SPREADING single super on pastures is an annual practice for graziers but understanding where that phosphorus fertiliser ends up is not fully understood.
A three-year project funded by Meat & Livestock Australia and Australian Wool Innovation is hoping to answer this, looking at the short-term and long-term fate of P fertiliser, and its availability to plants.
Phosphorus use efficiency is much lower on sheep and beef properties than cropping, so the research is hoping to find ways to lift this efficiency and promote more pasture growth. It is widely believed that much of the P added to pastures is 'fixed' by the soil, and that less than 15pc is taken up by pastures.
The good news from first-year results undertaken by the University of Adelaide, CSIRO and the University of New England is that a much higher amount of the fertiliser applied is taken up by clover pastures than previously thought in the year of application.
Field experiments were carried out at two sites - the Kybybolite Research Station in SA (a P non-responsive site) and the CSIRO Ginninderra Experimental Station in the ACT (a P responsive site). A sub-clover pasture sward was established at both sites and single superphosphate containing an isotopic tracer was applied to the soil surface to track P in pasture systems.
Researchers found that about 95 per cent of the P had dissolved from the fertiliser granules, which demonstrated that most of the P left the granule and entered the soil.
At both sites, a surprisingly high phosphorus use efficiency of 34pc and 40pc of the fertiliser applied was found in the plant tops grown at Ginninderra and Kybybolite, respectively. About 30pc of P was found in the 0 to 4-centimetre layer of the soil and a considerable proportion of this remained in a plant-available form. Less than 5pc was found in the 4-8cm layer.
The P that was unaccounted for was believed to be in the root biomass, which will require further investigation.
Despite the relatively high P use efficiency of clover, the soil is still the primary source of P to clover pastures.
In long-term studies, an analysis of soil P was carried out on soils collected at the long-term field site at the CSIRO Ginninderra Experimental Station. This showed that about 80pc to 95pc of P had accumulated in the 0-20 cm layer.
The differences between short-term and long-term results show that P appears to be highly recycled through the pasture system. This is likely to include the addition of P to the soil surface from pasture senescence of under-utilised pasture, and animal dung.
Speaking at the recent NRM SE Pathways to Productivity Expo at Bordertown, the University of Adelaide's Research Fellow in Soil Nutrient Dynamics, Dr Tim McLaren, said P was one of the major nutrients limiting pasture growth and that many properties were still operating under the critical level required.
He used the example of a soil survey in the Adelaide Hills and Fleurieu Peninsula of more than 200 paddocks on 91 properties which showed more than half the sites were P-responsive, indicating the importance of soil phosphorus testing.
Dr McLaren said the cycling of P between soil inorganic forms (similar to that in fertiliser) and soil organic forms was complex but they were seeking to better understand this movement. Initially, the results show no difference in the relative composition of soil organic P in soil with and without fertiliser P.
The short-term results showed that most of the P remained in an inorganic form, whereas analysing longer-term data showed inorganic and organic P levels significantly increased with annual fertiliser applications.
* Full report in Stock Journal, April 17, 2014 issue.