Variable Rate Nitrogen and Yield Map Interpretation

Variable rate application is justified because the yield maps are consistent across three years and the soil test results confirm real differences in nutrient status between zones. The sandy rise has low phosphorus and low nitrogen, and its low yield is likely limited by both nutrients and by water-holding capacity, while the flats have adequate to high levels. A flat application would over-fertilise the flats, wasting money and increasing the risk of nutrient loss, while under-fertilising the sandy rise and failing to address its constraint. A zonal strategy should reduce or omit phosphorus on the flats, where 28 mg/kg Colwell is above the critical range for wheat, and apply a higher rate on the sandy rise to build soil phosphorus and support early root growth. For nitrogen, the flats can receive a moderate rate, but the sandy rise needs careful management: its low water-holding capacity means nitrogen applied early may leach before the crop can use it, so a split application timed to rainfall is preferable. In a decile 3 year, the overall nitrogen rate should be conservative, because yield potential is limited by water, and the flats should not be pushed to high nitrogen rates that cannot be supported.

A blanket cut is too blunt. In a dry year, the highest-yielding flats are likely to be limited by water rather than nutrients, so reducing nitrogen there may save money with little yield penalty. However, the sandy rise is already low in nutrients and has poor water-holding capacity, so cutting fertiliser further could reduce its already modest yield and delay pasture or crop recovery. Phosphorus is also a capital nutrient: reducing phosphorus on the sandy rise in a dry year may reduce the soil phosphorus bank and hurt future yields, whereas phosphorus applied to the flats, which are already above critical, can be safely reduced. A more nuanced approach is to maintain or slightly increase phosphorus on the sandy rise to build the bank, reduce phosphorus on the flats, and split nitrogen applications so that the crop receives nitrogen only when soil moisture is adequate. The grower could also use in-season monitoring, such as NDVI or tissue testing, to adjust nitrogen rates as the season unfolds rather than committing to a blanket cut at sowing.

Yield monitor data can contain errors from calibration, grain flow lag, header width assumptions, moisture variation and positioning errors, so the grower should validate before relying on the maps. The first step is to check that the monitor was calibrated for the grain type and that calibration loads were collected across a range of flow rates. The second step is to compare the total harvested grain weight from the monitor with weighbridge or silo records for each paddock; a difference of more than a few percent suggests a calibration problem. The third step is to inspect the maps for artefacts such as stripes, edge effects or abrupt changes that do not match soil or topography, and to remove or correct these before zoning. The grower should also ground-truth the zones by soil sampling and by inspecting the crop in the low and high zones, and ideally compare the yield map with an independent source such as satellite imagery or a hand-held yield check. Only after the data are validated should the zones be used to set variable rates, and the zones should be reviewed each season as the maps accumulate.