Foliar urea feeding is one of the fastest ways to deliver nitrogen directly into plant tissue, bypassing soil processes. Urea is absorbed through leaves within 4–6 hours after application. This makes it indispensable under stress conditions, rapid nutrient deficiency, or when there is a need to influence crop quality in a short time.
Why Urea is the Best Choice for Foliar Feeding
Not all nitrogen fertilizers are equally suitable for foliar application. Ammonium nitrate causes burns at high concentrations due to the nitrate ion. UAN requires mandatory 1:3–1:5 dilution with burn risk if the protocol is violated. Urea at concentrations up to 15% is safe for most crops — the amide form of nitrogen penetrates well through the cuticle and does not cause osmotic burn when rates are observed.
The key requirement for urea used in foliar feeding is biuret content not exceeding 0.25% (agricultural grade). Biuret is an impurity formed during overheating in production. It is biuret, not urea itself, that causes leaf necrosis at higher concentrations.
How Foliar Urea Nutrition Works
Urea (CO(NH₂)₂) enters the leaf through stomata and the cuticle. Inside the cell, the enzyme urease hydrolyzes it to ammonium (NH₄⁺) and CO₂ within 30–60 minutes. Ammonium is directly incorporated into amino acids and proteins. The absorption rate is 5–8 times higher than root uptake through soil solution.
Practical effect: foliar urea feeding at the flag leaf stage increases protein content in wheat grain by 0.5–1.5 percentage points without increasing the root nitrogen dose.
Concentration and Application Rates by Crop
| Crop | Solution Concentration | Urea Rate, kg/ha | Solution Volume, L/ha |
|---|---|---|---|
| Wheat, Barley, Rye | 10–15% | 20–30 | 150–250 |
| Maize | 8–12% | 20–30 | 200–300 |
| Sunflower | 8–10% | 15–25 | 200–250 |
| Rapeseed | 8–12% | 15–20 | 150–200 |
| Sugar Beet | 5–8% | 10–20 | 200–300 |
| Potato | 5–8% | 10–15 | 150–200 |
| Tomato, Pepper, Cucumber | 5% | 8–12 | 150–200 |
| Fruit Trees (on open leaves) | 0.3–0.5% | 2–5 | 400–600 |
| Fruit Trees (autumn, dormant buds) | 3–5% | — | As per sprayer rate |
Optimal Conditions for Foliar Application
Foliar Urea Feeding Timing by Crop
Winter Wheat
Most effective with two foliar applications:
- 1st — stem elongation stage (BBCH 31–32): 15–20 kg/ha urea, concentration 10–12%. Increases productive tiller density and grain number per ear.
- 2nd — flag leaf stage (BBCH 39–41): 20–30 kg/ha, concentration 12–15%. The decisive treatment for protein accumulation in grain. Applied combined with a fungicide against ear diseases.
Maize
Foliar urea feeding at the 6–8 leaf stage (BBCH 16–18): 20–25 kg/ha, concentration 8–10%. Combined with herbicide application. After row closure, foliar application is difficult due to limited sprayer access.
Rapeseed
Foliar urea feeding in spring at the start of stem elongation (BBCH 30–32): 15–20 kg/ha, concentration 8–10%. May be combined with a fungicide and boron (boric acid 150–200 g/ha) to prevent pod shatter.
Fruit Trees
Two fundamentally different urea treatments are used on orchards:
- Spring application on open leaves: 0.3–0.5% solution — a safe concentration for apple, pear, and cherry leaf surfaces. Promotes vegetative growth.
- Autumn application on dormant buds (after leaf fall): 3–5% solution — prevents scab and moniliosis. Accelerates decomposition of fallen infected leaves.
How to Prepare the Urea Working Solution
Compatibility with Pesticides
| Pesticide Group | Compatibility | Note |
|---|---|---|
| Fungicides (triazoles, strobilurins) | Good | Standard practice on cereals — feeding + fungicide in 1 pass |
| Insecticides (pyrethroids, organophosphates) | Good | Jar test required for the specific product |
| Growth regulators (chlormequat etc.) | Good | Often recommended by plant growth retardant manufacturers |
| Alkaline herbicides (pH > 9) | Caution | Possible urea hydrolysis, reduced pesticide efficacy |
| Micronutrients (chelates Zn, Mn, Cu) | Good | Enhances foliar micronutrient uptake |
| Oil-based products | Caution | Possible separation — jar test mandatory |
Common Mistakes in Foliar Urea Feeding
- Exceeding concentration — the most frequent mistake. Doubling the rate "for better effect" results in leaf margin necrosis and yield reduction.
- Treatment in hot weather — rapid water evaporation concentrates the solution on the leaf surface above the safe level.
- Urea with high biuret — industrial urea (grade A) with biuret 0.3–0.6% causes burns in foliar applications. Only agricultural grade (≤0.25% biuret) must be used.
- Late cereal top-dressing — after milk ripeness (BBCH 73+), nitrogen from urea can no longer be incorporated into grain protein. Effect is zero, costs are wasted.
- Application in dew or before rain — solution is washed off before absorption or diluted to an ineffective concentration.