Second year update on CCC macadamia soil amendments trial
28 October 2025
The second year of the Clean Coastal Catchments (CCC) macadamia orchard soil amendments trial has been completed at the Centre for Tropical Horticulture in Alstonville.
This long term research trial, delivered through the Department of Primary Industries & Regional Development (DPIRD), has been designed to support innovation in sustainable agriculture, with the ultimate goal of improving water quality in the marine estate.
The research team is investigating the role of organic soil amendments in improving orchard soil health. The expectation is that the use of organic amendments will result in less fertiliser moving off the orchard, which will therefore reduce the environmental impact of excess nutrients flowing into coastal aquatic systems.
Double ground mulched organic soil amendments more likely to stay in place in the orchard
The amendments applied in this trial consist of a blend of manure and woodchips, processed into double-ground mulch (as opposed to single coarse woodchip blended with manure). The six different soil amendment treatments currently being assessed in the trial are detailed in our February 2025 article: Assessing the benefits of organic nutrition for macadamia production.
“Double-ground mulch offers distinct advantages over a single-chip mulch,” said DPIRD Macadamia Development Officer, Jeremy Bright.
“While single-chip mulch tends to float away during heavy storms or strong water flow, double-ground mulch, which has a finer texture, remains more stable and integrates more effectively into the soil,” said Mr Bright.
Because of the smaller particle size, double-ground mulch also breaks down more easily, adding carbon into the soil and improving nutrient cycling and retention.
The research team believes the double ground woodchip and manure mix is more likely to stay in place in the soil, when compared to synthetic fertilisers, particularly as the roots of the macadamia tree grow into the soil amendment. (See references at the end of this article for relevant research regarding nutrient mobility from Claassen, V.P. and Al-Bataina, B.B.)
In contrast, synthetic nitrogen fertilisers are often in a granular form that can easily wash away. The release of nutrients from these products can also occur more rapidly than the rate at which the tree can take up the added nutrients, allowing excess nutrients to escape into the atmosphere as gas (including as greenhouse gases), or to leach out beyond the active root zone into the broader environment.
High cation exchange capacity helps soil hold on to nutrients
Organic matter also has a high cation exchange capacity (CEC) which helps soil hold nutrients such as ammonium, potassium, calcium, and magnesium. Additionally, wood chips have a high carbon-to-nitrogen (C:N) ratio. This high C:N ratio in the woodchip blend used in this trial promotes nitrogen immobilisation (Chaves, De Neve, Boeckx, Van Cleemput & Hofman, 2007, cited in Al-Bataina, B.B., et al. International Soil and Water Conservation Research (2016), https://sci-hub.st/10.1016/j.iswcr.2016.07.003 )
The nitrogen in the wood chip blend is temporarily immobilised as organic matter is consumed by soil microbes during the initial decomposition process. As decomposition progresses and the microorganisms die off, the immobilised nitrogen is gradually released back into the soil where it can be taken up by plant roots.
This gradual release should reduce the leaching of nitrogen into groundwater. It should also reduce nutrient concentrations in surface water flowing off the orchard into rivers and estuaries.
Benefits of the trial expected to become more apparent in coming seasons
While early results have shown no significant differences in yield or nut quality between macadamias grown with organic soil amendments (compared to macadamias grown only with conventional chemical fertilisers), the research team expect the broader benefits of annual amendment applications will become more apparent in coming seasons.
In perennial cropping systems like macadamias, the full effects of soil amendments often take several years to become evident. This is due to the inherently slow and complex nature of tree physiological responses and soil ecological change.
Gradual improvements in tree health anticipated
Dr Suzy Rogiers, Plant Physiology Principal Research Scientist with the Department of Primary Industries and Regional Development (DPIRD), is working on the CCC soil amendment trial with Macadamia Development Officer, Jeremy Bright.
Dr Rogiers’ research focus is on key indicators of long-term system health, including:
- Soil biology and microbial activity
- Photosynthetic capacity and tree carbohydrate reserves
“These metrics will help determine the cumulative value of soil amendments in building more resilient and productive macadamia orchards,” said Dr Rogiers
“It is still early in the study and so far, no marked treatment effects on photosynthesis have been detected. Likewise, seasonal trends in plant non-structural carbohydrates were similar across the trial treatments.”
