The effect of micronutrient sprays on the leaf nutrient levels of Leucospermum and Protea

Micronutrient deficiencies, mostly zinc (Zn), manganese (Mn) and iron (Fe) are often noticed on Leucospermum and Protea shoots, mostly at the middle to end of the growing season (summer and autumn). Symptoms include interveinal chlorosis, smaller leaves and/or shortened internodes. The application of micronutrients is currently uncommon, and sprays are occasionally applied correctively in Proteaceae production, whereas in fruit production micronutrient sprays form part of a balanced fertiliser and foliar spray program. The rate of uptake of micronutrients into leaves is also unknown.

The aim of this trial was to evaluate the effect of micronutrient foliar sprays to selected Leucospermum and Protea cultivars during the vegetative growing season. For Leucospermum and Protea the sprays commenced when shoots were 5–10 cm long and were repeated twice, six weeks apart. The spray volume was 700 L per hectare for each application. The cost to the growers to apply these sprays was calculated, to assess the benefits and cost factor along with the improvement in visual shoot appearance, health and/or growth.

Leucospermum cultivars included in this trial was ‘Succession II’, ‘Soleil’, ‘Ayoba Sun’ and ‘Tango’. The All cultivars were grown on Piket-Bo-Berg with ‘Succession II’ and ‘Soleil’ treated at Pomona farm and ‘Ayoba Sun’ and ‘Tango’ at Bugler’s Post farm. Protea cultivars included was ‘Madiba’, ‘Susara’ and ‘Carnival’. ‘Madiba’ was grown at Bergsig farm, ‘Suara’ at Eureka farm outside Paarl, while ‘Carnival’ was grown at Klokberg farm in Paardeberg. Zn and Mn were selected as mineral element for evaluation. Treatment 1 was an unsprayed control. Treatment 2 was amino acid complexed Mn and Zn at the same rate as the traditional salt products (Table 1). Treatment 3 was amino acid complexed Mn and Zn at the supplier recommended rate (Table 1). Lastly, treatment 4 was a combination of straight products, zinc sulphate (ZnSO4) and manganese sulphate (MnSO4). Fulvic acid was added to Treatment 4. For all treatments the tank water was buffered to pH 5 using Aquaright 5 and a surfactant, Supawett, was added. Sprays were applied using a motorised backpack spray at 700L water/ha in a complete randomised block design with four blocks and five plants per block. Products were applied three times during late spring and summer. Leaf samples were collected on 17 April 2023 and sent to an accredited lab for nutrient analysis. Each sample consisted of four subsamples taken from each block. Samples contain at least thirty leaves Protea or 50 leaves for Leucospermum. Leaves were not picked from flowering shoots and was free from dust or chemical residue.

For Leucospermum cultivars (‘Ayoba Sun’, ‘Succession’, ‘Tango’, ‘Soleil’), zinc and manganese concentrations were measured across four treatments: control, amino acid chelated (recommended and higher rates), and salt. Control treatments generally exhibited the lowest zinc concentrations (10–16 mg.kg⁻¹), while both amino acid chelated and salt treatments produced higher zinc levels (27–33 mg.kg⁻¹), with the recommended amino acid rate showing the lowest values among these. ‘Tango’ demonstrated the highest zinc concentrations in amino acid treatments (27-29 mg.kg⁻¹). However, ‘Soleil’ was exceptional, with its control leaves showing the highest zinc concentration (25 mg.kg⁻¹) and all other treatments recording similar or lower values (15–22 mg.kg⁻¹). Manganese concentrations varied considerably across cultivars and treatments, ranging from 89 to 328 mg.kg⁻¹. ‘Succession’ showed the lowest manganese levels (89–107 mg.kg⁻¹), while ‘Ayoba Sun’ ranged from 148 mg.kg⁻¹ (control) to 263 mg.kg⁻¹ (salt treatment). ‘Soleil’ again had the highest control concentration (312 mg.kg⁻¹), and ‘Tango’ maintained consistently high values across all treatments (301–328 mg.kg⁻¹).

For Protea cultivars (‘Susara’, ‘Carnival’, ‘Madiba’), zinc levels in ‘Susara’ and ‘Carnival’ ranged from 4 to 16 mg.kg⁻¹, with controls generally lowest. ‘Carnival’ ranged from 4 mg.kg⁻¹ (control) to 16 mg.kg⁻¹ (higher amino acid). ‘Susara’ showed values from 10 mg.kg⁻¹ (control) to 13 mg.kg⁻¹ (higher amino acid). ‘Madiba’ exhibited substantially higher zinc concentrations (54–84 mg.kg⁻¹), with the control recording the highest value. Manganese concentrations in Protea ranged from 50 to 168 mg.kg⁻¹. ‘Carnival’ ranged from 78 mg.kg⁻¹ (control) to 100 mg.kg⁻¹ (salt). ‘Susara’ showed similar levels across treatments (50–66 mg.kg⁻¹). ‘Madiba’ recorded the highest manganese values, with 168 mg.kg⁻¹ in the control, followed by 125 mg.kg⁻¹ for salt and 63 mg.kg⁻¹ for recommended amino acid treatment. The recommended amino acid treatment (R37.75) and salt treatment (R29.53) had similar costs, while the higher amino acid rate was nearly double (R61.71).

‘Modern’ amino acid chelated foliar nutrients are not necessarily taken up better by plants when compared to ‘traditional’ salt of sulphate products with the addition of fulvic acid. This preliminary investigation shows that a thorough investigation into leaf norms and the uptake of different mineral nutrient products is warranted for Leucospermum, Protea and perhaps also Leucadendron.

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