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Identifying the issue

Most improved pastures across Australia are constrained by underlying soil phosphorus deficiency.

While nitrogen is often the most limiting soil nutrient through its impact on legume production (and associated nitrogen fixation), phosphorus fertiliser management drives overall pasture productivity.

The end goal is to use strategic inputs of phosphorus to accelerate the cycling of phosphorus and generate the fertility conditions required for maximum production.

Maximum (95%) spring pasture growth occurs at a critical Colwell P level of 35mg/kg.

Strategic phosphorus fertiliser application underpins the growth rates of high quality pastures (comprising 20–40% legumes), which drives the optimal animal weight gains that underpin red meat enterprise profitability.

Understanding the critical phosphorus (P) target of your soils is central to strategic phosphorus management.

Critical P is the soil test phosphorus value that supports near-maximum pasture growth (Figure 1).

Reaching and maintaining critical P levels, in conjunction with addressing other existing nutrient deficiencies, not only optimises pasture and animal production on a per hectare basis, but also optimises the use of existing farm resources (i.e. land and water/rainfall).

Monitoring soil phosphorus levels through regular soil testing and fertilising to hit and maintain critical P targets is central to success.

Figure 1. Soil phosphorus level to achieve maximum pasture growth

Key research findings:

  • Maximising pasture growth rates with strategic phosphorus fertiliser inputs will optimise animal growth rates and soil and water use efficiency (WUE).

  • Regular soil testing develops a database of information to underpin a strategic phosphorus fertiliser program for your soils. Building a bank of knowledge for the whole farm will not only drive better decisions about how much fertiliser to use, but also determine in which paddocks the fertiliser is most likely to have an impact.

  • Use the phosphorus buffering index (PBI) soil test to determine the critical Colwell P value of your soils. Apply phosphorus fertiliser to build soil fertility towards your critical P target or maintain current soil phosphorus levels.

  • The main reasons to apply phosphorus to pastures are to increase or maintain stocking rate. Applying phosphorus without having sufficient stock to use the additional pasture may not be profitable.

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