Chasing sandy soil profitability without chasing risk

Reducing phosphorus and redistributing nitrogen are providing profitable returns without increasing risk for Mallee growers

Group of four

Trials at the Mallee Sustainable Farming site at Karoonda, South Australia, are assessing the most profitable fertiliser strategies for the common soil zones in the Mallee. (From left) Gemma Walker, Mallee Sustainable Farming, Michael Moodie, Mallee Sustainable Farming, Dr Rick Llewellyn, CSIRO and Dr Therese McBeath, CSIRO.

PHOTO: Emma Leonard

Mallee growers generally have large cropping programs that run across varying soil types due to the dune/swale formations that dominate this area. Many have already taken a spatial approach to the allocation of fertiliser, particularly phosphorus.

Now, research supported by the GRDC and run by CSIRO with Mallee Sustainable Farming (MSF) indicates that the timing and distribution of nitrogen are crucial if profitability in these soil types is to be increased without increased risk.

Dr Rick Llewellyn from CSIRO works with a team of soil scientists and research agronomists, including Dr Therese McBeath, Dr Sean Mason, Dr Ben Jones, Michael Moodie and Bill Davoren, that runs a series of fertiliser and rotation trials at the MSF trial site at Karoonda, South Australia.

There are two trials at Karoonda evaluating the effects of varying nutrient input levels. The Cereal Strategies trial has 150-metre trial strips running along the swale to dune, and a trial looking at combinations of different nitrogen and phosphorus rates is located on each of the key swale, mid-slope and dune soil types on 20m plots. This design aims to help demonstrate the most profitable agronomic and fertiliser strategies for each soil zone. Yield and soil samples are taken at intervals in the extra-long plots and are complemented with continuous assessments of biomass using crop sensors.

Dr Llewellyn says: “Traditionally local growers have applied about 50 kilograms per hectare of diammonium phosphate (DAP) at seeding when sowing a cereal crop, which supplies 10kg/ha of phosphorus and 9kg/ha of nitrogen.

“Over the seasons since 2010 our trials suggest that this is too much phosphorus on most soil types and not enough nitrogen on sandy soil types.”

The fertiliser trial consists of five rates of nitrogen applied as urea at seeding (0, 10, 20, 40 and 80kg N/ha) and four rates of phosphorus applied as triple superphosphate (0, 5, 10 and 20kg P/ha) on each of the three soil types (dune – deep sand, mid-slope sand over clay loam and swale/flat – clay loam over clay). The trials have had the same treatments applied to a given plot for three seasons (2010–12).

Phosphorus

Nil to low levels of phosphorus on the heavy swale have shown no yield penalty in the first three years of both trials, with variable responses on sandy soil types.

Dr McBeath explains: “A response to fertiliser phosphorus has been difficult to predict but we have found some responses on the sandier soils at the Karoonda site.

“On the heavier swale, where soil tests are showing a plentiful supply of phosphorus, a zero rate of phosphorus is probably the most cost-effective. However, monitoring these soil-phosphorus levels is the only way to ensure this approach is sustainable.”

Nitrogen

A similar trend has emerged in relation to nitrogen, with a zero rate of nitrogen being the optimal rate on the swales where soil nitrogen has accumulated due to low rates of removal in the lower-rainfall seasons of the Millennium Drought.

The district practice fertiliser rates have over-supplied nitrogen in the swale but under-supplied nitrogen to the dune and mid-slope areas that have lower inherent fertility and higher nitrogen demand.

“In our Cereal Strategies trial, applying 50kg of DAP and 67kg of urea at seeding increased the productivity of these areas by at least 750kg/ha compared with district practice of 50kg/ha of DAP with no urea,” Dr Llewellyn says.

In 2011, even greater benefits were achieved when the cereal crop followed volunteer medic pasture. Again, this illustrated the value of additional nitrogen.

The treatments also compared upfront versus top-dressed nitrogen. In these low-rainfall environments, in high-nitrogen-demand sandy soil types, it was found that delaying nitrogen to growth stage 30 (GS30) produced a considerably reduced benefit compared with a similar amount of nitrogen applied upfront across three seasons.

“Basically, we think that in these low-rainfall regions where the crops have been underfed, upfront fertiliser ensures nitrogen is available when required by the crop for early growth and after winter rain events on these typically infertile sandy soils,” Dr Llewellyn says.

“Trying to achieve the perfect timing for top-dressing across large cropping programs, when rainfall is unpredictable and crop growth varies across the landscape, is very hard.”

Redistributing nitrogen inputs from the swales to the dune and mid-slope region so that each area receives fertiliser inputs more closely matched to production requirements is proving to be a method of increasing profitability without all of the added risk that comes with blanket increases in nitrogen application in these low-rainfall dune/swale areas.

More information:

Dr Rick Llewellyn, CSIRO Ecosystem Sciences,
08 8303 8502,
rick.llewellyn@csiro.au

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