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What Happens to Phosphorous When You Spread it On Pasture?

01 April 2014

New research is shedding light on how clovers take up applied phosphorus (P) in pastures, write consultants at Meat and Livestock Australia.

University of Adelaide research fellow Dr Tim McLaren is part of a team investigating the fate of P fertiliser in pasture systems in the research project being jointly funded by MLA and Australian Wool Innovation (AWI).

He will share his first year’s findings at the MLA Bordertown Pasture Update on 8 April, underlining the importance of soil testing and stocking rates in managing P levels.

“It has long been believed the efficiency of P fertilisers in pasture systems is quite low – about 10-20 per cent fertiliser use efficiency,” Tim said.

“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 just sitting there.”

Researchers have tested this ‘conventional wisdom’ by directly measuring phosphorus use efficiency (PUE) in sub-clover pastures at Naracoorte in South Australia and Hall in the ACT.

A 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 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 P had gone.

“The pasture at the two sites was able to recover 34 and 40 per cent of fertiliser P added, which was quite high compared to what we expected,” Tim said.

“Further research is needed to see if this reflects seasonal conditions during the experiment, or whether we should revise our expectations of how much P the clover can get from the added fertiliser in the short-term.

“The message for producers 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.”

Researchers will now test whether PUE changes with a soil’s P fertility, and whether there are benefits to applying fertiliser P below the soil surface, rather than on top. They will also consider timing of fertiliser P application, by testing the benefits of spring versus autumn application.

Tim will join a full program of speakers at the Bordertown Pasture Update, with topics ranging from succession planning to water innovation. The keynote speaker on the day will be Michele Lally from Savannah Lamb.

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