Foliar Fertilization in Hazelnuts: When and How to Apply Leaf Fertilizer

What Is Foliar Fertilization?

Foliar fertilization means delivering nutrients to the plant dissolved in water and sprayed onto the leaf surface. The nutrient enters the tissue by passing through the waxy cuticle covering the leaf and through the stomata (pores).

The Pistachio Research Institute of the Turkish Ministry of Agriculture and Forestry summarizes the mechanism this way: the soluble salts in leaf fertilizers are largely taken up through the stomata; stomata open in light and close in darkness, and under very hot, dry conditions the plant closes its stomata to conserve water. That single fact explains why foliar fertilization must be done during the cool hours of the day.

Does It Replace Soil Fertilization? — No

This is where growers go wrong most often. The Hazelnut Research Institute’s grower training leaflet on hazelnut fertilization is explicit: plants take up most of the nutrients they need through their roots and only a very small part through their leaves. The Pistachio Research Institute publication agrees — foliar feeding is an “excellent complement” to soil applications; it is not possible to feed a plant through its leaves alone or to cover a macronutrient deficit entirely by foliar spraying. Micronutrient requirements, on the other hand, can in most cases be met through the leaves.

	Soil Fertilization	Foliar Fertilization
Amount of nutrient delivered	High (main source)	Low (top-up)
Speed of effect	Slow (days to weeks)	Fast (hours to days)
Duration of effect	Long	Short, needs repeating
For macronutrients	Suitable	Insufficient on its own
For micronutrients	Depends on soil condition	Usually more effective
Affected by soil pH	Strongly	Barely at all

The right structure is therefore: run your fertilization programme through the soil, and add foliar application on top of it as fine tuning.

When Does It Actually Help?

The Hazelnut Research Institute leaflet defines the use case as “situations where nutrient uptake from the soil is restricted”, and highlights two in particular: (1) supplying nutrients that convert in the soil into forms the plant cannot absorb, and (2) applying nutrients that are needed only in very small quantities.

In practice these correspond to the following situations:

• When acute deficiency symptoms appear. Once you see chlorosis, interveinal yellowing or reduced leaf size, waiting for a soil application to take effect costs you the season. Foliar spraying is the fastest intervention available.
• When the root zone is not working properly. High pH and free lime (which lock up zinc, iron and phosphorus), waterlogging or drought, a high water table, root rot and collar problems all block root uptake. If the roots are not working, spreading more fertilizer on the soil solves nothing.
• For micronutrient top-ups. Boron, zinc, iron and manganese are needed in gram-scale quantities per decare. Distributing such small amounts evenly in the soil is difficult; foliar application is both cheaper and more precise.
• During critical phenological windows. Flowering, fertilization and fruit set are short windows in which nutrient demand spikes. Speed of effect matters here.
• On sloping ground where nitrogen losses are high. The study by Özkutlu and colleagues (2020) in Ordu notes that because hazelnut land is largely sloping, nitrogen losses via surface runoff are high and the leaves end up inadequately fed.

Which Nutrients Can Be Applied Through the Leaf?

The answer is determined by mobility within the plant. If the nutrient entering the leaf can be translocated, its effect spreads through the tree; if it cannot, it stays in the leaf you sprayed.

Element	Mobility in plant	Note on foliar application
Nitrogen (N)	High	Very well absorbed in urea form; the most common leaf fertilizer
Potassium (K)	High	Absorbed and translocated; burn risk is high, so keep the dose low
Magnesium (Mg)	Medium–high	The sulphate form is widely used
Phosphorus (P)	Medium	Absorbed, but quantities are limited; soil application remains essential
Sulphur (S)	Medium	Usually arrives together with sulphate-based fertilizers
Zinc (Zn)	Low	On calcareous and clayey soils, foliar application beats soil application
Iron (Fe)	Very low	Foliar application clearly superior to soil application
Manganese (Mn)	Low	Effective through the leaf
Boron (B)	Very low	Applied at flowering and fruit set
Calcium (Ca)	Very low	Reaches only the sprayed tissue; expect no systemic effect

The key conclusion: for immobile elements such as calcium, boron, zinc and iron you must match the application to the tissue showing the deficiency and to the correct phenological stage — a late boron spray will not make it in time for fruit set.

