Seeing what NDVI can show: Monitoring potassium responses at Ballidu
By Kate Parker, WMG Project Officer
Late-season NDVI imagery from the 2025 Ballidu potassium demonstration site, part of the GRDC funded K Extension Project, provided useful insights into treatment performance that were not visible through ground assessments alone. While early-season biomass cuts and plant tissue results showed minimal separation between potassium rates, the NDVI sequence revealed subtle differences in canopy development across soil types and treatments later in the season.
Early Season: Uniform NDVI Across Treatments
NDVI imagery from July and early August showed relatively uniform crop vigour across both trial sites. This aligned with field observations and GS30 biomass data, where no K treatment effects were detected. Early uniformity was expected, as potassium responses typically appear later in the season and soil type often plays a greater role than fertiliser rate at early stages.
Late Season: NDVI Shows Increasing Variation
From September onwards, NDVI began to show clearer differences between treatments, particularly on the gravelly clay-loam (Site 2). While these differences did not translate into statistically significant treatment effects in biomass or tissue tests, the NDVI maps indicated variation in canopy density and greenness that was not evident during ground-based assessments.
These late-season contrasts suggest that NDVI is a useful tool for detecting subtle nutrient interactions and soil-related variation before yield measurements or further tissue analysis.
Satellite imagery of NDVI of the trial site on 17/10/2025 taken from Data Farming website.
Reappearance of Historical N-Banking Strips
An unexpected outcome was the re-emergence of an old nitrogen-banking strip at Site 2 in the NDVI imagery. The grower reported that this strip had not produced visible responses or yield differences for several years, yet it became apparent in the 2025 imagery. This may reflect:
- increased mid-season rainfall improving nutrient mobility,
- the interaction between residual N and the applied K treatments,
- or improved root exploration in the gravelly profile.
This observation highlights the value of NDVI in identifying legacy nutrient effects that may not be visible on the ground.
Practical Implications for Growers
The NDVI monitoring at Ballidu demonstrated several practical points:
- NDVI can detect small differences in canopy growth that field inspections may miss.
- Potassium responses may not be strongly expressed early in the season, especially in sandy soils.
- Historical nutrient treatments can reappear under favourable seasonal conditions.
- NDVI is a useful tool for guiding targeted sampling, identifying management zones, and supporting interpretation of nutrient trials.
Summary
While ground measurements from the Ballidu trial did not show strong potassium treatment responses, NDVI provided an additional layer of evidence that helped identify subtle variations across soil types and historical nutrient zones. As a complementary monitoring tool, NDVI can improve understanding of nutrient behaviour and assist growers in evaluating trial performance and spatial variability within paddocks.
