Zinc Sulfate (ZnSO₄) on Hazelnut: Deficiency, Dose and Timing

What Does Zinc Do in the Hazelnut?

Zinc (Zn) is a micronutrient the plant needs in milligrams, not kilograms. The quantity is tiny but the job is critical: zinc is a structural part of dozens of enzymes, and its best-known role is participation in the synthesis of auxin (IAA), the growth hormone that governs shoot elongation and cell division.

In the orchard this translates into:

• Shoot growth — with insufficient zinc, internodes shorten and shoots stay weak
• Leaf area — new leaves stay small and narrow, cutting the bush’s photosynthetic capacity
• Pollination and fruit set — pollen viability and fertilisation success are sensitive to zinc
• Kernel filling and fruit quality — the share of blank, shrivelled and undersized kernels is decided at this stage

The last point matters most in hazelnut, because what reaches the grower’s pocket is not only in-shell yield but the sound kernel ratio. The Turkish research below measured exactly that relationship.

Symptoms of Zinc Deficiency

Zinc has low mobility inside the plant. It cannot easily be moved from old leaves to young ones, so symptoms appear first on shoot tips and young leaves. The classic hazelnut picture is:

Symptom	Where it appears	Explanation
Little leaf (rosetting)	Shoot tip	Leaves stay far below normal size and bunch up at the tip
Shortened internodes	Current-season shoot	The shoot cannot elongate; leaves are crowded together
Interveinal chlorosis	Young leaves	Veins stay green while the tissue between them yellows
Wavy leaf margins	Young leaves	The blade develops crooked, narrow and asymmetric
Tip dieback	Advanced deficiency	The terminal shoot dries back and lateral shoots break
More blank and shrivelled kernels	At harvest	Defective fruit ratio rises, sound kernel ratio falls

Why Is It So Common on the Black Sea Coast?

Zinc deficiency in hazelnut is not incidental — it is structural. In the survey by Özkutlu and colleagues (2019), 130 soil samples (0-30 cm) were taken from orchards stretching from the Ünye district boundary to the Gülyalı boundary in Ordu province. Based on DTPA-extractable available zinc, 89% of the hazelnut area was found to be zinc deficient. The same study reported a boron deficiency rate of 85%.

In the same screening, available soil zinc ranged from a low of 0.05 to a high of 1.94, with a mean of 0.45 mg Zn/kg. The classification used in the study and the distribution of samples were as follows:

DTPA-Zn (mg/kg)	Rating	Share of Ordu samples
below 0.2	Very low	11%
0.2 – 0.7	Low	78%
0.7 – 2.4	Sufficient	11%
above 2.4	Excessive	0%

Several factors sit behind this picture:

1. Antagonism with phosphorus. Phosphorus accumulated in the soil blocks zinc uptake. Because applying phosphate fertiliser every year without analysis is a widespread habit, this is the region’s number one zinc lock.
2. High pH and liming. As pH rises, zinc shifts into a form the plant cannot take up. The figure the same study gives is striking: a one-unit rise in soil pH reduces zinc availability 30- to 100-fold. When lime is applied to acid soils, part of the zinc is held on the surface of the CaCO₃ particles. This is why liming done without need, or in excess, can sharpen an existing zinc deficiency.
3. Rainfall and leaching. Ordu receives roughly 1,042 mm of rain a year. Under that rainfall zinc leaches readily out of the topsoil — particularly on sandy and sandy loam textures. That is why zinc deficiency in the region is not confined to calcareous soils.
4. Low organic matter. Organic matter is the key buffer, raising both the solubility of zinc and its movement through the soil. On sloping land, losing topsoil to erosion weakens that buffer.
5. Continuous harvest, nutrients never returned. Zinc leaves the orchard in the crop every year; macronutrients get replaced, micronutrients are never considered.

For the phosphorus–zinc relationship see our DAP (18-46-0) and Urea Phosphate (18-44-0) guides: both stress that phosphorus should be applied once every two to three years, on the basis of analysis. That is, in itself, a zinc precaution.

Think of it alongside boron: the same survey found 85% of Ordu soils also deficient in boron. In hazelnut, zinc and boron are usually short together — for the other half of that pair, see our boron fertilisation guide.

Diagnosis: Do Not Apply Zinc Without Leaf Analysis

Soil analysis shows the zinc reservoir in the soil; leaf analysis shows how much the plant has actually taken up. With zinc the two often disagree — because the problem is usually not an absence of zinc but an inability to absorb it. That is why leaf analysis is the decisive test for zinc.

