Agriculture Reference
In-Depth Information
on the fine earth fraction (
2-mm sieve fraction).
Gravel
(2-600 mm) and
boul-
ders
(
600 mm) are larger than the fine earth.
■
The proportion of sand, silt, and clay-size particles in the fine earth determines
the
soil's texture
. Texture is an important property that affects the soil's response
to water (soil stickiness, consistence, and permeability), temperature, and cultiva-
tion (soil tilth and compaction). Texture can be assessed in the field.
■
The clay-size fraction is divided into the
crystalline clay minerals
(layer-lattice min-
erals or phyllosilicates) and the
accessory minerals
, comprising various Fe and Al
oxides (sesquioxides), Mn and Ti oxides, SiO
2
, and calcite. The basic crystal struc-
ture of the phyllosilicates consists of sheets of Si in tetrahedral coordination with
O [SiO
2
]
n
and sheets of Al in octahedral coordination with OH [Al
2
(OH)
6
]
n
.
Crystal layers formed by the sharing of O atoms between contiguous silica and
alumina sheets give rise to different minerals, depending on the Si:Al mole ratios:
1 (
imogolite
and the
allophanes
), 1:1 (
kaolinites
), and 2:1 (
illites
,
hydrous micas
,
vermiculites
, and
smectites
). Isomorphous substitution (Al
3
Si
4
and Fe
2
,
Mg
2
Al
3
) in the lattice results in an overall net permanent negative charge.
■
The edge faces of the clay crystals, especially kaolinite, and the surfaces of Fe and
Al oxides (e.g.,
goethite
and
gibbsite
) bear variable charges depending on the asso-
ciation or dissociation of H
ions at exposed O and OH groups. The surfaces are
positively charged at low pH and negatively charged at high pH. These charges
are therefore called pH-dependent.
■
The cation and anion charges adsorbed (mols of charge per unit mass) measure a
mineral particle's cation exchange capacity,
CEC
, and anion exchange capacity,
AEC
, respectively.
CEC
ranges from 3 to 150 cmols charge (
) per kg for clay
minerals, and
AEC
is 30-50 cmols charge (
) per kg for sesquioxides. Imogolite
and allophane have roughly comparable
CEC
and
AEC
values at pH 6-7.
■
The smaller a particle's size, the larger is its specific surface area. Depending on
the mineralogy of the clay, the specific surface area ranges from 5 to 1000 m
2
/g.
Clay particles (
2
m) are much more effective than silt (2-50
m) and sand
m) in adsorbing water, ions, and solute molecules. In expanding-
lattice clays, the total surface area is the sum of the internal and external areas;
this can be as high as 750 m
2
/g for a fully dispersed Na-montmorillonite (a smec-
tite clay).
■
Inputs of C as plant litter, dead roots, animal remains, and excreta are substrate
for a heterogeneous population of soil organisms. As the organic residues decom-
pose, releasing a proportion of the C as CO
2
to the atmosphere, soil organic mat-
ter (
SOM
) accumulates in a humified state. Soil humus consists of resistant plant
residues (lignins), as well as complex new organic compounds synthesized by soil
microorganisms.
■
The living organisms
(biomass
) consist of the
reducers
, invertebrates such as mites,
springtails, insects, termites, earthworms, nematodes, and molluscs, and the
de-
composers
, microorganisms (bacteria, actinomycetes, fungi, and algae) and micro-
fauna (protozoa). The most important reducers are
earthworms
in temperate soils,
especially under grassland, and
termites
in many soils of the tropics and subtrop-
ics. These organisms of diverse size and function comprise an interdependent “food
web” in the soil.
■
Annual rates of litter fall are 0.9-1.2 t C/ha in vineyards of low planting density,
but up to 4 t C/ha in temperate forests. Dead roots and carbon substrates de-
posited in the rhizosphere also contribute to biomass substrate. Earthworm bio-
(50-2000
