Soil Type vs Nitrogen Rate: What On-Farm Sugarcane Trials Reveal About Yield in the Wet Tropics

When sugarcane yield varies across a block, the instinct is often to reach for more nitrogen. On-farm nitrogen rate trials across the Mulgrave and Russell River catchments tell a more useful story: across multiple sites, soil type and drainage frequently mattered more for yield than the nitrogen rate applied. Understanding that distinction is the difference between fixing a yield problem and fertilising past it.

How the trials were run

Trials compared a range of nitrogen rates — typically 50, 100, 150 and 200 kg/ha — across different soil types within each block, using randomised, replicated designs. All other nutrients were applied to soil-test requirements so that nothing else limited growth, isolating the nitrogen response. Several sites also carried additional treatments adding 10 kg/ha of phosphorus to test for a phosphorus effect. Yields were measured through 9-month biomass sampling and, where possible, commercial mill data, with baseline biomass sampling done before treatments to capture the starting differences between soil types.

Site 1 — Deeral: soil type outweighs nitrogen rate

The Deeral block spanned two contrasting soil types: Timara (a heavy soil, 52% clay) and Malbon Prior (29% clay), both very poorly to poorly drained with a water table around 1.4 m and indications of waterlogging at 1.2–1.6 m depth.

Across both 2023 and 2024 there were no significant differences between nitrogen rates. Yet the Malbon Prior end consistently out-yielded the Timara end — by 10.1 t/ha (and 1.7 t/ha sugar) in 2023, and 5.2 t/ha (and 1 t/ha sugar) in 2024. The conclusion: factors other than nitrogen rate, such as inherent soil characteristics and micro-environment, drove yield. Site-specific soil assessment, not extra nitrogen, is the lever here.

Site 2 — Babinda: a clear soil split and a 100 kg/ha optimum

The Babinda block compared Wanjuru and Bulgun soils. Wanjuru carried a dark organic peat layer to 0.6 m over heavy grey clay from 1.3 m (the grey colour indicating poor aeration), a high water table (0.7–1.4 m) and very poor drainage; Bulgun was poorly drained but better off. Both soils returned a high pachymetra spore count (1,020,360).

The difference between soils was statistically significant — a 21.6 t/ha yield gap and 3.3 t/ha sugar gap in favour of the better-drained Bulgun end. For each soil type, around 100 kg/ha of nitrogen tended to give the highest results, reflecting the SIX EASY STEPS rate for both soils (soil organic carbon was above 2.4% throughout). The more waterlogged Wanjuru soil, with heavy clay below the peat and a higher water table, was the lower performer. The trial was terminated in its second year after Ex-Tropical Cyclone Jasper — the wettest tropical cyclone on record — severely affected the block.

Site 3 — Gordonvale: season and crop age dominate

The Gordonvale site shifted focus to yield variation over time. It revealed significant year-to-year differences but no substantial differences between soil types, underlining how strongly seasonal variability and crop age influence performance — a reminder that a single season's result can mislead.

Demonstration trial — Mirriwinni: nitrogen does lift yield where soil allows

A demonstration trial at Mirriwinni, set up as a backup nitrogen rate trial, evaluated crop response across nitrogen rates. Here higher rates — 150 kg/ha and 200 kg/ha — significantly increased yield and sugar yield compared with lower rates. The site also held a near-neutral soil pH of 6.77, underscoring how good nutrient availability lets the crop convert higher nitrogen into yield. The contrast with Deeral and Babinda is the whole point: nitrogen rate moves yield when soil and drainage aren't the binding constraint.

The pattern across all sites

A consistent thread runs through the program: the poorer-performing soils were the ones with more severe waterlogging and drainage limitations. The soil differences at the trial sites coincided with differences in water-table height and wet-season waterlogging, confirmed through site classification with the Queensland Government Department of Resources — conditions that are not unusual in the Wet Tropics sugar industry.

What it means for fertiliser strategy

Three practical conclusions follow:

• Diagnose before you dose. Where yield varies across a block, check soil type, drainage and water table before assuming a nitrogen deficit. At Deeral and Babinda, the lower-yielding ground had a drainage problem, not a nitrogen problem.

• Around 100 kg/ha was the practical optimum at Babinda, matching the SIX EASY STEPS rate — higher rates did not pay where drainage limited the crop.

• Nitrogen lifts yield when conditions allow it, as at Mirriwinni with its near-neutral pH and higher rates. The skill is knowing which situation you are in, which comes back to understanding within-farm variability.

Key takeaways

• Soil type and drainage frequently outweighed nitrogen rate as yield drivers — clearest at Deeral (Malbon Prior out-yielded Timara by 5–10 t/ha despite no nitrogen-rate effect).

• At Babinda the better-drained Bulgun soil out-yielded waterlogged Wanjuru by 21.6 t/ha; ~100 kg/ha N was the optimum, matching SIX EASY STEPS.

• At Mirriwinni, higher rates (150 and 200 kg/ha) did significantly lift yield, alongside a near-neutral pH of 6.77.

• Diagnose soil and drainage before adding nitrogen; understanding within-block variability is the central lesson.

Frequently Asked Questions

Does soil type or nitrogen rate matter more for sugarcane yield?
Across several Mulgrave Russell trial sites, soil type and drainage mattered more than nitrogen rate. At Deeral, the Malbon Prior soil out-yielded the Timara soil by 10.1 t/ha (2023) and 5.2 t/ha (2024) despite no significant difference between nitrogen rates. Poorer-performing soils were consistently those with worse waterlogging and drainage.

What was the optimal nitrogen rate in the trials?
At Babinda, around 100 kg/ha of nitrogen tended to give the highest results for both soil types, matching the SIX EASY STEPS rate. At the Mirriwinni demonstration trial, higher rates of 150 and 200 kg/ha significantly increased yield and sugar yield, alongside a near-neutral soil pH of 6.77.

How much did waterlogging affect yield in the Babinda trial?
At Babinda the better-drained Bulgun soil out-yielded the more waterlogged Wanjuru soil by a statistically significant 21.6 t/ha in cane and 3.3 t/ha in sugar. Wanjuru had a peat layer over heavy grey clay and a higher water table, causing more wet-season waterlogging.

Why did some nitrogen trials show no response to higher rates?
At Deeral and Babinda, drainage and soil characteristics were the binding constraints on yield, so adding nitrogen above the SIX EASY STEPS rate did not increase yield. Nitrogen rate lifted yield only where soil and drainage were not limiting, as at Mirriwinni with its near-neutral pH.