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Beyond the Bale : June 2014
ON-FARM 41 June 2014 BEYOND THE BALE Interim results from a three year project funded by AWI and Meat & Livestock Australia show that when phosphorus (P) fertiliser is applied to clover pasture, more of the fertiliser is taken up by the plants in the year of application than was previously thought. University of Adelaide research fellow Dr Tim McLaren, who is part of the research team including CSIRO and the University of New England, presented the results from the first year of the project to the 'Pathways to Productivity' forum at Bordertown, South Australia in April. "It has long been believed the efficiency of P fertilisers in pasture systems is quite low -- about 10--20 per cent fertiliser use efficiency," Dr McLaren said. "So if you're a producer applying 100kg of single superphosphate per hectare and only 10 per cent is being used by the pasture in that year, that's a lot of money in the soil rather than in the pasture." However the results from a short-term field trial show that recovery of fertiliser P by clover pastures was relatively high at the two field sites used for this study: 40 per cent at Naracoorte (SA) and 34 per cent at Ginninderra (ACT). The importance of fertiliser P to clover growth was higher at low soil P fertility. "The results indicate that fertiliser P can be utilised by pastures and does not rapidly become unavailable upon application. However, despite higher than expected FAST FACTS l Preliminary results from an AWI and MLA funded project indicate that fertiliser P does not become immediately unavailable to plants when added to pasture soils as previously thought. l 34--40 per cent of fertiliser P was recovered by clover pastures at two sites in the ACT and South Australia. l Producers should still ensure they do soil testing to determine their P levels before applying fertiliser. uptake of P fertilizer, the soil was still the largest source of P for clover growth, so soil testing is important," Dr McLaren said. The majority of fertiliser P in the soil was found in the surface 0--4 cm layer and existed as inorganic P. A considerable proportion (about 20 per cent) of the applied fertiliser P in the soil fraction remained in a plant available form at the end of the growing season. The short-term field trial involved adding single superphosphate to the soil surface in winter, then taking multiple cuts throughout the growing season to simulate grazing. Researchers measured how much of the P fertiliser was directly taken up by the pasture. Remnants of the fertiliser were recovered and soil samples taken to see where the remainder of the fertiliser P had gone. An analysis of soils collected from a long-term field trial was also undertaken at Ginninderra, ACT. Results showed that the vast majority of fertiliser P was recovered in the 0--20 cm layer. Both inorganic and organic P increased with the addition of fertiliser P. Adding fertiliser P above optimum levels for pasture growth only further increased soil inorganic P, particularly the slowly plant- available P pools. Humus P was the main organic P form that accumulated. Essentially, inorganic P is associated Research into phosphor us uptake A site involved with the AWI and MLA funded project that is helping determine the efficiency of phosphorus fertilisers. with metals (i.e. Al, Fe, Ca, etc.), and organic P is P associated with carbon. "During a growing season most of the fertiliser P remains as inorganic P, but much is converted to organic P over the long- term. This is likely to be associated with soil organic matter, which has implications for other soil parameters such as soil structure." The short- and long-term results suggest that a significant proportion of the fertiliser P used for pasture growth is actually recycled and returned to the soil surface. Dr McLaren said that further research will investigate what effect fertiliser timing (autumn and spring), placement (surface and deep), and the role of soil P status has on phosphorus use efficiency. "We will go to more field sites to look at P in the whole system, including fertiliser P in soil fractions 0--4 cm and 4--8 cm, P in the fertiliser granule, P in the pasture cuts, and P in roots. We will also investigate the availability of soil P forms to pastures when P fertilisation stops." But the message for producers currently remains the same: ensure they do soil testing to determine their P levels before applying fertiliser and set their stocking rate for optimal pasture utilisation. "There doesn't appear to be much benefit adding fertiliser P at rates well in excess needed for optimum pasture growth," Dr McLaren added.