Author: Lee Menhenett, Incitec Pivot’s Technical Agronomist

With ongoing uncertainty about fertiliser supply, accurate nitrogen (N) budgeting is more important than ever. Deep soil testing remains the foundation of these decisions.

Without measured soil N, a N budget is inherently imprecise. In a season where input efficiency is critical, this introduces unnecessary risk.

Key considerations for 2026

  • Prioritise pre-sowing sampling to inform early N allocation
  • Delay sampling until April to allow mineralisation rates to stabilise
  • Expect elevated mineralisation following recent rainfall and warm soil conditions
  • Sample to depth (≥60 cm) and segment to understand N distribution
  • Include sulphur analysis to fully close nutrient budgets.

Seasonal context: elevated mineralisation potential

Late February to early March rainfall across the southern cropping zone, combined with sustained warm soil temperatures, has likely driven significant N mineralisation. This follows an extended period of below-average rainfall (summer, autumn and spring), where mineralisation was constrained. As a result, there is potential for a larger-than-normal N flush from soil carbon.

Mineralisation rates will vary depending on:

  • Soil carbon levels and labile carbon fractions
  • C:N ratios
  • Soil moisture and temperature
  • Previous crop type (with pulses and canola typically contributing more available nitrogen than cereal stubbles).

Residual N from 2025 fertiliser applications may also contribute, depending on application rates, application timing, and crop removal.

Sampling timing: managing mineralisation effects

Sampling too soon after rainfall can capture N while mineralisation is still actively occurring. This provides a snapshot of nitrogen in transition, rather than a stable estimate of plant-available supply, and may not reflect the full contribution of ongoing mineralisation.

Elevated ammonium-N levels (>5 mg/kg) can indicate that sampling has occurred before mineralisation has stabilised.

Allowing time for soils to cool and dry following rainfall will improve confidence that the results reflect the N available to the crop.

As a guide:

  • Sample once soils begin to cool and dry
  • Allow sufficient time after rainfall for mineralisation to occur
  • Ammonium-N levels >5 mg/kg may indicate sampling occurred too early.

Pre- vs post-sowing sampling

Both pre-sowing and post-sowing sampling approaches are valid, however, pre-sowing is the preferred sampling strategy for 2026.

It enables:

  • early and more accurate N budgeting,
  • informed fertiliser procurement decisions, and
  • strategic allocation of limited inputs.

Post-sowing sampling may better reflect in-season mineralisation but introduces practical constraints (e.g. trafficability) and delays decision-making.

Depth and segmentation: improving resolution

While most of the mineral N is typically located in the 0–30 cm layer, N may accumulate deeper in the profile due to:

  • high historical N application rates,
  • leaching following rainfall events, or
  • lighter soil textures.

Sampling to at least 60 cm (or rooting depth) and segmenting by depth provides greater resolution of N availability. This supports:

  • more accurate fertiliser rate decisions,
  • improved spatial allocation across paddocks or management zones, and
  • identification of early N constraints in the topsoil.

Early N availability is critical for crop establishment and yield potential, and this information is particularly valuable.

Figure 1: Various deep soil sampling segmentation techniques. Source: Lee Menhenett, Incite Pivot Technical Agronomist, January 2026

Sulphur: a secondary but essential consideration

Sulphur is often distributed deeper in the soil profile due to its mobility and lower plant uptake rates. Without measurement, sulphur budgets remain incomplete. Including sulphur in deep soil testing improves overall nutrient use efficiency and maximises the value of sampling.

Summary

Deep soil testing underpins nitrogen fertiliser decision-making in winter cropping systems.

In 2026, seasonal conditions and external supply pressures increase the importance of:

  • quantifying existing soil nitrogen,
  • accounting for mineralisation dynamics, and
  • allocating fertiliser inputs with precision.

Failure to do so increases the risk of both under- and over-application, with direct implications for yield potential and input efficiency.

Further information

If you would like to learn more about deep soil testing, contact Incitec Pivot Technical Agronomist, Lee Menhenett on 0412 565 176 or via email: lee.menhenett@incitecpivot.com.au