Application Conditions: The Details That Decide the Outcome

In foliar feeding the result is determined far less by the brand of fertilizer than by how and when you spray it.

Time of day, temperature and weather

The Hazelnut Research Institute leaflet is clear: foliar fertilization should be carried out on cool, cloudy days, so that the plant is not burned by the solution. The Pistachio Research Institute publication gives the same reasoning — in sunny weather the water evaporates rapidly, the salt left on the leaf surface becomes concentrated and scorches the tissue. According to that source, the best time to apply is towards evening or in the early morning.

• Do not spray in windy weather — you get both drift and uneven coverage.
• Do not spray ahead of rain — the solution washes off and the work is wasted.
• Be careful when leaves are wet with dew; on a very wet surface the solution runs off and the dose is diluted.
• In very hot, dry periods in unirrigated orchards the application can stress the tree; the Pistachio Research Institute notes this can cause leaf scorch and leaf drop.

Coverage, droplet size and wetting agents

According to the Hazelnut Research Institute, the smaller the droplets at the moment of spraying, the lower the risk of burn and similar damage. The same leaflet recommends adding an appropriate amount of sticking agent to the spray solution; the Pistachio Research Institute publication completes the reasoning: the best uptake occurs when the nutrient remains on the leaf surface as a thin dissolved film, and what makes that possible is a spreader-sticker that lowers the surface tension of the solution.

The underside of the leaf must also be wetted — in the hazelnut trial by Özkutlu and colleagues, the spray was applied so that both the upper and lower leaf surfaces were wet.

Water quality and tank mixing

• Fill the tank with clean water; sediment-laden water clogs nozzles and ruins distribution.
• The Pistachio Research Institute publication advises that when spraying together with crop protection products, dissolve the fertilizer first and add the pesticide afterwards.
• Run a compatibility test: mix a small batch in half a litre of water. If you see precipitation, curdling or heating, do not tank-mix. Calcium products with sulphate or phosphate products typically precipitate.
• Water pH affects uptake and stability for many products; if the label states a required range, stay within it.
• Do not store the mixed solution — use it up the same day.

Application Timing in Hazelnuts

According to the publication of the Samsun Provincial Directorate of the Ministry of Agriculture and Forestry, foliar application in hazelnut is carried out between May and July, 2–3 times at 15–20 day intervals, spraying dilute solutions until no dry spot remains on the leaves. The same source positions this as a complementary practice that boosts the effect of the compound fertilizers applied to the soil in February–March and the calcium ammonium nitrate fertilizers applied in May–June.

Mapped onto phenological stages:

Period	Phenological stage	Dominant requirement
March – April	Bud break, leaf development	Nitrogen; rapid build-up of leaf area
Late April – May	After fertilization, start of fruit set	Boron and zinc; set and blank nut problems
May – June	Fruit formation and growth	Nitrogen plus micronutrient top-up
June – July	Kernel filling	Potassium-weighted top-up
September (post-harvest)	Before leaf fall	Building nitrogen reserves

In the hazelnut trial of Özkutlu and colleagues (2020) the application followed exactly this logic, three times a year: (i) in autumn immediately after harvest (end of September), (ii) in spring after the leaves had formed, and (iii) at the beginning of the initial fruit formation period.

Dose Logic and the Risk of Leaf Burn

In foliar feeding the danger lies not in the total amount but in the concentration. When the water on the leaf surface evaporates, the salt left behind becomes concentrated; above a certain threshold it damages the cell membrane and produces burn starting at the leaf margins.

The concentration ranges cited in the sources are as follows:

Source	Range given
Hazelnut Research Institute, “Fertilization in Hazelnut” leaflet	The sprayed solution should be as dilute as possible, 0.1–0.5%
Pistachio Research Institute, foliar fertilization article	1–2% for macronutrients, 0.1–0.2% for micronutrients
Özkutlu et al. (2020), foliar urea trial in hazelnut	0.25% urea applied three times a year gave the best result

In the two-year trial by Özkutlu and colleagues on the Çakıldak cultivar at Gölköy, Ordu, control, 0.125%, 0.25% and 0.50% urea rates were compared; a spreader-sticker at 0.03% was added to the solution and the sprays were made in the evening hours, towards sunset. Averaged over two years, the 0.25% urea treatment raised leaf nitrogen concentration from 1.58% to 2.01% (a 27% increase) and leaf potassium from 0.40% to 0.80%; yield rose from 157 kg to 205 kg per decare, an increase of 30.6%.

