Chelated Foliar Nutrition Products

Factors that lead to phytotoxicity

According to Santos et al., (2017), the phytotoxic effect of compounds is directly related to the environmental conditions at the time of application. In the case of mineral oils, it is specifically the level of radiation, e.g., the higher the light intensity, the higher the risk of phytotoxicity. Furthermore, highly concentrated sprays, especially those including salt-based fertilisers, can potentially cause leaf burn and/or drop (Qureshi & Stansly, 2021). The risk of phytotoxicity is higher when salts with a low point of deliquescence (POD) are used. This is because residues of salts with a low POD on the leaf cuticle will dissolve at lower humidity levels, raising the risk of its concentration becoming excessive (Schönherr, 2001). See Table 1 for PODs of commonly used foliar nutrition formulations.
Formulations of mineral element solutions that include adjuvants can improve the uptake and/or bioactivity of the nutrients applied to the foliage. The risk of it causing phytotoxicity might, however, be either decreased or increased. So, when a foliar nutrient formulation is developed, fine-tuning of the nutrient active ingredients, the adjuvant compounds, and their relative concentrations are necessary to give reproducible uptake without damage to the plant. Foliar nutrition manufacturers cannot theoretically predict the performance of a mineral nutrient element in combination with a particular adjuvant. Due to the risk of phytotoxicity to leaves, they should submit every new formulation to rigorous empirical testing. Therefore, citrus producers should only use products and formulations of which the efficacy and optimal application rates have been established through proper experimentation for use on citrus trees.

Guidelines for foliar applications that maximise uptake of nutrients and minimise the risk of plant phytotoxicity

• Adjust the pH of the spray solution An acidic pH of the foliar nutrient spray solution improves the leaves’ absorption of nutrients. A solution pH of 5.0–6.5 is suitable for most foliar applications (Coetzee, 2007). However, the optimal pH will vary between nutrients. For example, the optimal pH range for phosphorus application is 3.0 to 3.7, while the highest uptake rates by citrus leaves of foliar urea were at a solution pH of 5.5 to 6.0 (El-Otmani et al., 2000). Acidification is not always required and might even be detrimental (always read the product label carefully). Also note that residues remaining on the leaves might result in phytotoxicity if the pH of a future spray (applied within approximately seven days) is not suited.
• Use of surfactants A good surfactant will function as a spreader, reducing the surface tension of the spray solution. This will spread the product across a larger leaf area. A wide range of surfactants is available for foliar fertilisation that might affect citrus cultivars, according to the epicuticular and cuticular wax composition of each one. Therefore, selection of a surfactant for regular commercial foliar application of mineral nutrients should be done with caution (Singh & Khan, 2012).
• Ensure compatibility of product mixtures Take note of the product specifications from the manufacturer. When combined with some adjuvants and plant protection products, the performance of foliar fertilisers can change. Use a proven combination, since the relative efficacy of a foliar-applied nutrient-adjuvant-agrochemical mixture cannot be predicted. With no specific information on the label, small quantities of the products should be mixed with water in a jar and sha-
  ken to determine the risk of precipitation. Any new mixture or individual product should be sprayed on a few trees a week, before the intended commercial treatments. Given incompatibilities among various products, combining foliar fertilisation with pesticide is discouraged. Therefore, the product labels must be carefully read to assess compatibility. For good practice, avoid combining a high rate of any fertiliser with summer spray oil and emulsifiable concentrate forms of pesticides (Qureshi & Stansly, 2021).
• Apply foliar nutrition in suitable
  environmental conditions
  A contact period of at least 15–20 minutes is ideal for uptake into the leaf. The lower the temperature during application, the better. Do not apply foliar nutrition when the ambient temperature exceeds
  30 oC or the wind speed exceeds 15 km/hour. This will avoid increased susceptibi-lity of the leaves to a mixture of high concentrations that might otherwise not harm the leaves. Under these conditions, as a spray solution dries on the leaf surfaces, the applied salts’ (foliar fertilisers) concentration increases and may damage leaves. Avoid early morning sprays in areas where the ambient temperature rises rapidly. Night foliar sprays are more effective with lower night temperatures and higher relative humidity (RH). It is always safer to apply foliar sprays at night or late afternoon (Woolfolk et al., 2002).
• Limit spray volumes Avoid spraying the leaves to the point of runoff. The accumulation of the nutrient mixture on the leaf tips and edges will result in necrosis in these areas when the solution dries.
• Do not exceed concentrations of spray mixtures beyond the safe limit Concentrated sprays might scorch leaves. Trees in humid climates generally have thinner cuticles, which makes them more susceptible to phytotoxicity. So spray mixture concentrations must be considered more prudently in these areas. Evaluate new products to know if a foliar spray contains sufficient nutrient, to obtain – cost-effectively – a concentration in the tree that will result in a positive plant response. Refer to Table 1 for the required concentrations at which nutrient elements should be applied to be effective as foliar sprays. Any new product can be evaluated according to these guidelines, i.e., to be effective, after diluting the benchmark chemical, per label specifications, a product must have a nutrient concentration of at least 80% of the stipulated norms in Table 1. So, although additives can improve absorption, unless the nutrient concentration is ≥80% of the benchmark concentration, proof of any claim of improved or effective uptake must be available (Coetzee, 2007). The mixture provided in Table 2 may be used safely if application of multiple micronutrients is required.
• Avoid applications during sensitive phenological stages, i.e., full bloom to fruit set, and on late mandarins, after colour break.

