be platelets that pack together
closely. Their combination of very small size and
chemical characteristics makes clay soil sticky when

The shape of the particles varies from gritty to more
weathered, rounded grains ('soft'). Colour varies