Planning your phosphorus management this season


Todd Buck

Phosphorus (P) is an essential macro-nutrient for crop and pasture growth. In cropping systems, P supply needs to be managed before/at sowing because most P is taken up by the crop in the first six to eight weeks.  Sufficient P supply will maximise tiller number and head size and consequently yield potential. P deficiency reduces seedling root growth, decreasing access to water and other nutrients, and makes them susceptible to weed competition, pests and disease. Yield potential cannot be recovered if there is an early P deficiency. With high P fertiliser prices, it is important to optimise fertiliser allocation through understanding your soil P status and ensuring a robust P replacement strategy.

Know your Soil Phosphorus levels

An effective P management strategy starts with measuring your current soil P status and comparing them to relevant critical soil test values. Critical values tell you what level of soil P will be “non-limiting” to crop growth. These values will vary by test type (e.g. Colwell P), crop, region and soil type (or Phosphorus Buffering Index, PBI) and have generally been established by research agronomists using P response trials.

Where soil P levels are below the critical value, it is recommended that capital rates of P are applied. Capital P is the fertiliser required to increase the soil P levels to a target value and can be applied through broadcast application prior to sowing. Depending on the rates required, such applications can be split over several years.

In contrast, where the Colwell P levels in the soil are above the critical values it may be possible to reduce P fertiliser use and maintain or mine the P reserves in the soil. However, even with high soil P levels some starter P fertiliser is still required at sowing. When P levels match or are close to critical values, maintenance applications of P will offset crop export and other losses that occur naturally over time.

Key Message Know the potential responsiveness of each paddock by soil testing. A soil Colwell P test and a soil PBI, will enable a capital P budget to be constructed to apply P most efficiently.

Developing a P replacement strategy

The major loss pathway of P from cropping systems is through nutrient removal in grain. Accurate yield data can be used to calculate how much P has been exported by the previous year’s crop. This can be done at the paddock scale or further refined by mapping yield variation across the paddock. Ensuring that your header is set up prior to harvest will help ensure high quality and easily accessed yield data, streamlining the process of developing an accurate P replacement strategy. Nutrient removal can be calculated using indicative grain quality data, or further refined by grain nutrient analysis as grain P concentrations can be quite variable. For example, across a number of studies, wheat ranged from 2.3 – 3.6 kg P/t grain, barley ranged from 2.0 – 3.3 kg P/t grain and canola ranged from 3.6 – 7.2 kg P/t grain.

However, an effective P replacement strategy should still be based on some level of knowledge of the soil P starting point and PBI to avoid accidentally maintaining an inadequate (or excessive) level of P.

Key Message Knowing grain offtake can be useful in defining P removal and as an adjunct to soil testing. 

What might my precision soil P management journey look like?

Factors such as soil type and crop yield can vary substantially between different parts of a single paddock and therefore P fertiliser requirements are unlikely to be uniform either. An effective multi-year soil P management approach could involve an initial variable rate capital application, that supports an ongoing P replacement strategy undertaken for several seasons.

Nutrient mapping will measure initial soil P variability and ensure that adequate capital or maintenance rates are applied to meet critical levels and achieve production targets across the paddock. Figure 1-Year 1 is an example of the level of Colwell P variability (26-75 mg P/kg) that is commonly observed.  With this degree of variability, a uniform fertiliser application would result in over- and under-fertilisation in different areas of the paddock. Soil P maps can drive variable rate fertiliser applications (VRA) that match actual soil P status and PBI. An example of a capital P map is shown in Figure 1-Year 1 with P rates ranging from 0 to 250 kg/ha SSP which will allow this paddock to achieve a minimum Colwell P of 35 mg/kg.

Figure 1: An example of a 3-year P management program utilising capital P fertiliser applications based on nutrient mapping in Year 1 plus P replacement strategies using yield data in Years 2 & 3.

Once soil P levels have been ameliorated to remove initial constraints through variable rate capital P, a P replacement map can be developed using good quality yield data (Figure 1-Years 2&3). P rates are calculated using either indicative grain P concentrations or actual data if the grain quality was analysed. In the example (Figure 1-Years 2&3) yield ranged from <1 – 4.9 t/ha with the lowest rates observed in headlands and a replacement P map which ranged from 90 – 150 kg/ha SSP. This maintenance approach can be continued for several seasons before nutrient mapping is required to ensure that no new P constraints have developed.

Effective phosphorus management is critical to achieving yield potential, and efficient nutrient application is particularly important when fertiliser prices are so high. Effective and efficient P management starts with soil testing, is improved by accounting for paddock variability, and when combined with yield data a targeted VRA strategy can be implemented to achieve the best return for your fertiliser spend.

Article written by Dr. Kirsten Barlow – Head Scientist, Precision Agriculture

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