What are the impacts of foliar fertiliser?Research & Extension
Dr Gordon Brown reviews a research paper that indicates early spray application of copper sulphate or potassium nitrate to pears improves yield, fruit quality and reduces disease incidence.
In an environmentally conscious world, where nutrient runoff into the environment is not desirable, efficient ways of applying fertilisers to apple and pear trees are required. With more efficient fertiliser application, less fertiliser is needed for optimal growth, leading to reduced fertiliser costs. One of the most efficient methods of applying fertilisers is by spraying the foliage where it can be absorbed directly into the leaves where it is needed.
Research has been conducted in Pakistan on the impact of foliar fertiliser application on the yield and fruit quality of European pear trees. This research used a single spray application, just after fruit set, of potassium nitrate (KNO3) and/or copper sulphate (CuSO4) in an effort to also reduce the need for synthetic fungicides. Potassium is essential to regulate water uptake and also increase the trees’ resistance to pests and diseases. Copper, as well as being an excellent antimicrobial compound, plays an essential role in photosynthesis and several enzymes. For my PhD I studied the impact of copper nutrition on cherries and apples where I found that copper application increased cell wall cellulose, pectins and lignins. This was due to increased supplies of photosynthate leading to firmer fruit more resistant to rain-induced cracking and bruising supporting the concept being investigated in this Pakistani research.
The potassium nitrate was applied as a 0, 1, 2 or 3 per cent solution and the copper sulphate was applied as a 0, 0.2, 0.4, 0.6 or 0.8 per cent solution. All possible combinations of KNO3 and CUSO4 were applied in 240L/ha of water. This means that at the highest rate of KNO3, 312g/ha of nitrogen and 1080g/ha of potassium were applied. Assuming that copper sulphate pentahydrate was used, this represents 104g/ha of copper and 250g/ha sulphur for the 0.8 per cent solution. In this article I will only report the results for the highest rates of application of both compounds.
Copper sulphate applied by itself is very phytotoxic, causing severe leaf burning, which is why it is mixed with lime in traditional Bordeaux mixture. No mention of foliar damage is made in this paper; however, care would be needed if repeating this treatment to ensure no foliar damage is encountered. The potassium nitrate probably did not cause any damage to the foliage of the trees. I apply a saturated solution of KNO3 (approximately 12 per cent) to my tea plants on a regular basis, which causes the leaves to turn a healthy deep green.
Both KNO3 and CuSO4 increased yield per tree by about 30 per cent (Figure 1,) indicating that the foliar application of a small quantity of these materials soon after flowering alleviates a deficiency of these nutrients in the tree in spring. The quantity of nutrients supplied was extremely small compared to normal ground-based applications, demonstrating the efficiency of foliar nutrition. As mentioned earlier, CuSO4 has potential to burn the leaves such that these results suggest that if the leaves were damaged the effect was temporary.
The impact of KNO3 application
The application of 3 per cent KNO3 soon after flowering had a positive effect on all the fruit quality parameters measured (Table 1). Fruit weight, firmness, sugars and vitamin C were all increased, while fruit acidity and disease incidence on the fruit was reduced. In the introduction the authors state that an aim of the research was to investigate if the use of KNO3 could reduce the need to use synthetic fungicides for the control of black spot. The results for incidence of diseased fruit support this hypothesis. Unfortunately, the diseases observed on the harvested fruit were not identified.
How the KNO3 reduced the incidence of disease on the fruit was not explored. In addition to direct antimicrobial action of KNO3 on the disease, the treatment may have also improved the rate of photosynthesis, providing energy for cell wall building and defence and the production of plant phytoalexins such as polyphenols and terpenes. This mode of action is desirable as the disease control is not targeted, making disease resistance, prevalent with synthetic fungicides, less likely.
The impact of CuSO4 application
The results for the effect of CuSO4 on harvested fruit mimic those of KNO3 in that all fruit parameters were improved (Table 2). The superior control of diseases on harvested fruit for the CuSO4 treatment compared to the KNO3 treatment is not surprising as copper has been used for a long time for the control of black spot in apples and pears, as well as many other plant diseases.
It is a shame that no information on crop tolerance to the treatment was given as copper sprays are known to damage apple and pear foliage and to russet fruit. Raw CuSO4 is very damaging to most foliage and, normally, other less damaging forms of copper, such as copper oxychloride, copper hydroxide or Bordeaux mixture are used. To further ensure no damage to apples and pears, copper sprays are normally applied prior to budburst to avoid tree and fruit damage. Another issue with copper is that it is a heavy metal so levels in the soil will build up over time and may eventually become a problem.
Copper, however, remains an essential element that most plants have a growth response to when applied to the foliage. Copper is tightly bound to the roots and, even in soils where copper toxicity occurs, relatively little makes it up to the leaves where it is required for photosynthesis. When applying copper hydroxide at the recommended label rate for apples and pears, up to 800g/ha copper will be applied to the crop. In the trials reported here, a total of close to 100g/ha of copper was applied, representing a significant reduction in copper being released into the environment.
This research paper identifies a potential to use KNO3 and/or CuSO4 for reducing the incidence of black spot on pears. If the CuSO4 treatment provides similar control of black spot to the standard copper fungicide spray program, then this treatment can potentially replace that program and dramatically reduce copper contamination of the soil. The lack of any observations on foliar or leaf damage for CuSO4 is of concern and more studies are needed to confirm the treatment safety on the trees.
The mode of action of these treatments was not explored, however, it is probably a combination of direct and indirect effects on microbial spore germination and growth as well as improved tree health. This mode of action will not readily allow for the development of disease resistance to the treatment.
This research was based on the impact of a single application of the nutrients soon after fruit set. It is possible that the KNO3 treatment will be safe to use during the growing season, allowing for superior control of the diseases as well as providing additional nitrogen and potassium to the tree. As the levels of nitrogen and potassium applied in the treatment are small (less than 1kg/ha) compared to ground-based applications, multiple applications are feasible.
Take home messages
- Application of 0.8 per cent CuSO4 and/or 3 per cent KNO3 soon after fruit set reduces disease incidence on the harvested fruit as well as improving yield and fruit quality characteristics.
- No assessment of foliar or fruit damage due to the treatment was reported.
- Very small quantities of the nutrients were applied, which had a large impact on the trees, allowing for more efficient nutrient uptake and less potential environmental damage.
Effect of foliar application with potassium nitrate and copper sulfate on fruit yield and quality of pear (Pyrus communis L.) trees. www.tandfonline.com/doi/pdf/10.1080/15538362.2022.2117263
Supplemental foliar potassium applications with or without a surfactant can enhance netted muskmelon quality. https://journals.ashs.org/hortsci/view/journals/hortsci/41/3/article-p741.xml
This article was first published in the Autumn 2023 edition of AFG.