Interveinal yellowing in a Gold3 kiwifruit block
A 5.5 ha block of 6-year-old Gold3 kiwifruit on a free-draining sandy loam has shown interveinal yellowing on spring flush leaves across about 30% of vines, worst in the wettest rows. Soil test (0–15 cm) shows pH 5.4, Olsen P 12 mg/L, exchangeable K 0.35 me/100g, organic matter 3.2%, and high Mg. Leaf tissue from affected vines has N 2.1%, K 1.0%, Mg 0.25%. The orchard received 180 kg N/ha as urea in August and 60 kg K/ha as SOP. Irrigation is by micro-sprinkler at 4 mm/day; drainage is impeded in the worst rows after 25 mm rain. The packhouse has warned that green fruit colour and dry matter may be affected. Vines are otherwise vigourous, with strong shoot growth.
1. Identify the most likely cause of the yellowing and explain why the soil and leaf data support your diagnosis rather than a simple nitrogen deficiency.
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The most likely cause is magnesium deficiency induced by waterlogging and low soil pH, not nitrogen deficiency. Leaf Mg at 0.25% is below the usual adequacy range for kiwifruit, while leaf N at 2.1% is adequate and 180 kg N/ha was applied in August. Interveinal yellowing on spring flush leaves is consistent with Mg shortage because Mg is mobile and moves to growing points. The soil has high Mg, so the problem is uptake, not supply: pH 5.4 reduces root function, and impeded drainage in the wettest rows causes root hypoxia, limiting Mg uptake. Vigourous shoot growth also dilutes Mg in the leaves. Low exchangeable K (0.35 me/100g) and leaf K (1.0%) suggest a K–Mg imbalance may worsen the symptom, but waterlogging is the primary driver.
2. Compare two interventions-foliar magnesium sulphate versus improving drainage-and recommend a sequence.
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Foliar magnesium sulphate gives rapid symptom relief because Mg is taken up through leaves, but it is temporary and does not correct the root-zone cause. Improving drainage-installing mole and tile drains or lifting the worst rows-addresses the hypoxia that is blocking Mg uptake, but its effect on this season's canopy is slower. The sequence should be: first identify and fix the drainage in the worst rows before winter; second, apply foliar MgSO₄ at 2–3 kg/ha to affected vines at 10–14 day intervals for two sprays to protect fruit colour and dry matter; third, apply lime to raise pH toward 6.0 and retest soil and leaf tissue next season. Do not apply more Mg fertiliser to soil until drainage is corrected, because the soil already tests high in Mg.
3. Explain how the orchard's irrigation management may be contributing to the problem and propose a monitoring trigger.
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Applying 4 mm/day by micro-sprinkler regardless of rainfall is likely over-irrigating the wettest rows, keeping the root zone saturated and worsening hypoxia. Irrigation should be scheduled from soil moisture, not a fixed daily rate. Install tensiometers or capacitance probes at 15 cm and 30 cm in good and poor rows. Trigger irrigation only when the 15 cm reading falls below about 60% of field capacity and no rain is forecast. In the impeded rows, reduce the irrigation rate or isolate those rows until drainage is improved. Record rainfall and drainage so the block's water balance can be reviewed at the end of the season.