Soil fertility and sustainability - Fundamentals of the Physical Environment - page 463

Structureless, single grain Clay domain

Individual grains; characteristic of
loose sand.

Quartz grain

Structureless, massive

No structure visible; clods poorly
developed. Organic matter

Crumb

More or less rounded, fine (<2mm)
aggregates.

Blocky

Figure 19.2 'Bridges' provided by clay and organic matter in
binding quartz particles.
Source: After Emerson (1959)

More or less equi-dimensional peds,
either angular or sub-angular in outline.

Prismatic

Vertically elongated peds with planar
faces. illustrates how colloids of clay and humus provide various

types of 'bridges' or linkages between the coarser mineral
particles. This is the Emerson model of structure forma-
tion and illustrates that attraction between particles
depends upon electrostatic forces on the surfaces of
colloids. The role of organic matter lies in providing a
strong and stable structure, because humic colloids are
hydrophobic , i.e. water-repellent. This gives stable struc-
tural units, more likely to survive disruptive forces of
wetting and of raindrop impact. The grade of structure
is its stability, an important property in studies of soil
erosion; usually three grades of structure are recognized
- weak, moderate and strong.
The kinds or shapes of aggregates are summarized in
Figure 19.3 . Structureless single grain consists of loose,
individual particles as found in raw sands. Structureless
massive consists of a large mass of compacted soil with no
recognizable aggregates. It may be found in wet and raw
clays. Crumb is characteristic of soils with mull humus and
a very active soil faunal population, especially earth-
worms. Blocky structure , either angular or sub-angular, is
common in many arable topsoils and subsoils in medium-
textured soils. Prismatic structure is characteristic of
subsoils in clay soils affected by shrinking and swelling.
Columnar structure shows some slaking (dispersion) and
is found in subsoils of clay solonetz soils with large sodium
contents (see p. 451). Both prismatic and columnar
structures are very hard and dense on drying, making
them difficult to cultivate with farming implements,
impervious to water infiltration, and resistant to root
penetration. Platy structures occur in a variety of soils.
They are characteristic of compacted clay soils, and also
of silts. In the latter case drying and crusting after early

Columnar

As above, but with rounded tops due
to slaking.

Platy

Horizontally aligned plates.

Figure 19.3 Shapes of common soil structures.

spring rain gives platy aggregates which can cause serious
problems for the emergence of crop seedlings ( Plate 19.1 ).

Porosity and density

The total volume of pore space in soil is its porosity or air
capacity . It is calculated from the bulk density , the weight
of soil per unit volume. Assuming that the mineral density
of soil particles is 2·65 g cm -3 , then:

BD BD

—— 100 = —— 100

PD 2.65

where BD = bulk density and PD = particle density. Bulk
density values range between about 0·8 g cm -3 and 2·4 g
cm -3 , equivalent to a range of soil porosity from 70 per
cent to 10 per cent respectively.