Urea (46-0-0) Fertilizer: When, How and How Much to Use on Hazelnuts

What Is Urea (46-0-0)?

Urea is a white, odourless nitrogen fertilizer with the chemical formula CO(NH₂)₂, sold as granules or prills. Its fertilizer grade is 46-0-0: it contains 46% nitrogen (N) and no phosphorus or potassium.

At 46% N it is the most concentrated solid nitrogen fertilizer on the market. That single fact explains its popularity in hazelnut orchards: delivering a given amount of pure nitrogen requires far fewer bags to carry, store and spread than any alternative.

Property	Value
Formula	CO(NH₂)₂ (46-0-0)
Nitrogen (N)	46%
Nitrogen form	Amide nitrogen (NH₂-N)
Phosphorus / Potassium	None
Solubility	Very high (dissolves quickly even in cold water)
Physical form	White granule / prill
Effect on soil pH	Acidifying after hydrolysis
Main weakness	Ammonia volatilization if left on the surface

Why Is It So Widely Used?

• Lowest cost per unit of nitrogen. Fertilizers should be compared by the pure nitrogen they carry, not by the price of the bag. A bag of urea carries more than twice the nitrogen of the same weight of ammonium sulfate, so on a “cost per kilogram of actual N” basis urea is usually the cheapest option.
• Transport and storage advantage. Supplying 20 kg of pure nitrogen takes about 43 kg of urea, versus 95 kg of ammonium sulfate. In steep orchards far from a road, that difference is measured in sacks carried on your back.
• High solubility. It dissolves rapidly even in cold water, which makes it suitable for both soil and foliar application.

What Happens to Urea in the Soil? Urease and the Conversion Chain

The nitrogen in urea is not in a form the plant can use directly. Roots take up nitrogen mainly as ammonium (NH₄⁺) and nitrate (NO₃⁻). Urea reaches those forms in three steps:

1. Amide → ammonium (hydrolysis). The urease enzyme, abundant in soil and in plant residue, breaks urea down into ammonium, carbon dioxide and ammonia (NH₃). In warm, moist soil this step is often complete within 1–3 days.
2. Ammonium is held. Ammonium carries a positive charge, so it binds to soil colloids (clay and organic matter) and resists leaching reasonably well.
3. Ammonium → nitrate (nitrification). Soil bacteria convert ammonium into nitrate. Nitrate is the fastest-absorbed form but carries a negative charge and does not bind to soil — under the wet Black Sea climate it can leach below the root zone.

The Heart of the Matter: Nitrogen Lost to Ammonia Volatilization

Urea’s low price loses its meaning when the fertilizer is applied badly. Part of the nitrogen released during hydrolysis turns into ammonia gas (NH₃). If the granule is sitting exposed on the soil surface, that gas escapes straight into the atmosphere — nitrogen you paid for never reaches the tree.

Research compiled by Manitoba Agriculture on surface-applied urea shows the scale of the problem:

Condition	Ammonia loss over 5–7 days
15 °C, urea spread on the surface	Under 7%
25 °C, urea spread on the surface	38–46%
30 °C, residue-covered surface (zero tillage)	Up to 88%
30 °C, urea treated with a urease inhibitor (NBPT)	12%

The same review notes that for urea left on the surface for four days, losses doubled from 10% to 20% as soil pH rose from 6.5 to 7.5.

Conditions That Increase the Loss

• High temperature — losses at 25–30 °C are several times those at 15 °C. This risk is real for the late-May/June application.
• High soil pH — limed plots, or plots recently limed, lose noticeably more.
• A dry or rapidly drying surface — the worst case is a surface wetted by dew or light rain and then dried by sun: urea dissolves, hydrolysis starts, but there is no water to carry it into the soil.
• Plant residue and leaf litter — the leaf-and-weed layer under hazelnut stools is very rich in urease and keeps the granule from touching soil. Urea spread on that layer carries the highest risk of all.
• Wind — it continuously carries ammonia away from the surface, so volatilization never slows down.
• Applying at the same time as lime — lime raises surface pH and increases the loss.

How to Reduce the Loss

What is a urease inhibitor? It is a coating additive (most commonly NBPT) that temporarily suppresses soil urease activity. It delays hydrolysis by a few days; if rain arrives in that window, the urea moves into the soil and no nitrogen is lost. As the table above shows, at 30 °C it cut losses from 88% to 12%. On steep plots where you cannot incorporate, the extra cost can be justified; where you can incorporate, it is unnecessary.

When Should It Be Applied to Hazelnuts?

The growing-technique recommendation of the Hazelnut Research Institute is that nitrogen fertilizer be applied in two split doses rather than all at once:

Application	Timing	Share	Phenological stage
1st application	End of February – early March	Half of the recommended amount	Bud swell, before shoot growth
2nd application	End of May – early June	The remaining half	Nut development, before kernel fill

With split applications the nitrogen is available at the moment the plant needs it rather than leaching away first, and the salt and volatilization risk of a single heavy dose is spread out. The Giresun Provincial Directorate of Agriculture and Forestry likewise stresses that splitting nitrogen fertilizer in two matters for plant development and nut quality.

How Should It Be Applied?

The Hazelnut Research Institute describes the application as follows:

1. Spread the fertilizer evenly in a ring-shaped band 30–40 cm wide under the drip line of the stool’s branches. Do not tip it against the trunk — the absorbing fine roots are concentrated under the canopy edge.
2. Work the fertilizer into the soil to a depth of 5–10 cm with a hoe. This step is not optional; it is what saves the economics of urea.
3. On sloping ground, hazelnut roots are not evenly distributed. The Institute’s recommendation is to place the most fertilizer at the sides of the stool, some on the downhill side and the least on the uphill side. Fertilizer piled above the stool washes downhill with the first heavy rain.

