Ammonium Sulfate (21-0-0) Fertilizer: Use in Hazelnuts and the Acidity Risk

What Is Ammonium Sulfate (21-0-0)?

Ammonium sulfate ((NH₄)₂SO₄) is a white-to-beige crystalline nitrogen fertilizer containing 21% nitrogen (N) and 24% sulfur (S). Because it looks like granulated sugar, growers across the Black Sea region and most of Anatolia call it “sugar fertilizer” (şeker gübre). Its grade is written 21-0-0: it contains no phosphorus and no potassium.

All of its nitrogen is in the ammonium (NH₄⁺) form, and its sulfur is in the sulfate (SO₄²⁻) form, which plants take up directly. Besides the crystalline product, a granular version is also sold; in the granular form the nitrogen content can drop to around 20.5%.

Property	Value
Formula	(NH₄)₂SO₄ (21-0-0)
Nitrogen (N)	21% — entirely ammonium nitrogen
Sulfur (S)	24% — in sulfate form
Phosphorus / Potassium	None
Solubility	Readily water-soluble
Effect on soil pH	Acidifying (physiologically acidic)
Physical form	White-beige crystal or granule
Common local name	Sugar fertilizer

What Does “Ammonium Form” Actually Mean?

Nitrogen fertilizers deliver nitrogen in three chemical forms: ammonium (NH₄⁺), nitrate (NO₃⁻) and urea (NH₂). Ammonium sulfate is entirely ammonium, and that has two practical consequences:

• It resists leaching better. The ammonium ion carries a positive charge and binds to soil colloids. Nitrate, being negatively charged, drains below the root zone with rainfall. Along the Black Sea coast, where annual rainfall exceeds 1,000 mm, this is a genuine advantage.
• It acts more slowly but lasts longer. Ammonium becomes fully available only as soil bacteria convert it to nitrate (nitrification). In cold soil that conversion slows down, so very early applications may not deliver the expected response.

There is a price, though: nitrification releases hydrogen ions (H⁺) into the soil. That is the real source of ammonium sulfate’s acidifying character — more on this below.

What Does It Do in a Hazelnut Orchard?

The Nitrogen Contribution

Nitrogen is the nutrient hazelnut consumes in the largest quantity, and the Hazelnut Research Institute’s fertilization guidance builds the whole annual programme around nitrogen timing. In hazelnut, nitrogen drives:

• Shoot growth — annual shoot length determines how many fruiting buds next season will carry
• Leaf area — larger photosynthetic capacity means a bigger “engine” for the bush
• Protein and chlorophyll synthesis — directly visible in leaf colour and kernel quality
• Cluster formation — in a trial in Samsun, sucker-control treatments using 21% ammonium sulfate and 26% calcium ammonium nitrate solutions positively affected the number of female flowers and clusters per shoot, while producing no difference in fruit quality measures (nut weight, kernel ratio, shell thickness) 📚 Serdar et al., 2017

The Overlooked Role of Sulfur

What separates ammonium sulfate from an ordinary nitrogen source is its 24% sulfur. Most fertilizer articles skip this part, yet sulfur matters for hazelnut:

• It is the building block of sulfur-containing amino acids such as methionine and cysteine; however much nitrogen you supply, protein synthesis stalls without sulfur
• It supports oil synthesis — the chemical basis of kernel quality in hazelnut
• It plays an indirect role in chlorophyll formation; sulfur deficiency shows as yellowing of the young leaves (nitrogen deficiency starts on old leaves — that is the distinguishing sign)

In high-rainfall regions sulfate sulfur leaches much like nitrate, so supplementing sulfur under Black Sea conditions is not unreasonable. But ammonium sulfate is not your only option: potassium sulfate carries 18% sulfur without acidifying the soil to the same degree.

The Acidity Question: Black Sea Soils and Ammonium Sulfate

This is the critical section. Ammonium sulfate is one of the most widely used nitrogen fertilizers in Turkish hazelnut growing — but being widespread does not mean it is right for your orchard.

What Is a Physiologically Acidic Fertilizer?

A fertilizer need not be acidic itself; if the transformations it undergoes in the soil acidify the medium, it is called a physiologically acidic fertilizer. That is exactly what happens with ammonium sulfate: H⁺ ions are released both when the plant takes up the ammonium ion and when soil bacteria oxidise ammonium to nitrate. The result is a gradual drop in soil pH.

