Farmers Turn to Sunn Hemp to Curb Rising Fertiliser Bills
With the conflict in the Middle East driving up global urea prices, many Australian growers are looking for ways to lower input costs without sacrificing yields. One solution gaining traction is the use of sunn hemp (Crotalaria juncea) as a nitrogen‑fixing cover crop. The legume draws atmospheric nitrogen into the soil through symbiotic bacteria, offering a renewable source of this essential nutrient.
Neil Maitland’s on‑farm results
Near Cairns, sugarcane farmer Neil Maitland has been planting sunn hemp during fallow periods for several years. By incorporating the legume into his rotation, he reports cutting his nitrogen fertiliser bill by more than $400 per hectare. Overall, he has reduced his total fertiliser use by about 25 %, noting that urea—the most expensive component—has seen price spikes that would have doubled his costs without the cover crop.
“It’s not that we’re not using nitrogen; it’s just that it’s coming from an organic source instead of an inorganic fertiliser,” Maitland said. He still applies phosphorus and potash as needed, but the nitrogen supplied by sunn hemp has allowed him to rely less on synthetic urea.
Department of Primary Industries trials
Researchers at the Queensland Department of Primary Industries (DPI) have been evaluating sunn hemp for both its fertiliser‑saving potential and its pest‑management benefits. Paul Grundy, a DPI agronomist, explains that the legume can fix the equivalent of 220 to 320 kilograms of urea per hectare, depending on growth conditions and soil health.
In Central Queensland cotton systems, Grundy’s team has used sunn hemp to suppress reniform nematodes—a soil‑borne pest that can cut cotton yields by 10‑20 %. Starting with roughly 3,000 nematodes per 100 grams of soil, a two‑and‑a‑half‑month sunn hemp cycle reduced the population to about 100 per 100 grams. Over three seasons, the data have shown a consistent nematode decline, prompting more growers to adopt the legume in their rotations.
Grundy cautions that further work is needed to assess whether sunn hemp could harbour diseases in cooler regions, noting that current trials have focused on the warmer climates of Central Queensland where disease pressure has been low.
Multi‑species cover‑crop experiments in North Queensland
North of Ingham, Lawrence Di Bella of Queensland Cane Agricultural Renewals is leading a trial that mixes sunn hemp with cowpea, lablab, and soybean. The multi‑species approach aims to maximise nitrogen cycling, carbon sequestration, and biodiversity in sugarcane fields.
Di Bella says the rising cost of urea makes it unaffordable to apply at traditional rates, so the trial seeks to quantify how much nitrogen the cover crop blend can contribute to the cane crop. Early observations indicate that the mixture provides a stable nitrogen supply while improving soil structure and suppressing weeds.
He adds that, had the trial begun a year or two earlier, the data could have guided immediate fertiliser reductions, but the results will still help shape future cost‑saving strategies for the region’s sugarcane producers.