In the leaf samples collected by Özkutlu and colleagues (2018) at the fruit formation stage in Ünye, Ordu, the sufficiency range used as a reference was reported as 15-80 mg Zn/kg. In that study:

Finding	Value
Sufficiency range (reference)	15-80 mg Zn/kg
Range found in samples	10-68 mg Zn/kg
Mean leaf zinc	21 mg Zn/kg
Share of deficient orchards (below 15 mg/kg)	22%

For sampling timing and method see our Soil and Leaf Analysis Guide: the leaf sample is taken in July, from the middle leaves of current-season shoots, from all four sides of the bush. Zinc adds one more rule: if a zinc-containing foliar fertiliser or fungicide was sprayed before sampling, the result is unreliable — residue on the leaf surface inflates the reading. If you have sprayed, wait at least three to four weeks and tell the laboratory.

Soil Application of Zinc Sulfate

The effect of soil-applied zinc sulfate on hazelnut has been measured directly in Türkiye. The study by Ete Aydemir, Akgün and Özkutlu (2023) was carried out in 2018-2019 at the Turnasuyu (Gülyalı) location in Ordu province, in a grower’s orchard at 50 m elevation, on the ‘Palaz’ hazelnut cultivar. The trial soil was clay-textured, with pH 6.20, 0.67% lime, 1.70% organic matter and available zinc of 0.42 mg/kg — that is, an orchard in the “low” class according to the table above. The rates were applied in a randomised block design, in 3 blocks with 5 replications, to a total of 45 bushes. The rates tested were:

Treatment	Rate
T0 (control)	0 kg ZnSO₄·7H₂O/da
T1	1.25 kg ZnSO₄·7H₂O/da
T2	2.5 kg ZnSO₄·7H₂O/da

The trial measured the sound kernel ratio together with defective kernel ratios (cracked fruit, small kernel, black-tipped kernel, shrivelled kernel, abortive, mouldy, rotten and twin kernel). The two-year results:

Treatment	Sound kernel, yr 1	Sound kernel, yr 2	Defective kernel, yr 1	Defective kernel, yr 2
T0 (control)	68%	93%	31%	6%
T1 (1.25 kg/da)	53%	94%	38%	4%
T2 (2.5 kg/da)	66%	95%	23%	2%

The conclusion is unambiguous: soil-applied zinc sulfate raised the sound kernel ratio and lowered the defective kernel ratio. From the first to the second year, the increase in sound kernel ratio was 37% in T0, 77% in T1 and 44% in T2 — that is, the greatest improvement came at the T1 rate. Second-year defective kernel ratios fell with rising rate to 6%, 4% and 2% respectively.

Averaged over the two years, the mix of defects also shifted: cracked fruit fell from 6% in the control to 3% in T2 (roughly a halving), abortive kernels from 4% in the control to 2% in T1, mouldy kernels from 4% to 3% in T2, and rotten kernels from 4% to 2% in T2 📚 Ete Aydemir et al., 2023

Why Doesn’t the Highest Rate Give the Best Result?

In the Palaz study the highest sound kernel ratio (95%) was recorded in T2, but T1 and T2 were statistically similar to the control, and the proportional improvement was greatest in T1.

The same team’s doctoral work measuring yield on the Çakıldak cultivar completes the picture. In a two-year trial in a zinc-deficient orchard (0.21 mg Zn/kg), the two-year mean yield was 166 kg/da in the control, 215 kg/da with 1.1 kg Zn/da applied to the soil (100 g ZnSO₄ per bush) and 230 kg/da with 2.2 kg Zn/da (200 g per bush). Even so, the study’s economically based recommendation is the lower rate of 1.1 kg Zn/da — the extra yield from doubling the rate did not cover the extra fertiliser cost 📚 Ete Aydemir, 2023 (doctoral thesis)

To that you must add zinc’s narrow safety band: zinc accumulated in the soil does not leach away like nitrogen, and an excess disturbs the uptake of elements such as iron. You will see the same pattern in our foliar fertilisation guide: in the foliar urea trial on hazelnut, the highest rate also performed worse than the middle rate. With micronutrients, “let me add a bit more, it can’t hurt” simply does not hold.

Application Practice

In the Palaz trial the fertiliser was not broadcast but placed locally: around each bush, 15-25 fertiliser holes 5-10 cm deep were opened according to the size of the bush, and the fertiliser was distributed into those holes. In the same trial, base fertilisation was applied in the second week of February as the plants came out of dormancy, while the nitrogen top-dressing was split in two and applied on 15 March and 15 May.

Foliar Application of Zinc

Zinc is one of the elements for which foliar application makes the most sense. The reason is simple: high pH, lime and accumulated phosphorus lock zinc up in the soil, and none of those locks apply when it is delivered through the leaf. The Hazelnut Research Institute’s fertilisation publication likewise recommends foliar application specifically for nutrients that “are converted in the soil into a form the plant cannot take up” and that “must be supplied in very small amounts” — zinc sits squarely inside that definition.