The striking part is this: raising the rate to 0.50% did not improve the result, it made it worse (170 kg/da). The “more is better” instinct simply does not hold in foliar fertilization.

The same study states that when urea is applied to the leaves, the biuret content must be 1% or lower, because fertilizers with a high biuret content cause leaf burn. When buying urea for foliar use, that is the first thing to look for on the label.

Test before every application: spray your prepared solution on 2–3 bushes at the edge of the orchard and wait 48–72 hours. If there is no scorching, browning or necrosis at the leaf margins, extend the application to the whole orchard.

Common Mistakes

1. Cutting back soil fertilization and trying to make it up through the leaves. The leaf simply cannot cover a tree’s macronutrient demand.
2. Spraying at midday with the sun overhead. This is the moment of maximum burn risk.
3. Doubling the rate on the assumption that “a small dose gives a small effect”. In the hazelnut trial, 0.50% urea performed worse than 0.25%.
4. Assuming one spray is enough. Only a small fraction is absorbed each time; the sources recommend 2–3 repeats.
5. Wetting only the upper leaf surface. Uptake happens largely through the stomata on the underside.
6. Skipping the spreader-sticker. The solution runs straight off and very little stays on the leaf.
7. Tank-mixing without a compatibility test. A mixture that precipitates clogs nozzles and causes burn.
8. Spraying when rain is forecast. A washed-off solution wastes both money and labour.
9. Using unlabelled “cure-all” blends. Do not expect results from a product whose contents you do not know; read the guaranteed analysis.

The Link with Leaf Analysis: What to Measure and When

Foliar fertilization is the most expensive fertilization method there is when applied blind. The correct structure is this: soil analysis shows the supply, leaf analysis shows what the plant is actually taking up. Neither replaces the other.

As reported in the paper by Özkutlu and colleagues (2020), Jones et al. (1991) give critical threshold values for hazelnut leaves: a nitrogen concentration of 2.00–2.29% is “low”, while 2.30–2.60% is considered sufficient. The same paper reports that in leaf samples taken by Özkutlu et al. (2018) from 130 different hazelnut orchards in Ordu province, the average nitrogen concentration was 1.91% and 97% of the samples were inadequately supplied with nitrogen. In other words, nitrogen deficiency is the rule rather than the exception across the region — but the only way to know is to test.

The same discipline applies to micronutrients. In the study by Silvestri and colleagues (2021) on the Tonda di Giffoni cultivar, spraying boron and zinc together throughout fruit set and the early fruit stage markedly increased yield and quality — but that result holds in an orchard where those elements were deficient. Hazelnut has a narrow safety margin for boron; both deficiency and excess cause damage. Decide on boron only on the basis of an analysis result.

Summary: Foliar Fertilization Checklist

• Foliar feeding is complementary; it never replaces soil fertilization
• Base the application on a symptom or an analysis result, not on routine insurance
• Spray on cool, cloudy days, early in the morning or towards evening
• Do not spray in windy weather or ahead of rain
• Wet both the upper and lower leaf surfaces with fine droplets
• Add a spreader-sticker to the solution
• Repeat 2–3 times at 15–20 day intervals; one spray is not enough
• Keep the concentration low; raising the rate does not raise yield, it burns
• When buying urea for foliar use, check that the biuret content is 1% or lower
• Test on a few bushes before the full application and wait 48–72 hours
• In tank mixes, dissolve the fertilizer first, then add the pesticide, and test compatibility
• Do not increase nitrogen rates after July

For the full annual fertilizer calendar see our Hazelnut Fertilization Programme guide, and for the analysis process our Soil and Leaf Analysis guide. If you want a closer look at two products that can also be applied through the leaf: Potassium Sulphate (0-0-51) and Urea Phosphate (18-44-0).

📚 Özkutlu F. et al. (2020), Ordu Univ. J. Sci. Tech. 10(1):23-31 📚 Hazelnut Research Institute - Fertilization in Hazelnut (Grower Training Series) 📚 Ministry of Agriculture and Forestry, Samsun Provincial Directorate - Fertilization Timing in Hazelnut 📚 Pistachio Research Institute - Foliar Fertilization 📚 Silvestri C. et al. (2021), Frontiers in Plant Science