Further comments to consider

• A new mixture or product must always first be tested for possible side effects or phytotoxicity by spraying a few trees a week before the intended commercial treatments.
• Avoid mixing copper oxychloride with zinc or magnesium nitrates – the last two are usually buffered in an acidic medium, which will dissolve more copper from the copper oxychloride suspension and increase the concentration of soluble copper to phytotoxic levels. As an after-petal-drop foliar nutrition application, during which copper oxychloride might also be sprayed, it is suggested that ZnSO4 or MgSO4 is used instead (Coetzee, 2007).
• Foliar sprays containing >2 000 mg Cu/L water are potentially dangerous and could be toxic, causing leaf burn and leaf drop. Suspensions, e.g., copper oxychloride and copper hydroxide, contain more total Cu than soluble or chelated products. This increases the risk of dark blemishes. However, any copper product can darken blemishes on the rind of citrus fruit. Even on mature fruit, Cu sprays will accentuate any fresh blemish. It is recommended that Cu foliar nutrition should ideally be done postharvest, together with the LB urea application. On later mandarin cultivars, e.g., Nadorcott/Tango, it can be applied on small fruit, after 100% petal drop.
• Never mix Borates and sulphur containing chemicals with any type of oil (Coetzee, 2007).
• Avoid applying oils four weeks after sulphur-containing products had been sprayed (Coetzee, 2007). Also, refrain from spraying any product within seven days after foliar nutrition was applied – re-wetting the residues on the leaves can cause scorching, especially if the pH of the applied product is not appropriate for the product already on the leaves (C. Fraenkel – personal communication).
• MnSO4 is compatible with MgNO3, sodium borate and urea.
• Dursban and boron foliar fertilisers are not compatible (Coetzee, 2007).
• Due to the risk of fruit granulation of certain mandarins, KNO3 should not be applied within three weeks of an application of Corasil-P®/Maxim®.
• Importantly, do not apply a foliar product containing phosphate (P) on late mandarins at, and after colour break (Marais 2017).
• After spraying, rinse the sprayer and all its parts thoroughly with clean water.

Conclusions
When a foliar nutrient is applied, various factors that lie beyond the physical-chemical properties of the fertiliser formulations and the effectiveness of treatments also apply. The prevailing water pH, temperature, humidity, and leaf age/surface properties in different plant production locations may play a significant role in the performance of foliar nutrient sprays, or the risk of phytotoxicity. It is important to develop a thorough control system, and to record spray conditions on the farm, to reduce the risk of human error when applying foliar spays, e.g., applying them under low relative humidity, high temperatures, at high wind speeds, and when stomata are closed.
Fully consider the interaction of the environmental conditions, the product and the plant, to avert the risk of phytotoxicity from chelated and non-chelated products. Notably, the proven efficacy of mineral nutrient salts (see Table 2) and their very low risk of phytotoxicity and cost-effectiveness make them ideal for obtaining the desired horticultural impact in your fruit tree.

Acknowledgements

We recognise the significant inputs of: Hannes Coetzee, Paul Cronjé, Tim Grout, Hannes Bester, Coenraad Fraenkel, Andre Combrink, Jan van Niekerk.

References

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