Chemistry Reference
In-Depth Information
considerable untapped potential in the quest to increase crop yields. Seedbed
slumping not only decreases the storage of water and transport of gases that
are essential for crop growth, it also increases the strength of soil upon drying.
9
The impedance to root growth caused by soil strength increases from drying
has a major impact on yield.
71
d
n
1
r
2
n
g
|
8
Soil compaction exacerbates the problem
further.
3 Measuring Soil Physical Constraints
Crop productivity is limited by the following soil physical constraints:
N
Hypoxia - depletion of oxygen at the root-soil interface. The oxygen
concentration or redox potential of soil can be measured directly to assess
hypoxia. Often a value of 10% air-filled porosity is assumed to be the cut-off
for severe hypoxia to occur.
72
This is assessed from the water content and
porosity of soil.
N
Water potential - capillary stresses of pore water that create suction in soil.
As suction increases, plants have to overcome a greater stress to extract
water from soil. A water potential of 21500 kPa defines the permanent
wilting point of soils, where crops can no longer access water, leading to
wilting and death.
1
Water potential is measured with tensiometers in the
field or from water-release characteristics obtained using pressure-plate
apparatus in the laboratory.
N
Mechanical impedance - roots push their way through the soil matrix to
increase the volume of soil they access and hence the capture of water and
nutrients. Increased soil suctions (drying), compaction and seedbed
slumping can increase the mechanical impedance of soil. At a threshold
value of 2 MPa, root elongation is impeded severely.
71
Mechanical
impedance is measured with a penetrometer, either in the field or the
laboratory under more controlled conditions.
N
Macropores - these larger pores in soil provide rapid transmission pathways
for gases and water, in addition to continuous void space for root growth.
73
They are formed either by the cracking and aggregation of soil through
weathering or by the action of soil biology, such as earthworms or plant
roots.
33,72
Macropores are measured either from water-release character-
istics (macropores drain at small suctions) or through visual methods such
as thin sections or X-ray computer tomography.
1
Although cut-off values for root growth and functioning have been provided
above, the values are not fixed and the different physical constraints are
interdependent.
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This is described in greater detail below.
Internationally there has been a recent surge in the development of soil-
quality indicators to measure physical constraints to crop productivity. Many
are based on the distribution of different pore classes in soil, as these provide
indicators of oxygen exchange and water transport to prevent hypoxia, water
storage
to
resist
drought,
and
the
presence
of
macropores
to
provide
