Agriculture Reference
In-Depth Information
gregate stability is favored by substantial inputs of organic matter, biological ac-
tivity, and minimal soil disturbance.
Tests of Aggregate Stability
The stability of aggregates is tested by their reaction to immersion in water. A
long-established test measures
water stable aggregates
(Klute 1986). This method
involves sieving, under water, an air-dry sample of soil on a vertical array of sieves
of different mesh size. After a standard time, the mass of aggregates remaining on
individual sieves is measured and expressed as a percentage of the total.
A useful field test of aggregate stability involves placing small air-dry aggre-
gates (0.2-0.5 g) in water of very low salt concentration (e.g., rainwater). If the
aggregate quickly collapses into subunits, which may be smaller aggregates or par-
ticles, it is said to
slake
. This indicates that the forces within an aggregate are not
strong enough to withstand both the pressure of air entrapped as the aggregate
wets up quickly and the pressure generated by clay swelling. The subunits pro-
duced by slaking may also be unstable if the swelling pressure in clay domains is
strong enough to force individual clay particles apart, a process of
deflocculation
(or dispersion). An interpretation of soil structural stability based on the slaking
and dispersive behavior of aggregates is given in box 3.2.
Flocculation, deflocculation, and swelling are clay-surface phenomena that are
discussed further in chapter 4. However, deflocculation has several undesirable ef-
fects on soil structure: deflocculated clay blocks infiltration and drainage pores
(chapter 7), translocated clay forms a dense clay B horizon, as in duplex soil pro-
files, and a hard surface crust forms when the deflocculated soil dries, impeding
infiltration and seedling emergence.
3.2.3
Soil Pans
Translocation of colloidal inorganic and organic material from the A horizon into
the subsoil is a major process in the creation of soil structure. In the changed phys-
ical and chemical environment of the subsoil, salts and oxides precipitate, clay par-
ticles and organic colloids flocculate, and deposits form on aggregate surfaces. Fe
and Mn compounds that are reduced and dissolved under anaerobic conditions
can be subsequently oxidized and deposited as oxidic coatings. These range in
color from black MnO
2
minerals through to blue, blue-grey, yellow, and red Fe
2
O
3
minerals, depending on the degree of oxidation and hydration of Fe. In dry en-
vironments, coatings of gypsum and calcite are found. Any of these coatings may
build up to such an extent that bridges form between soil particles and the small-
est aggregates, and a cemented layer develops in the soil. Such a layer or horizon
is called a
pan
, which is described by its thickness (if
10 mm it is called “thin”)
and its main chemical constituent.
Iron pans commonly form at the top of the B horizon in duplex soils that
are seasonally waterlogged. Over time and possibly as a result of climate change,
these pans become dehydrated and fragment into a layer of rusty ironstone gravel,
as shown in figure 3.5. Horizons that are cemented by silica deposits are called
duripans
: if massive, they may form a silcrete layer several meters thick, as found
deep in the profiles of highly weathered soils in the southwest of Western Aus-
3.2.4
