GCTV Ep#13 Phosphorus Uptake
Narrator: Jane Drinkwater Fertilisers are one of the single largest
variable cropping costs in the southern grain-growing region.
So when growers and their advisers were not seeing the improved crop yields they expected
their concern resulted in some action.
As part of GRDCÕs More Profit from Crop Nutrition initiative research into plant available phosphorus
was undertaken.
Roger Armstrong: Grains Innovation Park, DEPI Victoria
We would often see a vegetative growth response to applied phosphorus fertiliser early in
the growing season but as we came to grain harvest there was no actual grain yield response
to that fertiliser and therefore potentially the growers had lost their investment in terms
of fertiliser.
Narrator: Of the total pool of phosphorus in soil only
a small percentage is present in plant available form.
Most of it is tied up in slow release mineral or organic compounds.
So fertilisers are applied to boost levels of plant available P for the developing young
crops.
Jane Drinkwater Not all phosphorus fertiliser is taken up
by the young plants.
As their roots move deeper the unused P gets left behind either left in the top soil where
a lack of soil moisture limits the ability of the maturing cropÕs roots to access it,
or fixed meaning it is chemically unavailable.
Narrator: Soil testing is the standard mechanism used
to determine phosphorus levels and in the southern region particularly the Wimmera and
Mallee, the Colwell - P test has been widely used.
So here at DEPIÕs Horsham research facility a traceable radioactive isotope of phosphorus
P33, was watered into potted plants to quantify what the soil test was detecting.
Roger Armstrong: One of the hypotheses we had initially was
that crops were accessing sources of phosphorus in soil, potentially mineral maybe organic
phosphorus pools that the soil test was not detecting but the crop could access itself
and therefore meant the crop was not responding to Phosphorus fertiliser applications.
Narrator: A phosphorus soil test developed by the University
of Adelaide, the DGT - P test, was also used with apparent success.
Jane Drinkwater Recent research undertaken using P isotopes
indicates that DGT better reflect the P pool in soil accessed by wheat plants than other
commonly used soil tests such as Colwell.
This research may explain why field data collected in south eastern Australia by Sean Mason and
farmer groups over the past few years suggests that the DGT test better predicts P fertiliser
response than Colwell, especially on highly alkaline calcareous soils.
Narrator: Whether the same conclusion can be made for
non-alkaline soils is open for debate.
Another contributor to the phosphorus pool comes from what is on top of the soil and
research by a GRDC funded PhD student provided a significant insight.
Roger Armstrong: Sarah Noack, University of Adelaide, she focussed
on looking at the availability of phosphorus in crop residues and what impact that had
on the phosphorus cycle.
What Sarah found was there is a small but significant amount of phosphorus remaining
in plant residues at grain maturity but what SarahÕs work found using NMR was the popular
conception was most of that phosphorus was organic.
She actually demonstrated that it was inorganic in nature.
So that phosphorus was easily leached out if you had a reasonable sized twenty mil rainfall
event after maturity and that inorganic phosphorus would drop to the ground and be potentially
available to plants or microbial uptake.
Narrator: Field trials in South Australia and Victoria
followed the glasshouse work and focussed on the impact environmental factors, especially
rainfall and soil water, had on phosphorus availability to crops.
Jane Drinkwater What was found was that crops only source
five to twenty percent of their total phosphorus uptake from fertilisers.
The remainder comes from background soil available phosphorus.
So the P - fertiliser effectively stimulates the uptake of background soil - P rather than
meeting the cropÕs phosphorus requirements itself.
Narrator: Those field results were groundbreaking as
they lead to a better understanding of the impact environmental factors have on a cropÕs
response to phosphorus.
Roger Armstrong: And the really important thing that came out
of that was nitrogen nutrition was essential to determining your likelihood of a response
to phosphorus.
So it didnÕt necessarily reverse the other way but if you wanted to maximise grain yield
response to applied phosphorus you had to get your nitrogen nutrition right.
Narrator: As southern region growers prepare for the
new season they should consider what this research has shown and test for soil phosphorus.
If the results of a specific test are in doubt compare it with a different test.
Because knowing what is in your soil can potentially save you spending more on phosphorus fertiliser
than necessary or indicate when investing in additional P could result in improved yields.
Ends
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