Long-term trials of organic soil amendments in macadamia production were established by the Clean Coastal Catchments (CCC) Research project in 2023.

The trials have now been underway for almost 18 months at the NSW Centre for Tropical Horticulture near Alstonville.

The objective is to examine the nutritional contribution of the amendments to the trees and the soil. This analysis will determine if synthetic fertiliser applications could be reduced, or even replaced, by the addition of organic amendments. 

Reducing the use of synthetic fertilisers in tree crop farming could have significant benefits for environmental health.

Excessive use of synthetic fertilisers can lead to nutrient run-off, resulting in nitrogen and phosphorus being washed into sensitive creeks, rivers and estuaries. Nutrient pollution has the potential to cause algal blooms that deplete oxygen levels in aquatic ecosystems, which is detrimental to fish and other marine life.

Organic materials release nutrients more slowly than synthetic fertilisers, reducing the risk of leaching and runoff.

By replacing or reducing synthetic fertiliser with organic soil amendments, growers may be able to mitigate harmful nutrient pollution effects on the environment.

The nature and form of the organic amendments applied in this trial (a blend of manure and woodchips) are more likely to stay in place in the soil, particularly as the roots of the macadamia tree grow into the soil amendments.

In contrast, synthetic nitrogen fertilisers are often in a granular form which can wash away, escape into the atmosphere as gas (including as greenhouse gases), or leach out beyond the active root zone.

Through leaf, wood carbohydrate, and soil biology sampling, combined with ongoing monitoring of nut quality and yield at harvest, the CCC soil amendment trials are building a solid base of information about the relationship between production, soil and tree health, and nutrition (whether delivered by synthetic fertiliser or organic soil amendments).

The first round of preliminary results from the trial are expected to be released in late 2025.

Trial treatments

The organic soil amendment used in the CCC Research trials consists of a blend of manure and woodchips.

The synthetic fertiliser used in the trials is known as North Coast Macadamia Mix, a fertiliser product that is commonly applied to macadamia orchards by growers in northern NSW.

This product contains nitrogen, potassium, phosphorus, boron, sulphur and micronutrients.

For each of the six treatments, there are three replicates, with four trees in each replicate (adding to a total of twelve trees per treatment spread across two macadamia orchards at the Centre for Tropical Horticulture).

The trials commenced with the first soil amendment application in September 2023, followed by a second application in September 2024. An amount of 100 kg of organic soil amendment per tree was supplied on both occasions for all trees in the trials, except for the control treatment which received only synthetic fertiliser.

The synthetic fertiliser mix has been applied quarterly to relevant treatments since September 2023. The 100% rate of synthetic fertiliser for these trials consists of 620 grams of North Coast Macadamia Mix, supplied in 4 applications per year for an annual total of 2480 grams.

Trial progress

The CCC soil amendment trials were established in 2023 by macadamia development officer, Jeremy Bright from the Department of Primary Industries, Research & Development (DPIRD).

In 2024 DPIRD Plant Physiology Principal Research Scientist, Suzy Rogiers, joined the CCC Research team to assist in ongoing trial management and analysis of laboratory results.

Regular leaf nutrient, soil chemical, soil biology, and tree wood total non-structural carbohydrate (TNSC) samples are collected and submitted for laboratory analysis by independent horticultural consultant, Scott Hills from Healthy Hills Horticulture.

Annual nut yield and quality data at harvest are monitored for each of the treatments.

Given the slow rate of change expected in soil and tree health following the application of organic soil amendments, the research team anticipates it may take three to four years of data collection before differences start to emerge between each of the six treatments.

Agricultural and environmental benefits of organic soilamendments 

Organic matter creates improved soil structure allowing for better air circulation around the roots. This improves root respiration and helps prevent soil compaction. Organic matter also helps bind soil particles together, reducing the risk of erosion by water and wind.

Additionally, organic matter increases the soil's ability to retain moisture, which is beneficial during dry periods, reducing the need for more frequent irrigation.

One of the most significant advantages of organic amendments is their positive effect on root development. Active roots are more efficient at nutrient and water uptake. Organic matter also promotes the activity of beneficial soil microorganisms, which play a crucial role in breaking down organic matter and releasing nutrients in forms that plants can absorb. Increased microbial activity helps improve nutrient uptake, particularly for essential elements like calcium, magnesium, and trace minerals which are vital for overall tree health and productivity.

By improving soil fertility and water retention, organic amendments help ensure consistent nutrient availability, reducing the risk of nutrient deficiencies. Nutrients are involved in key processes like chlorophyll production which is vital for photosynthesis. Moreover, good soil with the right pH, organic matter, and nutrient levels strengthens tree immunity and resistance to pests, diseases, and environmental stresses such as drought or flooding.

The type of organic matter applied to an orchard can have a significant influence on productivity, due to as differences in nutrient content, decomposition rate, and impact on soil health.

Some organic materials are rich in specific nutrients that may be lacking in the soil, while others might contribute to improving overall soil fertility as the organic material breaks down. For example, animal manure (such as cow, chicken, or horse manure) is often rich in macronutrients such as nitrogen (N),phosphorus (P), and potassium (K), as well as trace minerals.

Compost is a more balanced source of nutrients, compared to fresh manure, with a relatively lower concentration of nitrogen, but a broad spectrum of essential nutrients. Compost improves soil microbial activity, nutrient retention, and soil structure.

Organic materials like fresh grass clippings and manure tend to decompose quickly, releasing nutrients faster than aged compost. This can be beneficial for crops that need a quick supply of nutrients, however more frequent applications may be required. 

Materials like straw, and wood chips decompose more slowly. While they provide long-term improvements to soil structure and moisture retention, they might not supply nutrients as quickly as fast-decomposing organic matter. However, they are essential for long-term soil fertility and building organic matter into soil structure.

The type of organic matter also affects the cation exchange capacity (CEC) of the soil, which is the soil’s ability to hold onto essential nutrients like calcium, magnesium, and potassium. Organic matter with higher CEC improves the soil’s nutrient-holding capacity, reducing nutrient leaching and improving overall soil fertility.

A blend of manure and woodchip has been used in the CCC soil amendment trial, as this blend offers both short- and long-term release of nutrients, binds well to prevent erosion, and is widely available in macadamia growing regions.

Beyond the benefits of organic amendments for soil health and production in the orchard, there are wider environmental benefits that should be considered by growers interested in ensuring ongoing social licence for agricultural production in coastal regions. Minimising the application of synthetic fertilisers in tree crop orchards has the potential to reduce environmental pollution.

Overuse of synthetic fertilisers can result in surplus nitrogen and phosphorus flowing into coastal waters where excessive nutrient concentrations can damage aquatic habitat, including valuable marine animals and plants. Organic materials, onthe other hand, release nutrients at a slower rate compared to synthetic fertilisers, thereby lowering the risk of nutrient leaching and runoff.

Farmers play a vital role in maintaining water quality in our coastal catchments. We hope that the outcomes of this research will help the macadamia industry fine-tune orchard management strategies to protect water quality while maintaining productive, high-quality crops.

Preliminary results from the CCC soil amendment trials are expected to be released in late 2025. Data will continue to be collected through till 2028 when the CCC Research project comes to an end.

The CCC Research project is delivered through the NSW Department of Primary Industries and Regional Development is funded by the NSW Government under the Marine Estate Management Strategy (MEMS). 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 NSW MEMS and its projects on the Marine Estate website.