This property is an advantage in calcareous, high-pH soils — as pH falls, iron, zinc and manganese availability improves along with phosphorus uptake. The problem is that this advantage does not apply to most Black Sea hazelnut orchards.

The pH Reality of Black Sea Soils

The Hazelnut Research Institute states that hazelnut develops normally in soils with a pH between 5 and 7. In much of the region, the reality sits at or below the lower end of that range: limestone is scarce along the Eastern Black Sea coast while acidic rocks such as granite and dacite are common, and heavy rainfall leaches calcium out of the profile, leaving naturally acid soils.

In samples taken from 71 hazelnut orchards across 40 villages in the Kumru district of Ordu, soil pH ranged widely from 4.53 to 7.87, and the orchards were found deficient in total nitrogen and in available phosphorus, potassium and zinc. The researchers’ conclusion is unambiguous: every orchard needs its own fertilization programme 📚 Karadeniz & Özkutlu, 2023

The Cumulative Effect Over Years

A single season of ammonium sulfate will not measurably move soil pH. The problem appears with repetition. Applying the same fertilizer every March for 10–15 years, with no liming, pulls pH steadily downwards. Once pH drops below 5:

• Phosphorus binds with iron and aluminium and becomes unavailable
• Aluminium and manganese can reach toxic levels, damaging fine roots
• Calcium and magnesium are leached out
• Soil microbial activity — and therefore organic matter breakdown — slows

You pay for the fertilizer, but the soil can no longer deliver that nitrogen to the plant.

Compensating With Lime

If you intend to keep using ammonium sulfate, liming is not negotiable. The Hazelnut Research Institute recommends liming in November–December, once every 3–5 years according to soil analysis, spread over a 40–50 cm band beneath the branch tips and worked into the top 5–10 cm of soil.

Liming has a measured payoff in hazelnut: in a field trial on acid soil, lime application raised in-shell yield by 29% over the control, while sugar industry waste sludge raised it by 41%; the highest yield (2.47 kg per bush) and 54% kernel ratio were obtained at a sludge rate of 1.5 times the lime requirement 📚 Özyazıcı, 2014

When Is It Applied in Hazelnuts?

Publications from the Hazelnut Research Institute and FİSKOBİRLİK recommend splitting nitrogen into two applications rather than one:

First Application — Late February / March

The period when buds begin to swell and root activity resumes. Half the annual nitrogen goes on now. The Giresun Provincial Directorate points to after 15 March for the first application.

Second Application — Late May / Early June

The period of flower bud formation and nut development. The remaining half is applied here. The Giresun Provincial Directorate suggests after 20 May for the second application.

How Is It Applied?

The application method determines how much of the fertilizer actually reaches the plant. Official guidance describes it this way: the fertilizer is spread evenly over a ring-shaped band 30–40 cm wide beneath the branch tips, then worked with a hoe into the top 5–10 cm of soil.

Incorporating the fertilizer is not a trivial detail: ammonium left on the surface volatilises as ammonia, especially in dry, warm weather, and on sloping ground it is physically washed away by rain.

How Is the Rate Determined?

There is no single rate that suits every orchard. The rate comes from three inputs:

1. Soil analysis — organic matter, total nitrogen and pH (see our soil analysis guide; provincial and district directorates run the analysis free of charge)
2. Tree age and bush size — a newly established bush and a 20-year-old bush in full production do not need the same nitrogen
3. Yield target and past yield — the more in-shell nuts you take out each year, the more nutrients you must put back

Converting Pure Nitrogen to Fertilizer Quantity

An agricultural directorate will usually give you a pure nitrogen (N) figure, not a fertilizer quantity. The conversion is simple division:

Fertilizer quantity = Pure nitrogen ÷ 0.21

In practice for ammonium sulfate: multiply the required pure nitrogen by roughly 5 (100 ÷ 21 = 4.76).

Pure nitrogen required	Ammonium sulfate needed	Sulfur supplied alongside
100 g	~476 g	~114 g S
200 g	~952 g	~229 g S
4 kg	~19 kg	~4.6 kg S
6 kg	~28.6 kg	~6.9 kg S

An Equivalence Derived From Official Recommendations

The Giresun Provincial Directorate of Agriculture and Forestry gives the following per-bush figures for two split applications: CAN (26% N) → 650 g per application, 1,300 g total, or urea (46% N) → 375 g per application, 750 g total.

