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Beyond the Bale : June 2013
40 40 40 on-farm Phosphorus (P) is an expensive and limiting factor on improved pasture production, or is it? New research funded by AWI and the Victorian Department of Environment and Primary Industries (DEPI) is exploring how to maximize pasture production with low or even no P inputs. The research has stemmed from woolgrowers raising in recent times the issue of ever-increasing fertiliser costs. Woolgrowers’ profitability may well improve if they don’t need to invest so heavily in phosphorus-based fertilisers and instead found grass and legume species that better use stored or “locked” P in the soil. Better management and efficiency of P would change the way pasture production is addressed in many areas. Early work is proving promising with novel species such as plantain (see image above) maintaining surprisingly high growth rates in soils where no P has ever been applied. However, full results won’t be complete until the results are proven to be both significant and repeatable. Researcher Dr Malcolm McCaskill said the current measurement of P in soil such as Olsen and Colwell do not accurately reflect the amount of ‘stored or sparingly soluble’ P in the soil, which is ‘available’ to some species not to others. This knowledge provides the opportunity to tailor P management strategies accordingly so that fertiliser rates can be decreased significantly without negatively impacting production. The Long Term Phosphate Experiment Grow ing wool w ith less Phosphor us at the DEPI site at Hamilton is proving invaluable. “Soil from the site is representative of a range of fertility levels, and has been used for several pot and soil core experiments,” Dr McCaskill said. “We are currently planting out a range of both naturalised and improved species into the small plots on the site to determine their individual P requirements.” By using radioactive P isotope in a pot experiment, research collaborators Dr Roger Armstrong and Katherine Dunsford at DEPI Horsham were able to directly measure how much of this ‘stored’ soil P was used by a wide range of pasture species. “Under a previous GRDC-funded project, we found that canola was able to access forms of this soil P that were unavailable to wheat; the project also found that new soil tests such as DGT could more accurately account for these forms of soil P compared to the traditional Olsen and Colwell tests,” Dr McCaskill said. According to Dr McCaskill, we know that perennial ryegrass species will only persist above Olsen P levels of 8 mg/kg (Colwell P 19 mg/kg) but other species are able to survive quite happily below this. “While arable country may be best served with fertiliser applications and more productive, improved species, the less accessible country and less fertile areas could be better served with certain naturalised species and legumes tolerant of lower P levels to keep the levels of nitrogen up. These more marginal areas are the focus of this project. “Plants such as redgrass (Bothriochloa macra) persist in areas of lower fertility for a reason and the limit to their production can often be nitrogen, so finding legumes that persist at lower P levels should help increase the productivity of these areas. “In a previous AWI-funded project we found some volunteer legumes that persisted at low P rates. Their growth rates were low but they would be important in fixing nitrogen for other plants within the pasture. One of these was suckling clover (Trifolium dubium), which we are testing in this new project. “On the Long Term Phosphate Experiment we found the economic optimum P application rate for a perennial ryegrass-subterranean clover pasture was about 18 kg/ha of P. This rate is at least four times more than is needed to replace P taken off in product, and the remainder accumulates in the topsoil. The new study is exploring ways of reducing the amount of P required to maintain our pastures.” Maximising pasture production under low P inputs is a project due to be completed in 2015. June 2013 BeYoNd the BaLe Dr malcolm mccaskill of Victorian Department of Primary Industry at Hamilton demonstrates the surprisingly high growth by plantain in soils with no previous fertiliser (left), relative to the medium and high (right) fertility levels. olsen P levels are 5, 8 and 39 mg P/kg, and colwell P levels 8, 19 and 113 mg P/kg. fast facts l New research explores how different species respond and utilise Phosphorus (P). l P isotopes can track which species use P that is not accessible to other plants. l Novel and native species may be able to utilise stored or “locked” P.