High grain yields mean high rates of nutrient removal, in particular, nitrogen (N) and phosphorous (P). Soil fertility in the paddock declines unless nutrients are replaced. Soil and grain tests help you make better nutrient budget decisions.
Reference values for nutrient removal are just an indication (especially for P):
- A 6-tonne wheat crop with 12 per cent protein removes up to 125 kg/ha of N, along with 18 kg/ha of P, 21 kg/ha of potassium (K) and 7 kg/ha of sulphur (S).
- Baling the stubble removes a further 56 kg/ha of N, 6 kg/ha of P, 109 kg/ha of K and 9 kg/ha of S.
- Check removal rates of other crop types as they can be very different for N, P, K and S.
Get the basics right
Make sure that soil pH, soil structure, P, K, and microbes are in the ranges they should be – before pushing N. Be wary of declining organic matter levels. After a good year with high rainfall crops often ‘mine’ N from organic matter reserves.
Test topsoil and subsoil for nutrient budgets. Mobile nutrients can leach, and good subsoil moisture allows plants to access more nutrients at depth. Particularly in the north, subsoil P, K and S testing is beneficial as this is where crops get these nutrients from. In lighter, acid soils, K is more likely to be limiting. Soil sampling methods and soil analysis need to be informed by the paddock history and plans for the current crop.
P offtakes from high yielding crops are a lot higher than normal. This could be reflected in soil test results. In most soils, P is highly immobile so sampling needs to mainly focus on where the crop is going to be sown (e.g. interrow of 2016). Interrows may still have residual P levels from previous seasons.
Beyond the N removed by the previous harvest, stubbles left in the paddock can immobilise N.
This becomes available again later in the season, but too late for the young crop.
Consider increasing at-seeding N rates by up to 15 per cent of their N budget. Be careful with fertiliser placement to avoid toxicity. If P inputs also need an increase, consider using MAP/DAP. Then monitor for further top-ups throughout the season. An N-rich test strip in the paddock is a good way to monitor if more nitrogen would help the crop.