Both recommendations amount to roughly 340 g of pure nitrogen per bush per year. Delivering that same nitrogen as ammonium sulfate would require roughly 1.6 kg per bush per year (about 800 g per application). This figure is not an official ammonium sulfate recommendation — it is the arithmetic equivalent of the official CAN/urea rates, and your orchard’s actual requirement may differ.

Ammonium Sulfate, CAN or Urea?

A comparison of the three nitrogen fertilizers from a hazelnut perspective:

Criterion	Ammonium Sulfate (21-0-0)	Calcium Ammonium Nitrate (26-0-0)	Urea (46-0-0)
Nitrogen content	21%	26%	46%
Nitrogen form	All ammonium	Half ammonium, half nitrate	Urea (amide)
Additional nutrient	24% sulfur	Calcium + magnesium (lime)	None
Effect on soil pH	Acidifying	Near-neutral / buffering	Slightly acidifying
Leaching risk	Low (ammonium)	Moderate (nitrate fraction)	Moderate
Volatilisation (ammonia) loss	Yes, if left on the surface	Low	High — must be incorporated
Quantity needed for the same N	Highest	Moderate	Lowest
On acid soil	Not recommended	The recommended choice	Usable
On alkaline/calcareous soil	Best fit	May be ineffective	Usable

A practical decision rule:

• Below pH 5 → stay away from ammonium sulfate. Choose calcium ammonium nitrate and plan a liming programme.
• pH 5–6.5 → CAN first; use ammonium sulfate occasionally where sulfur is deficient, on condition that you lime regularly and monitor pH.
• Above pH 7 → ammonium sulfate is a sound choice; it supplies nitrogen, pulls pH toward the ideal range and improves micronutrient uptake.
• If haulage and cost are the priority → urea is the cheapest per unit of nitrogen, but it must be incorporated into the soil.

Common Mistakes

1. Using it without an analysis. “My neighbour spreads sugar fertilizer, so I will too” is the most expensive mistake in the book. Applying an acidifying fertilizer without knowing your pH deepens the very problem you are trying to solve.
2. Using the same fertilizer for a decade. Locking on to one product changes the soil in one direction only. Rotate fertilizer types according to analysis results.
3. Skipping the liming. If you use ammonium sulfate, a liming programme is a mandatory companion, not an optional extra.
4. Top-dressing at the wrong time. Nitrogen after July prolongs shoot growth and increases frost sensitivity. Applying too early, into cold or frozen soil, fails to deliver because nitrification is slow.
5. Leaving the fertilizer on the surface. Spreading and walking away donates part of your nitrogen to the atmosphere and, on slopes, to runoff.
6. Over-applying nitrogen. Excess nitrogen lowers kernel quality, softens leaf tissue and increases susceptibility to powdery mildew, and leaves shoots unprepared for winter. “More fertilizer means more yield” does not hold in hazelnut.
7. Applying it together with lime. This loses nitrogen as ammonia.

Summary: Ammonium Sulfate Checklist

• Get a soil analysis first; do not decide before you know your pH
• Above pH 7 ammonium sulfate is a good choice; below pH 5, switch to CAN
• Split the nitrogen in two: March and late May–early June
• Spread over the 30–40 cm band at the drip line, incorporate to 5–10 cm
• The 24% sulfur is a bonus — valuable in orchards with sulfur deficiency
• If you use it, lime every 3–5 years (November–December)
• Never apply lime and nitrogen at the same time
• No nitrogen after July
• Confirm the rate with your provincial/district directorate or an agricultural engineer

For the full annual fertilizer calendar see our Hazelnut Fertilization Programme guide, and for sampling and reading results see our Soil Analysis Guide. For phosphorus sources look at Urea Phosphate 18-44-0, and for potassium and sulfur at Potassium Sulfate 0-0-51.

📚 Hazelnut Research Institute 📚 Giresun Provincial Directorate of Agriculture and Forestry 📚 Trabzon Commodity Exchange 📚 FİSKOBİRLİK 📚 İGSAN - Ammonium Sulfate 21%