Orchard trees operate on much longer biological cycles than annual crops. Many of the soil improvements from organic amendments, such as increased microbial activity, enhanced nutrient cycling, improved soil structure, and better water-holding capacity, build gradually over time.
“Initially, organic amendments may not dramatically alter nutrient availability in the way that fast-acting synthetic fertilisers do. Instead, their benefits accrue through incremental improvements in soil health that support deeper rooting, better resilience to stress, and enhanced biological interactions in the rhizosphere,” said Dr Rogiers.
These improvements can eventually influence whole-tree processes such as photosynthetic efficiency, carbohydrate storage, flowering, and fruit set, but the lag time between initial application of organic amendments and the eventual results, may span multiple seasons.
Additionally, mature macadamia trees have large internal nutrient and carbohydrate reserves, which can buffer the tree against short-term changes in soil condition. As a result, physiological or productivity differences driven by soil management may be masked in the early years of a trial.
Impacts of extreme weather variation on trial progress
Environmental variability, such as the adverse weather experienced during the first two years of this trial (2023-24 and 2024-2025), can also overshadow treatment effects. This could further delay the emergence of a clear differentiation in the effects of different nutrition treatments, when comparing the trees treated with organic amendments, against the trees treated with chemical fertilisers.
The past two seasons have presented considerable challenges for growers. Adverse weather conditions have affected both yield and quality.
Encouragingly, the current 2025 -2026 season is shaping up to be more favourable for growers, offering a valuable opportunity to assess the CCC soil amendment trial under more typical growing conditions.
A more ‘normal’ year will provide critical insights into how soil amendments perform over time, particularly in relation to soil function, tree health, and long-term productivity.
Better soil structure and nutrient retention should reduce diffuse pollution downstream
“Organic soil amendments improve soil structure, water infiltration, and nutrient retention, while boosting microbial activity and reducing erosion in orchards,” explained Dr Rogiers.
These improvements typically reduce sediment and nutrient runoff into waterways, helping to prevent algal blooms and hypoxia. (Hypoxia occurs when the oxygen level in a water body such as a lake or river falls to less than 2-3 milligrams of oxygen per litre of water, which can cause stress or death in aquatic animals.)
Reduced sediment movement and erosion from agriculture should also improve water clarity in aquatic habitats like mangroves, saltmarshes, seagrasses, and coral reefs.
“Cleaner waterways support safe recreation, strong local fisheries, and healthy coastal ecosystems where wildlife and communities can thrive,” said Dr Rogiers.
The CCC project’s continued investment in this research is helping to build a stronger evidence base for sustainable practices in the macadamia industry and other orchard crop industries.
The CCC project is delivered through the NSW Department of Primary Industries and Regional Development and is funded through the NSW Government’s Marine Estate Management Strategy (MEMS) Water Quality Initiative.
The ten-year strategy was developed by the NSW Marine Estate Management Authority to coordinate the management of the marine estate. Find out more about the strategy and its projects on the Marine Estate website.
This article was compiled by Dr Suzy Rogiers - Department of Primary Industries and Regional Development (DPIRD) Principal Research Scientist, Jeremy Bright - DPIRD Macadamia Development Officer, and Meg Strang - DPIRD Clean Coastal Catchments Research Project Communications Officer.
Nutrient mobility references
- Claassen, V.P. and Carey, J.L., 2007. Comparison of slow‐release nitrogen yield from organic soil amendments and chemical fertilizers and implications for regeneration of disturbed sites. Land Degradation & Development, 18(2), pp.119-132: https://onlinelibrary.wiley.com/doi/epdf/10.1002/ldr.770
- Al-Bataina, B.B., Young, T.M. and Ranieri, E., 2016. Effects of compost age on the release of nutrients. International Soil and Water Conservation Research, 4(3), pp.230-236: https://sci-hub.st/10.1016/j.iswcr.2016.07.003
