An integrated approach to crop nutrition in macadamias

Tony Dyne, Sales Agronomist

Macadamias require relatively high quantities of the macronutrients, nitrogen, phosphorus, potassium, as well as strategic ‘top-ups’ of calcium, boron, magnesium and zinc, throughout the growing season for optimal yield. Any deficiency of these nutrients, even for a short period of time, can adversely affect yield and harvest quality.

Macadamia trees are different from most nut-producing trees in that they are evergreen and have several vegetative flushes per year, with peaks in spring and late summer. Most of the feeder roots of macadamia trees are in the top 30 cm of soil, meaning fertigation is an ideal option for many nutrients. Conversely, some nutrients are relatively immobile in the soil or the plant, meaning foliar application is the most appropriate method of delivering these nutrients to where they are needed.

Nitrogen has a large influence on canopy health, crop vigour, nut yield and oil production. Nitrogen requirements are based on crop demand, which in turn, is driven by the previous crop yield. As a guide, about 10.5 kg of nitrogen should be replaced for every tonne of nut in shell (NIS) harvested.

It is also important to consider nitrogen form, application rate and timing. Supplying nitrogen as nitrate can improve nitrogen use efficiency by reducing volatilisation, leaching and soil acidification. The application of nitrogen during the oil production phases can encourage excessive vegetative growth and reduce nut yield and oil production.

Phosphorus is essential for the production of carbohydrates and energy transport, and thus supports root and plant growth, flower initiation and nut set. Soil phosphorus levels need to be maintained at about 85 to 100 mg/kg in most soils. As a guide, about 1.23 kg of phosphorus should be replaced for every tonne of nut in shell harvested.

Potassium is needed for starch formation, oil production, translocation of sugars and to regulate turgor pressure (i.e. the opening and closing of stomata). Mild to moderate potassium deficiency is expressed as slow growth, which reduces yield potential, while severe potassium deficiency causes leaf margin necrosis of older leaves.

Macadamia trees have high requirements for potassium, with 9 kg removed for every tonne of nuts harvested. Peak demand occurs during nut growth and oil accumulation but due to the high mobility of potassium in the trees, it can be readily re-translocated from other plant parts to supply requirements.

Calcium plays a critical role in maintaining cell membrane integrity and cell wall strength. Deficiencies are usually found at the growing points because calcium is immobile within the plant.

Calcium uptake by plants is dependent on new root growth and effective transpiration. This means it is important to match calcium applications with new root growth to maximise calcium uptake. The form of calcium applied is also important, as water solubilities vary greatly.

Magnesium is required for many plant processes, particularly chlorophyll production, which drives the plant’s ability to produce energy for growth, nut development and oil production.

Magnesium is relatively mobile in the soil and the ratio between calcium, potassium, magnesium and ammonium (NH₄) availability needs to be managed to ensure adequate nutrient availability. Heavy rainfall events or fertiliser programs that oversupply sulphate sulphur will decrease magnesium availability.

Boron is critical for cell wall development, cell division, sugar transport and hormone development, and thus is important during pollination, seed set and nut development. Boron is relatively immobile in macadamia trees, meaning a combination of soil and foliar application is required to ensure adequate plant tissue levels are maintained.

Zinc plays an important role in nitrogen metabolism and amino acid production required for growth, flowering and seed set. Zinc deficiency is common in sandy soils containing low organic matter, alkaline or high pH soils or those receiving heavy applications of manure. Foliar application is an efficient method to supply zinc and maintain adequate levels.

Yara markets a comprehensive range of high quality fertilisers that can adequately supply the nutritional requirements of macadamias. These include YaraRega and YaraTera water-soluble NPK fertilisers, YaraLiva calcium nitrate granular fertilisers and YaraVita foliar micronutrients.

Depending on leaf analysis, a typical program for macadamias starts with the application of YaraLiva Nitrabor (80–100 kg/ha) at flowering to supply a rich source of nitrogen, calcium and boron.

This is followed by applications of YaraTera REXOLIN APN (2–4 kg/ha), YaraRega 13-2-21 (100 kg/ha as one or two applications) and YaraVita ZINTRAC (1–2 L/ha) during the spring flush.

YaraTera REXOLIN APN is a high quality combination of chelated micronutrients, including boron, copper, iron, manganese, molybdenum and zinc. It is ideal for application via fertigation in combination with YaraRega, which supplies highly soluble forms of nitrogen, phosphorus and potassium. YaraVita ZINTRAC is a highly concentrated flowable formulation of zinc that is easy to  measure, pour and mix.

Subsequent applications of YaraRega 9-0-30 (50 kg/ha) and YaraRega 13-2-21 (100 kg/ha as one or two applications) are made during the nut growth and summer flush periods, respectively.

Finally, YaraVita BUD BUILDER FL (10 L/ha) is applied as a foliar spray during nut drop or after harvest to ensure sufficient levels of boron, magnesium and zinc are present to enhance flowering and fruit set in the following season. 
Any crop nutrition program must be supported by good orchard management practices, such as irrigation scheduling, orchard floor management and effective pest, disease and weed management.

Yara provides technical support to growers and their advisors via a team regionally-based sales agronomists and customer service representatives. Our goal is to deliver integrated crop nutrition solutions, backed by innovative decision-making tools, that help growers to get the best return from their investment in quality crop nutrition.