Download Optimal soil structure for plant growth

F6 1. How do we maintain and improve soil fertility?
Optimal soil structure for plant growth:
field evaluations and management
guidelines for improved soil quality
A spade method to assess soil structure is described. This can help identify optimal structure for
plant growth. Soil loosening techniques for improving sub-optimal structure are described.
Dr Bruce C. Ball
SAC Crop and Soil Systems Research Group
Edinburgh
United Kingdom
[email protected]
The preservation of good soil
structure is essential to maintain
root penetration, uptake of nutrients by roots, soil water storage
and flow and soil gas exchange.
A quick and simple field spade
method of Visual Soil Structure
Quality Assessment (VSSQA)
is presented which requires little equipment or expertise to
use. The structural quality can
be used to determine any need
to change crop or soil management. Soil structural variability
can be visually related to crop
variability and can be assessed
as the standard deviation of
measurements of VSSA.
The VSSA method is easily
learned. It involves digging out a
block of soil with a spade, breaking it up, examining the soil and
scoring the structure by comparison with photographs in a visual key, available from the author. Table 1 shows photographs
of typical soils after break-up in
each of the five score categories
of structural quality (Sq). The
breaking-up of the soils produces fragments called aggregates which are not necessarily
a representation of aggregates
present in the field. Each score
category also has a distinguishing feature (Table 1). For example, the best quality score (Sq1)
has fine aggregates whereas the
worst score (Sq5) usually contains soil of grey-blue colour
typical of anaerobism. The Sq
score is also confirmed by consideration of the ease of block
extraction, shape and size of aggregates, porosity and distribution of roots. The test takes 5-10
minutes per location and enough
replicates are obtained for statistical comparison of datasets. See
Ball et al. (2007) for further details.
The method has been tested
to confirm reproducibility and to
assess sensitivity to differences
in soil management (e.g. due to
compaction, tillage, residue incorporation). For example, the
Table 1. The five score categories used in field evaluation of soil structure.
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1. How do we maintain and improve soil fertility? F6
Table 2. Draft guidelines for improving quality in soil where the structure is damaged (Sq4 and 5), adapted from Spoor (2006).
test relates well to differences in
vane shear strength and cone resistance.
The test is a descriptor of
soil structure. Attempts to relate
Sq scores to conditions for crop
growth and to produce indicator and management thresholds
need the opinions of experts.
Discussion with colleagues indicates that the optimum soil
structure for cereal root growth
would be ~15 cm of Sq 1 overlying a slightly more compact
zone (Sq 2-3) to provide support and prevent compaction of
the layer below. Sq1 is often stabilised under grass by the grass
roots. In the absence of roots,
a layer of soil of coarser structure at the surface or crop residues may protect the soil from
slumping or erosion. However,
fine aggregates in the seedbed
are needed for good soil-seed
contact for moisture uptake, particularly for grass. Our data indicate that soil strength should
not restrict growth at scores between Sq 1 and 3. Scores ≤ 3 are
considered acceptable.
After scoring, any areas and/or layers with consistently low scores (Sq4 or 5) are
identified. These areas may require improvement by tillage,
drainage or different cropping.
Such degraded soil is shown as
pans, clods, smeared surfaces
and smelly layers. In temperate
countries it is usually associated
with compaction damage. Restoration of continuous macroporosity is then a priority. Some
guidelines for improvement are
given in Table 2. Tillage to restore soil structure depends
mainly on tensile soil failure
and should only be done when
the soil is dry enough to permit
such failure.
The roots of some crops, particularly those with tap roots, can
penetrate pans and restore structure. This generally takes some
time as the roots have to decay
before the macropores can function. Roots can also stabilise the
surface soil structure.
The VSSA test is also useful
in guiding the need for subsoiling and for the suitability of the
soil for minimum tillage. The
test is also useful for guiding the
depth and location of any further soil measurements required
to diagnose soil problems. One
such integrative structural test is
the Least Limiting Water Range,
which integrates soil aeration,
field capacity, soil mechanical
resistance and wilting point into
one variable. Other simple tests
such as macroporosity and dry
bulk density may also be useful.
References
Ball BC, Batey T & Munkholm
L. 2007. Field assessment of
soil structural quality – a development of the Peerlkamp
test. Soil Use and Management 23: 329-337.
Spoor G. 2006. Alleviation of
compaction: requirements,
equipment and techniques.
Soil Use and Management
22: 113-122.
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