Mulch trials: Which types work best?

Four years of ‘citizen science’ mulch trials on lime farms in the Mareeba region has shone a light on what kinds of mulch are the best performers – and it has also led to farming changes.

Mutchilba horticulturalists Michelle and Ante Milicevic are now using Rhodes grass hay annually right across two orchards and, as a result, they’ve improved soil health and significantly reduced their watering and spraying regimes.

“We’d put tea tree and hay in before but not every year,’’ Michelle said. “Since starting annual mulching we’ve been watering a lot less. We used to water every two to three days in the hot dry weather. Now we can push it out to every four or five days, and even a little bit more if we need to.

“Before the trials, we sprayed to keep the weeds down so that our sprinklers worked well. Now we’re not spraying as much because the weeds aren’t coming up. And the ground is a lot easier to dig up. The soil is softer, even where it’s being worked outside the mulch row.”

Four mulches trialled

Terrain NRM and the farmer-led Wet Tropics Soilcare group trialled four mulches – hay, tea tree, pine wood chip and organic matter from mowing the inter-rows – on two farms in the Mareeba district.

Hay was the best performer, followed by tea tree and organic matter. Terrain NRM agronomist Sally Fields said the long-term soil health benefits of woodchip were also evident.

“The trials were on farms with sandy loam soils with a low capacity to hold nutrients (a cation exchange capacity of less than two on both sites at baseline testing in 2021). We took a bunch of measures, full soil tests, leaf sap testing, penetrometer readings and worm counts, and monitored soil health parameters of infiltration. An average yield per tree was also recorded on three occasions at Milinder Farming.

“Growers are recognising more and more the importance of ground cover – keeping your soil covered at all times protects it from water and wind erosion and maximises water infiltration. But many orchards continue to spray under trees and have bare ground under the canopy. We took on this project with Wet Tropics Soilcare because growers were curious to see how much mulch mattered.”

Healthier, more productive trees

Ms Fields said one telling result was the “phenomenal” difference between using side-throw from the mower and mulching – with noticeably healthier and more productive trees under hay and tea tree mulch.

Soil analysis results from both farms showed soil health had also improved under mulch.

She said the trials showed the soil health benefits of woodchip mulch in particular, but also the need to carefully select and apply your woodchip.

“The best choice is ramial (branch) woodchip because the nutrient loads are much higher in the branch-growing region of a tree. Woodchip taken from the trunk will have higher carbon and nitrogen rates and it’ll need more time to break down before using it. Hardwood is better than softwood for orchards. Softwoods like pine need to sit for at least six to 12 months before application to make sure tannins and chemical compounds have leached out.