101 Questions answered
Using Yield Maps for Phosphorus Decisions: From Replacement Rates to Variable Applications
Calculating the nutrient export off a paddock is all well and good (see Question 92. ‘How much nutrient is exported from the paddock after a bumper year?’), but how do you actually use the numbers?
One way is to set the replacement fertiliser rate, i.e. how much you need to apply to replace what was removed. For example, a 2.2 t/ha wheat crop exports roughly 3 kg of phosphorus (P) for every tonne of grain. The crop has removed 2.2 x 3 = 6.6 kg of P.
The replacement P strategy is therefore to apply P at a rate to supply 6.6 kg of P /ha, which equates to 30 kg MAP/ha or 33 kg DAP/ha.
Another way is to generate variable rate application maps based on nutrient removal. For each yield point, run the simple P balance again to get variable P application maps. The maps below show an example variable rate P application map and the corresponding MAP map.
This is a starting point. Other nutrient deficiencies, water stress, disease, etc. drive yield patterns and the paddock might need more or less than a simple replacement strategy.
The phosphorus buffering index (how the soil locks up P) should also be considered. Phosphorus replacement strategies based on estimated P removal in grain only work on soils where P is not limiting growth, i.e. those with high soil P and low PBI.
Recent work in the Mid-North/Yorke Peninsula has shown that high PBI and resulting low soil P tests highlight areas that are still responsive to P applications after a long period of uniform P applications. These metrics have been more powerful than grain yield maps alone at determining required P rates.
Soil tests will show if:
- Nutrient levels are high enough that a replacement strategy will suffice.
- Soil PBI is moderate or higher and higher P rates are needed.
- Soil P has been mined and higher levels are needed to rebuild the soil P bank.
