Chemistry Reference
In-Depth Information
of soil''.
44
Soil structure includes the complex interactions between the many
minerals, organic substances, organisms, gases and water in soils at a range of
organisational levels, ranging from the interactions between clay particles and
domains (typically 10
27
m in diameter) to large-scale cracks and field-scale
hydrological processes (often .100 m in size). Within this physical framework,
multiple chemical and biological processes affecting soil fertility occur and
roots function to support the growth of the whole plant.
Soil structure is created by a range of biological, hydrological and mechanical
processes, often intimately associated with the growth and activity of root
systems. Structure is formed over time by a combination of dispersion and
aggregation of soil particles, driven by cycles of wetting and drying, additions of
organic compounds, faunal burrowing, root penetration and the bonding of soil
mineral surfaces. These processes are accentuated at the root-soil interface,
resulting in a more distinct and physically stable structure than in the bulk soil.
Roots mechanically deform soils as they pass through them and the flora, fauna
and exudates associated with them alter bond energies between particles, water
surface tension and soil physical behaviour.
45
Structure formation is evident
from the sheath of soil (sometimes referred to as a ''rhizosheath'') that generally
adheres to excavated roots and has been shown in many studies to be more
structurally stable than bulk soil.
46,47
Root hairs also play an important role in
bonding soil to root surfaces, which increases contact and hence the potential
uptake of water and nutrients.
48,49
Secondary metabolites from soil microbes are
also a major driver of structure formation close to roots, with arbuscular
mycorrhizal fungi also playing a role.
50,51
Alternate cycles of water extraction by roots and wetting by rain induce
shrinking and swelling in many soils that can cause them to crack and slake.
52
Such cracks form large pores (macropores) in soil that act as pathways for
rapid transmission of gases and water between upper and lower soil layers; in
this way, water and dissolved solutes can be moved rapidly downwards to
groundwater, by-passing the bulk of the soil. However, plant roots can also
exploit large pores as means of by-passing obstructions, such as compacted soil
layers induced by inappropriate cultivation (e.g. plough pans), thereby gaining
access to nutrients and water in subsoils.
53
The ability of roots to deform soil during elongation influences the
mechanical resistance to root penetration and, ultimately, the size of the root
system and the performance of crops and pastures. Several studies have
measured the distribution of soil porosity or particles after a root has
penetrated a soil core and showed changes in bulk density at up to 4-5 mm
away from the root surface.
54
For maize roots growing in sand, particle image
velocimetry has shown density increases of up to 30% adjacent to growing root
tips, with an approximately exponential variation in particle displacement as a
function of distance from the root surface.
55
Local variation in sand density
was associated with root cap frictional properties, so that roots with a root cap
shedding mucilage and border cells deformed the soil radially, with density
increases generally confined to the flanks of the root, whereas roots from a
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