Phosphate, unlike nitrogen, does not typically leach from the root zone. Phosphorous binds to sediment particles and is lost from a block or property via erosion or a rainfall runoff event. A phosphorus nutrient budget allows you to identify areas where soil phosphate levels are increasing or decreasing and target phosphorus nutrients to where they are required.

Fertiliser applications can be classified in three categories:

• Capital fertiliser to increase soil fertility
• Maintenance application to hold the current status quo, or
• Reduced applications to mine excess nutrients from the soil profile.

These three management approaches are based on how much fertiliser is being applied compared to how much the crop is removing from the paddock.

1. Capital fertiliser applications are appropriate in low fertility situations where based on a soil test, additional nutrients are required to achieve optimum crop production.
2. Where the soil test values are within the agronomic optimum range for the crop rotation*, the nutrients removed by the crop from the paddock should be replaced with fertiliser inputs.
3. Where the soil test is above the rotation optimum, ground applied applications should be reduced to less than crop removal to allow the crops to “mine” the excess nutrients and to help bring the soil test back into ‘Balance”.

*Each crop has an optimum soil test threshold where little economic value will be measured with additional fertiliser application. When a series of different crops are being grown in a paddock in rotation, the crop rotation will have an optimum soil test value that is equivalent to the highest crop agronomic optimum value.

Where the soil test has an Olsen P above the crop optimum, some crops can still show a response to additional phosphate applied at planting especially in low soil temperature situations. Consideration should be given on how to apply a starter Phosphate product most effectively in these situations without continually increasing the soil fertility beyond the crop rotation optimum.

Let’s look at the Phosphorous Nutrient Budget Template.

We start with a soil test value for Olsen P, the amount of plant available phosphorous (phosphate) in the soil. We look up our crop in the Nutrient Management book and for our anticipate yield we find the amount of extra phosphorous that is recommended.

We select our fertilisers, timings and rates and calculate the total we expect to apply.

Remember, most phosphate is applied as a base dressing before planting, but some crops seem to respond to a supply of fertiliser applied at planting. Phosphate applied as a side-dressing later will be poorly taken up by the crop.

If the recommendation is to add no more P Fertiliser, calculate how much P you expect to remove in the harvested crop. This is how much you would apply to maintain the same soil P levels over time.

If your soil P (Olsen test value) is high, apply less than the estimated crop removal and “mine your reserves” to slowly lower the excess.

At the end of the season, you can do another budget using your actual yield and actual applied phosphate and calculate the difference.

If we want to raise the amount of available phosphate in a paddock, we need to add more than we remove. Some of what we apply will be “Locked up” by the soil, so we need to add extra for that. the amount of applied phosphate required to raise available P depends on the type of soil you have.

To raise the Olsen P value by one unit (ug/mL) (0-15 cm soil depth) requires:

• 25 kg P/ha on volcanic ash (allophanic or granular soils) – range 20 – 30 kg P/ha
• 13 kg P/ha on peat and sands (organic and recent soils) range 10 – 16 kg P/ha
• 10 kg P/ha on sedimentary soils (recent and brown soils) range 3 – 16 kg P/ha