Is Phosphorus building up in your paddock?

a mountain of soil phosphorus with a stickman scratching his head.

Phosphorus may be over applied in Southern Australian grain farms. A recent survey by IPNI found 80% of paddocks had more P applied than removed over a 3–5 year period. Nutrient Performance Indicators (NPIs) revealed that on average, 0.69 kg P is removed per kg P applied.

Some farms may be able to reduce their P inputs without compromising soil fertility. To work out if this applies to your paddock you need to consider:

  • NPIs for your paddock
  • P availability – through appropriate soil testing
  • Spatial distribution of P in the soil.

NPIs to calculate

NPIs are used to understand trends in nutrient management. They are calculations based on crop inputs and yield. Use NPIs foremost to track performance in specific paddocks over time. NPI values will vary with seasons, cropping intensity and rotation sequences so it is important to take a long term view (5 years or more).

Partial Factor Productivity-P

PFP-P = grain produced (kg) / P applied (kg)

PFP-P describes the grain yield benefit from P fertiliser. High PFP-P scores suggest a strong yield response to P fertiliser. The normal range for PFP-P is 100–250, although averages vary widely throughout the world and within Australia. In southern Australia the average PFP-P from the survey was 227.

If your PFP-P level is much higher or lower than the average for southern Australia, consider:

  • A very high PFP-P means the crop yield responded strongly to P fertiliser. Probably don’t reduce P rate if PFP-P is high. There might be room to increase the fertiliser rate.   
  • A very low PFP-P means there was little response to fertiliser, or other yield constraints limited the crop. Consider reducing P rate if PFP-P is low and the soil test P level is adequate.

Partial Nutrient Balance-P

PNB-P = P removed (kg) / P applied (kg)

PNB-P indicates whether a crop cycle left the paddock with higher, lower or stable P reserves. Soils could be being depleted if PNB-P is above 1. Measure PNB-P after each harvest and interpreted as a trend over years. An IPNI Crop Nutrient Removal Calculator can supply estimates for nutrient removal for different crops and straws.

The goal is for average PNB to approach 0.9–1, to avoid running down soil P reserves. Southern Australian PNB-P values ranged from 0.7 – 1.0.  A PNB >1 over multiple years indicates rundown of soil P reserves. Consider reducing P rates if PNB-P is below 0.9 for several years AND soil testing shows P levels are adequate.

Availability of phosphorus

Only a small proportion of P fertiliser (usually <20%) is taken up by the current crop. This years applied P contributes to the P reserve of the paddock, while the crop draws on prior year applications held as soil reserves. Productivity relies on soil P because it’s not agronomically practical to supply crop P needs during the season. Soil testing is the best way to assess available soil P. Where soil P is low it’s best to maintain P rates above crop demand to build the P reserve over time, regardless of NPIs.

Location of phosphorus

In the soil, P tends to stay where it is. Starter P can help, even when P is not limiting, because the seedling can only access nearby nutrients. In time, crop roots can grow to access soil P e.g. previous fertiliser bands. No-till paddocks can end up with P more concentrated at the soil surface, or in horizontal bands. The availability of that P to seedlings depends on the moisture status and their proximity to P rich zones.


Nutrient Performance Indicators for your paddocks

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Fertilizer and nutrient trends in Australia

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