Heavy November nut drop? The cause may be a hunger your trees don't show

Many macadamia growers follow a sound programme and still miss their yield, quality and crack-out targets. A KwaDukuza consultancy argues that the cause often starts in the root zone, with a hunger the tree does not show, and that a free field test is the place to start looking.

November is a testing month in a macadamia orchard. After flowering, the tree sheds much of its young crop. Some drop is normal, and pollination, heat, pests, water stress and crop load all play a part.

Two of those causes start below ground. A tree short of water at nut set drops nuts. A tree short of calcium or boron at flowering sets fewer and weaker nuts. Both depend on how well the roots can work, and both can be measured.

Most growers in KwaZulu-Natal and Mpumalanga lime, fertilise on schedule, spray on time and send leaf samples to the labor-atory. Yet many still fall short on yield and crack-out, the percentage of kernel recovered from nut-in-shell.

Mike Jackson, founder of Succession Soils in KwaDukuza, calls the cause hidden hunger. Jackson managed a commercial macadamia orchard from 2020 to 2025. Hidden hunger is a nutrient shortfall inside the plant that produces no visible symptom. The tree looks healthy, but it is short of some-thing it needs at the moment it needs it.

“A tree can be hungry for calcium and boron while it stands in soil that is full of calcium,” Jackson says.

“The soil report says you are covered. The tree is telling you something different, but only if you ask it the right way.”

Four layers, one diagnosis

Hidden hunger can start in the soil's structure, its biology or its chemistry. No single test sees all three. The first layer is the free Field Soil Health Assessor, which runs nine field tests of the soil's physical and biological con-dition. Four are physical: bare earth, water infiltration, the slake test, which times how long a dry clod holds together in water, and rooting depth. Five are biological: rhizosheaths, the soil that clings to fine roots, soil insects, earthworms, legume nodulation and soil smell.

The physical tests show whether water gets in and roots can reach it. That matters at nut set, when a water-short tree drops nuts, and at flowering, when calcium and boron arrive only in the water stream. The biological tests show whether soil life is cycling nutrients and keeping the soil open.

The other three layers measure chemistry in the laboratory. A soil analysis shows what is in the soil, not what the roots take up. Leaf tissue analysis shows what has built up in mature leaves over months. Plant sap analysis measures the nutrients moving in the leaf on the day of sampling.

Succession Soils compares young and old leaves. Mobile nutrients such as nitrogen and potassium move from old leaves to new growth. In morning samples on moist soil, a gap of more than 10% between them is an early warning of poor uptake. Calcium and boron travel only in the xylem, the water stream from the roots, and stay where they land. For these, the test is whether the young leaf is within range for the growth stage. A shortfall points back to the first layer: dry soil, compaction, root damage or low tran-spiration, the water loss through the leaves that pulls the stream upwards.

One block, four layers of data

Consider a six-year-old dryland macadamia block on the KwaZulu-Natal North Coast, at flower initiation. Soil and leaf samples were

Young macadamia nuts shed after flowering. Some drop is normal. Heavy drop is a reason to look at the root zone. Photo: Andrew Sheared

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taken in May, sap on 1 September and a field soil health assessment on 20 September.

Start with the spade. The free tests found half of the orchard floor bare: exposed soil with no living plants or mulch. Water soaked in at 306 mm per hour, so water gets in. The consultant's penetrometer, a steel probe that measures soil resistance, then found a hard layer at 165 mm, reaching 300 psi (2,070 kPa) in moist soil. Above it, the roots have a shallow zone to draw water from when a dryland tree is filling nuts through a dry spell.

The consultant's fuller assessment also measured the fungal-to-bacterial (F:B) ratio, the balance between fungal and bacterial biomass, at 0.3, and soil respiration, the carbon dioxide the soil gives off, at 0.33 mg per kilogram per hour. Neither has an agreed norm in macadamia orchards, so both serve as a baseline for measuring change.

Then the laboratory. Calcium filled 69% of the soil's nutrient-holding sites, so on paper it was plentiful.

Yet young-leaf sap held 855 ppm calcium, against the 1,000 to 1,400 ppm Succession Soils looks for at flowering. The calcium was in the soil, but too little reached the young growth where flowers form.

Boron, needed for pollen tube growth and flower set, was 8.22 ppm in young leaves against a flowering range of 25 to 45 ppm. Old leaves held 10.04 ppm, so the whole tree was short. Soil boron was not tested.

Nitrogen in the sap was almost all ammonium: 74.7 ppm against 5.5 ppm nitrate in young leaves. Ammonium com-petes with calcium for uptake at the root. Rain of 40 mm five days earlier may explain part of the ammonium, and it rules out dry soil as the cause of that day's calcium reading. That leaves compaction, weak biology and the ammonium load.

Biology before chemistry

“If a nutrient is missing from the soil, you add it,” Jackson says. “If it is there but compaction or weak biology is keeping it from the tree, more fertiliser costs money and does not reach the crop.”

The recommendation was to feed calcium and boron through the leaf during flowering, with ground cover, nitrogen form and the hard layer as longer-term work. The block will be sampled again, with soil boron added, and the results and nut set outcome reported in a future edition.

“I will not promise a grower a number,” Jackson says. “Yield, kernel quality and crack-out depend on cultivar, weather, pollination and pests as well as nutrition. What I can do is show where the tree is short, why it is short, and whether the fix is working.”

Start with a spade

The free Field Soil Health Assessor at fieldass.successionsoils.co.za runs the nine tests in 30 to 45 minutes per block. You need a spade or garden fork, a 150 mm diameter pipe 200 mm long with a mallet and wooden block, two 450 ml water bottles, a stopwatch or phone timer, a tape measure of at least 1.5 m, a glass of water, a dark tarpaulin or shade cloth, and a peg to mark the photo point.

Start with your best and worst blocks, while the soil is moist after spring rain. Each block gets a score out of 100, a band of Critical, Building or Func-tioning, and a list of what to fix first. The app and reports are free, and results are emailed to you. Scan the QR code to start.

FIND THE HUNGER YOUR TREES DON'T SHOW.

cross-checks every layer

Hidden hunger can start in the soil. A soil test shows what is in the soil, not what the roots can reach. The Field Soil Health Assessor runs nine field tests of your soil's structure and biology in 30 to 45 minutes per block. Laboratory soil, leaf and sap tests add the chemistry.

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Physical. Bare earth, infiltration, slake test and rooting depth show whether roots can get the water that carries calcium and boron at flowering and nut set.

Biological. Rhizosheaths, earthworms, insects, nodulation and smell show whether soil life is cycling nutrients.

Chemical. Laboratory soil, leaf and sap tests show which nutrients the tree is short of.

free, 30 to 45 minutes per block

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A score out of 100: Critical, Building or Functioning. Free report, emailed to you.

START WITH A SPADE.

fieldass.successionsoils.co.za

fieldass.successionsoils.co.za | info@successionsoils.co.za | +27 64 501 6603