In the study conducted at Ordu University on the Çakıldak cultivar, comparing zinc applied to the soil, to the leaf, and as a soil + foliar combination, the foliar treatment was applied three times a year with a 0.3% ZnSO₄·7H₂O solution. The trial orchard was selected after a screening of 130 orchards and had a low soil zinc concentration of 0.21 mg Zn/kg.

The results show that foliar application does the job even on its own: against a two-year mean control yield of 166 kg/da, three foliar applications of the 0.3% solution alone lifted yield to 200 kg/da. Where soil-applied 1.1 kg Zn/da was combined with the foliar treatment, yield reached 221 kg/da. Leaf zinc concentration rose from 22.95 mg/kg in the control to 37.45 mg/kg under the 1.1 kg Zn/da soil treatment 📚 Ete Aydemir, 2023 (doctoral thesis)

On timing, the critical window is just after fertilisation and during early fruit development. In the work of Silvestri and colleagues (2021) on the Tonda di Giffoni cultivar, spraying boron and zinc together through fruit set and early fruit growth markedly improved yield and fruit quality, with the blank nut ratio falling from 10% to a range of 0-3.3%. That result, however, was obtained in an orchard deficient in those elements.

All the spraying conditions — hour of the day, weather, droplet size, spreader-sticker, tank mixing order and the pre-application test — are shared rules, set out in detail in our Foliar Fertilisation Guide. Here we only repeat the three zinc-specific notes:

• Zinc is close to immobile inside the plant. It stays in the leaf it lands on, so newly developing shoots need a repeat application.
• Do not mix it in the same tank as Bordeaux mixture or copper products. Never build a tank mix without a compatibility test.
• Zinc sulfate solution is acidic. Prepared with hard, limy water it can lose effectiveness; use clean water.

Common Mistakes

1. Applying zinc without analysis. Yellowing leaves may be caused by iron, manganese, magnesium or a root problem. Applying the wrong element does not fix anything and costs money.
2. Trying to speed things up by raising the rate. In the Palaz trial, the largest gain in sound kernel ratio came not from the highest rate but from the middle one (T1).
3. Continuing to apply phosphorus every year without analysis. Accumulated phosphorus locks zinc up; applying zinc while still overloading phosphorus is filling and emptying at the same time.
4. Spreading zinc on the surface and leaving it there. Zinc that is not incorporated never reaches the root zone.
5. Taking the leaf sample straight after a zinc spray. Residue on the leaf surface inflates the result and produces a false “zinc is sufficient” reading.
6. Assuming one application settles it. Zinc is close to immobile; foliar programmes need repeats.
7. Never measuring the outcome. Only the next leaf analysis will tell you whether the zinc you applied worked.

Summary: Zinc Checklist

• Zinc is the most common micronutrient deficiency in hazelnut; the Ordu survey reported soil deficiency in 89% of orchards
• Symptoms start on young leaves: little leaf, rosetting, short internodes, interveinal chlorosis
• Leaf analysis is essential for the decision; the reference sufficiency range is 15-80 mg Zn/kg
• Take the leaf sample in July, at least three to four weeks after any zinc-containing spray
• For soil application, place the fertiliser in 15-25 holes 5-10 cm deep along the drip line
• Expect the effect in the second year; zinc moves slowly in the soil
• Raising the rate does not improve the result — the largest improvement in the Palaz trial (77%) came at the middle rate (T1), and the yield study’s economic recommendation also favours the lower rate
• Where pH is high or phosphorus has accumulated, prefer foliar application
• The foliar window is just after fertilisation and during early fruit development
• Apply phosphorus on the basis of analysis, once every two to three years — that is also a zinc measure
• Confirm the outcome with a leaf analysis the following year

For the full annual fertiliser calendar see our Hazelnut Fertilisation Programme, for the analysis process see Soil and Leaf Analysis, and for spraying rules see Foliar Fertilisation.

📚 Ete Aydemir Ö., Akgün M., Özkutlu F. (2023), Turkish Journal of Agricultural and Natural Sciences 10(2):450-456 📚 Özkutlu F. et al. (2019), Akademik Ziraat Dergisi - Zinc nutritional status of hazelnut soils in Ordu 📚 Özkutlu F. et al. (2018), Ordu Univ. Journal of Science and Technology - Zinc nutritional status of hazelnut by leaf analysis 📚 Ordu University Graduate School of Natural and Applied Sciences - Foliar and soil zinc applications in hazelnut 📚 Silvestri C. et al. (2021), Frontiers in Plant Science 📚 Hazelnut Research Institute - Fertilizing the Hazelnut (Grower Training Series)