Break crops Cropping |
Photo: Brad Collis
Economics: By changing soil chemistry different crop
rotations can influence the rate at which phosphorus
becomes available to subsequent crops.
Trials to reveal soil
phosphorus cycle
Break crops may be able to
make better use of phosphorus,
says Dr Ann McNeill, who is
leading research to investigate
the potential phosphorus
benefits of different rotations
At a glance...
▸▸By changing soil chemistry different
plants change the rate at which
phosphorus reserves are mobilised
▸▸ New research will identify how
much phosphorus is held in plant
roots and how rapidly it is released
▸▸ It will also evaluate the
potential short and long term
access to phosphorus for cereal
crops in different rotations
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A
better understanding of the
phosphorus cycling in soils is
the aim of a new research project
we are undertaking at the University of
Adelaide to help grain growers make the
most effective use of their fertiliser inputs
and select crop rotations.
Phosphorus is typically applied at
seeding to ensure adequate crop
production. But of the phosphorus
fertiliser applied, less than 30 per cent is
taken up by the current crop.
In economic terms, this is equivalent
to Australian grain growers each year
burying $5-$10 billion in their paddocks.
Over time this phosphorus will be
recycled into a plant-available form, but
unlike the more familiar nitrogen cycle,
the rate at which phosphorus is recycled
from the ‘soil bank’ depends on soil
chemical characteristics.
Plants exert influence on soil chemistry
in the zone around roots known as the
rhizosphere.
Unlocking the secret
Some plant roots, especially certain grain
legumes, excrete acids which solubilise
some of the phosphorus bank.
It is not known if this has long-term
benefits in the rotation by moving
phosphorus into pools which can be
accessed by following cereal crops.
Phosphorus is held in different pools
within the soil. These include the
microbial, plant-available (inorganic) and
labile organic pools.
Much of the organic and inorganic
phosphorus in the soil is released from
decomposing roots.
A study of how much phosphorus
exists in roots and how rapidly this is
released is an important part of the
research the University of Adelaide is
conducting with support from the Grains
Research and Development Corporation.
Crops have different root
growth and differ in their
rooting depth, so the
location and cycling of
phosphorus can vary in the
short and long term
Previous research has shown some
Australian soils have the potential to
supply the phosphorus requirement of a
two-tonne-per-hectare cereal crop from
their organic matter.
No. 241 February 2012 Farming Ahead
47
| Cropping Break crops
Plants provide phosphorous
Other work has identified decomposing
canola and legume stubble can provide
substantial amounts of phosphorus.
Building on these findings, this project
aims to assess the full impact of rotation
crops on phosphorus availability.
Investigations will focus
on the direct effect of
selected break crop or
pasture phases on the
cycling of phosphorus
Microbial influences and biochemical
cycling will also be assessed in the
rhizosphere under field conditions.
After consultation with stakeholders, a
range of short-term crop-sequencing
trials has been identified.
These are located across southern and
eastern Australia within a number of
agri-ecological zones.
The sites encompass a variety of
commonly-grown legume and pasture
crops including rye, barley, lupins, canola,
peas, vetch, chickpeas and beans, as well
as soils which differ in phosphorus
fertility and sorption properties.
Measuring Phosphorus
Measurements of phosphorus pools will
be made in the break-crop phase and
related to phosphorus pools in soil and
uptake in the subsequent cereal phase.
Plant-available phosphorus will be
measured using the DGT soil Phosphorus
test developed by researchers at the
University of Adelaide.
The sites and collaborating research
and grower groups are:
• Karoonda, South Australia — CSIRO,
Mallee Sustainable Farming
• Hopetoun, Victoria — BCG (formerly
the Birchip Cropping Group)
• Naracoorte, SA — the SA Research
and Development Institute, MacKillop
Farm Management Cropping Group
• Junee, NSW — CSIRO, FarmLink
• Longerenong, Victoria — Sustainable
Cropping Rotations in Mediterranean
Environments (SCRIME), Victorian
Department of Primary Industries
The fate of released phosphorus will
be evaluated in terms of its availability to
the subsequent wheat crop.
The benefit from microbial breakdown
and cycling of nutrients from below-ground
residues will be assessed as a potential
avenue for providing more controlled and
targeted release of phosphorus.
By looking at phosphorus release in
both time and space, better information
will be gathered on synchronising release
with plant demand and reducing the
impact of phosphorus fixation.
A common set of long-term trial sites
across the cropping regions of Australia
will also be selected.
At these sites the aim is to provide an
integrated assessment of the effects of
agronomic strategies on harnessing the
biological potential of agricultural soils. GRDC RESEARCH
GRDC Research Code
UA00119
Dr Ann McNeil
University of Adelaide
08 8303 8108
[email protected]
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Farming Ahead February 2012 No. 241
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