The macadamia tree, Macadamia integrifolia, is considered an ‘ecological giant’ in commercial agriculture in the horticultural sector. This may be ascribed to its efficiency to optimise water consumption, sequester carbon from the atmosphere, absorb phosphates and recycle organic waste material.

Macadamias are not heavy-feeders and the fine lateral feeder roots have the ability to absorb phosphates and calcium from soils, even in low concentrations. There is scope, therefore, for more ecologically friendly fertiliser applications.

Preliminary research findings indicate the average macadamia orchard absorbs more than 17 tonnes gross, and 14.5 tonnes net, of carbon per hectare per year from the atmosphere. Consequently there is a need to adopt sustainable on-farm practices to meet market demands, address environmental concerns, and simultaneously comply with the Kyoto Protocol of 2005 to limit carbon dioxide emissions.

This could lead to a reduction in the application of high dosages of fertiliser, excessive irrigation, and carbon dioxide emissions.

The demand for, and export of, macadamia nuts has increased significantly for South African growers over the past ten years. However, the quality of nuts, crackout percentages, yield and overall performance of trees, have not been altogether satisfactory. Since good soil health is fundamental to productive agriculture and may play a role in the long-term sustainability of macadamia orchards, scientists are studying various aspects of the soil in these orchards.

Focus shift

Focus has shifted to the state of the soil food web, the carbon and photosynthesis cycles, and the role of the food web in root nutrition and tree productivity. The food web is critically dependant on the regular addition and formation of organic matter as a source of organically active carbon and energy. Inadequate addition and formation of organic matter is responsible for a less complex and therefore depleted food web, with a concomitant negative effect on soil health and root nutrition. Well managed self-sustaining macadamia orchards will represent efficient micro-ecosystems, characterised by efficient nutrient cycles, sustained carbohydrate energy flows, stability, and provide protection against extreme climatic shifts.

The importance of balanced healthy soils for optimum tree performance and nut quality

Macadamia trees in their natural habitat, have a preference for frost-free coastal rainforests with only slight temperature shifts, grow close to rivers and in fertile volcanic soils rich in humus. Their fine feeder roots require large quantities of composted organic material for optimum growth. Healthy soils should contain complex and balanced microbial populations, responsible for the degradation and mineralisation of complex soil organic matter, and the retention of active carbon in the root zone. Restoration of the microbial physiological activity in orchard soils is a prerequisite for the sustainable nutrition of the root system, the retention of carbon, and providing resilience against extreme shifts in climate. The regular application of tree waste material as compost under the tree canopy provides organic carbon, improves root nutrition, and fuel for the microbiota in the soil food web.

The ideal would be to mimic conditions in the natural habitat and the forest floor under the tree canopy in the following way:

  • Orchard soil should be ripped to a depth of one meter before young trees are established to improve air penetration.
  • Application of a mulch, either as a plastic cover or composted organic mulch, to retain soil moisture and reduce weed growth.
  • Application and addition of organic material, to supplement organic and active carbon, which in turn supports and enhances feeder root growth.

 

Photo B: Macadamia orchard showing luxurious leaf growth on trees without symptoms of iron deficiency (Photo by P. van Niekerk).

 

Recommendation for growers:

  • Utilise all organic material that is available in orchards, including:
    • i) external husks and/or shells;
    • ii) carbonised husks (biochar);
    • iii) tree prunings;
    • iv) leaf litter;
    • v) composted cattle or sheep manure; and
    • vi) compost enhancing microbes.
  • Prunings, woodchips and husks should be milled and crushed as much as possible, to accelerate the composting process.
  • Apply husk, biochar, leaves, prunings and enhancer microbes as a mulch under tree canopies, with regular irrigation to promote the composting process.
  • Add composted cattle or sheep manure in small heaps, in the irrigation zone, under the tree canopy. Regular, but moderate, irrigation in the drip zone should enhance the composting process.
  • Establish grass cover between tree rows and maintain composted organic material under trees in the irrigation zone.
  • Growers should be observant, keeping an eye open for the formation of humus, which is the end product of composting on the orchard floor. Humus consists of stable organic matter with a dark brown to grey colour, a porous consistency, and a pleasant earthly smell.

 

Important factors to consider with regard to the iron-deficiency syndrome:

  • Avoid phosphate applications with a high dose of >30-50 mg/kg (Bray I), this may cause phosphate toxicity and contribute to the iron-deficiency syndrome.
  • Un-composted poultry manure is a common source of high soil phosphate levels, which could contribute to an iron-deficiency syndrome.
  • Macadamia trees are adapted to growing in acidic soils with low phosphate levels, and have developed fine feeder roots to increase absorption where there are low levels of phosphates. In soils with excess phosphates this ability may lead to iron deficiencies.
  • Test soils for high pH levels and adjust to 5, 5 – 6.0 pH (H20), or 4, 5 – 5, 0 pH (KCI). A combination of high pH and high phosphate levels will cause severe iron-deficiency symptoms.

 

Benefits of composting:

The importance of stable soil organic matter in the topsoil of orchards:

  • A stable soil structure will eliminate sub-surface compaction, soil crusting and water run-off, all of which prevent roots from penetrating fertile soil. Improved soil structure will result in sustainable water consumption and overall soil fertility.
  • Will improve the soil structure by increasing pore spacing and the creation of aggregates.
  • Provide improvement in the drainage, air and water penetration in the soil.
  • Water holding and buffer capacity of the soil will improve.
  • The nutrient recycling rate and the provision of nutrients for microbial and root metabolism will be accelerated.
  • There will be an improvement in the cation exchange capacity, as well as an increase in the retention of active carbon.
  • Stable soil organic matter will supplement carbohydrate reserves to support kernel oil formation.
  • The increase and maintenance of high levels of organic matter will ensure maximum nutrient cycling by:
    • i) increasing biological activity and biodiversity;
    • ii) increasing organic carbon levels;
    • iii) improving nutrient retention and re-cycling;
    • iv) improving root health due to better soil structure;
    • v) improving water infiltration, water retention and drainage; and
    • vi) the release of phosphorus.

Photo A: Macadamia orchard revealing an absence of composted organic material on the orchard floor and symptoms of iron deficiency (Photo by P. van Niekerk).

This research paper was authored by W.J. Botha & G. Nortje. W.J. Botha from the Agricultural Research Council, Plant Health and Protection, Biosystematics Division.

Dr Gerhard Nortjé, Environmental Sciences/Soli Science, Department of Environmental Sciences can be contacted at 083 501 8680 or by email at nortjgp@unisa.ac.za