Foliar Urea Application

Urea is one of the few nitrogen sources that can also be absorbed through leaves. A study on the Çakıldak cultivar carried out at Ordu University in 2018–2019 and published in the Ordu University Journal of Science and Technology applied increasing foliar urea rates (0%, 0.125%, 0.25%, 0.50%). The results:

• The 0.25% urea treatment raised total leaf nitrogen concentration from 1.58% to 2.01% (about a 27% increase).
• Leaf potassium concentration rose from 0.40% to 0.80%.
• With three applications of 0.25% urea, yield rose from 157 kg/ha in the control to 205 kg/ha (a 30.6% increase).

The authors recommend three foliar applications of a 0.25% urea solution to improve both nitrogen nutrition and yield.

Foliar application should be seen as a complement to, not a replacement for, soil fertilization. A tree’s full annual nitrogen requirement cannot be met through leaves.

Dose Logic: From Pure Nitrogen to Fertilizer Quantity

A soil analysis report tells you how much pure nutrient is needed, not how much fertilizer. The conversion is simple:

Urea quantity = required pure nitrogen ÷ 0.46 (i.e. pure N × 2.17)

Pure nitrogen required	Urea needed
5 kg N	≈ 10.9 kg
10 kg N	≈ 21.7 kg
15 kg N	≈ 32.6 kg
20 kg N	≈ 43.5 kg

In its advisory on nitrogen fertilization of hazelnuts, the Giresun Provincial Directorate of Agriculture and Forestry states that where the pure nitrogen recommended for the growing season is supplied as urea, 375 g in the first application and 375 g in the second — 750 g of urea per stool in total will be sufficient. The same advisory gives the calcium ammonium nitrate (CAN) equivalent as 650 + 650 g in two applications.

Urea vs. Ammonium Sulfate vs. CAN

Criterion	Urea (46-0-0)	Ammonium Sulfate (21-0-0)	Calcium Ammonium Nitrate (26-0-0)
Nitrogen content	46%	21% (+ ~24% S)	26%
Nitrogen form	Amide (NH₂)	Ammonium (NH₄⁺)	Half ammonium, half nitrate
Speed of action	Slow (hydrolysis required)	Moderate	Fast (nitrate portion acts at once)
Effect on soil pH	Acidifying	Strongly acidifying	Neutral / slightly raising (contains lime)
Volatilization (NH₃) loss	High if left on the surface	Low	Very low
Leaching loss	Moderate (rises as nitrate forms)	Low	High (nitrate portion)
Cost per unit N	Lowest	High	Moderate
Transport burden	Lowest	Highest	Moderate
When to choose it	Wherever you can incorporate it or apply before rain	Where sulfur is deficient and soil pH is high	On acidic soils and where fast action is needed

Because most Black Sea hazelnut soils are acidic, official publications often put CAN first. Urea, on the other hand, dominates in practice thanks to its cost per unit of nitrogen and ease of transport. The right answer is neither product by default but the pH figure on your soil analysis: in an orchard whose pH has already fallen below 5.5, relying permanently on urea and ammonium sulfate deepens the acidity. For a detailed comparison see our Ammonium Sulfate 21-0-0 and Calcium Ammonium Nitrate 26-0-0 guides.

Common Mistakes

1. Leaving the fertilizer on the surface. The most expensive mistake there is. Most of the nitrogen in urea spread over leaf litter on a hot day goes into the air.
2. Spreading without clearing the leaf and weed layer. That layer is both a source of urease and a barrier to soil contact.
3. Applying the whole year’s nitrogen at once. Splitting reduces both losses and salt risk.
4. Applying nitrogen after July. Shoots fail to harden off, and frost and mildew risk climb.
5. Applying at the same time as lime. It raises surface pH and increases ammonia loss — leave at least a month between them.
6. Piling fertilizer against the trunk. The absorbing roots are under the canopy edge; concentrated fertilizer at the trunk causes root burn.
7. Assuming more nitrogen means more yield. Excess nitrogen means excess shoots, shading, soft tissue, disease susceptibility and poor kernel quality.
8. Forgetting phosphorus and potassium. Urea supplies nitrogen only. For phosphorus see Urea Phosphate 18-44-0 or DAP 18-46-0; for potassium see Potassium Sulfate 0-0-51.

Storage

Urea is highly hygroscopic — it draws moisture from the air and cakes. Never leave a bag open; store it in a dry, cool, shaded place, off the ground on a pallet, and avoid stacking the bags too high.

Summary

• With 46% N, urea is the most concentrated nitrogen fertilizer and the cheapest per unit of nitrogen.
• Its nitrogen is in amide form; soil urease converts it first to ammonium, then to nitrate.
• Its biggest risk is ammonia volatilization: on a hot, dry, windy, residue-covered surface, losses can be very large.
• The fix is simple and free: incorporate it 5–10 cm deep, or apply before rain or irrigation.
• Split the application in two — end of February/early March and end of May/early June.
• Spread it in the 30–40 cm band under the drip line; on slopes, weight the application to the sides of the stool.
• The research rate for foliar spraying is 0.25% — higher rates risk leaf burn.
• Always set the dose from your soil analysis and consult your provincial or district agriculture directorate.

For the full annual calendar see our Hazelnut Fertilization Programme; for sampling steps see the Soil Analysis Guide.

📚 Ministry of Agriculture and Forestry, Giresun Provincial Directorate 📚 Hazelnut Research Institute - Hazelnut Growing Technique 📚 Özkutlu et al. - Foliar Urea Application in Hazelnut (DergiPark) 📚 Manitoba Agriculture - Volatilization Losses From Surface Applied Nitrogen 📚 Tarfin - Urea Fertilizer