Foliar Spray Application Guide & Calculator

Foliar feeding is one of the most efficient ways to deliver nutrients to a plant, especially when soil conditions make nutrient uptake difficult (such as high pH or cold temperatures). By spraying a nutrient solution directly onto the leaves, you bypass the soil entirely, allowing for almost immediate absorption.

However, foliar spraying is a "high risk, high reward" strategy. If your concentration is too high, you can cause devastating Leaf Burn (necrosis). If it's too low, you're wasting time and water.

In this guide, we'll focus on the most common foliar nutrient—Urea—and how to apply it safely.

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Why Foliar Feed?

While roots are the primary way plants drink, leaves have tiny pores called Stomata and a permeable cuticle that can absorb water-soluble nutrients.

• Speed: Nutrients can be absorbed in as little as 30 minutes.
• Precision: You can correct specific deficiencies (like Iron or Zinc) quickly.
• Efficiency: Near 90-100% of the nutrient is utilized, compared to as low as 10-20% in some soil applications.

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How Much Urea Per Liter? (The Golden Rule)

Urea (46-0-0) is the most popular foliar nitrogen source because it is a small, uncharged molecule that passes through the leaf cuticle more easily than nitrates or ammonium.

Safe Concentration Rates

For most agricultural crops (tomatoes, peppers, corn, wheat):

• Safe Range: 0.5% to 1.0% solution.
• In Grams: 5g to 10g of Urea per 1 liter of water.

Sensitive Plants

For sensitive plants (orchids, seedlings, or flowering ornamentals):

• Safe Range: 0.25% solution.
• In Grams: 2.5g of Urea per 1 liter of water.

High-Volume Application

If you are doing a large-scale application, a common rate is 5 to 10 kg of Urea per 1000 liters of water per hectare.

Warning: Never exceed a 2% concentration (20g/L) unless you are an expert grower with specific variety data. Most leaf burn occurs at the 2.5% threshold.

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The Best Time to Spray

Timing is everything in foliar application. The goal is to keep the leaves wet for as long as possible to maximize absorption time.

1. Early Morning (Best): The stomata are open, temperatures are cool, and humidity is usually higher.
2. Late Evening: Good, but can lead to fungal issues if the leaves stay wet all night in humid climates.
3. Cloudy Days: Excellent for foliar spraying as the solution won't evaporate quickly.

NEVER spray in the middle of a hot, sunny day. The water will evaporate instantly, leaving a highly concentrated salt crust on the leaf that will suck the moisture out of the plant cells, causing immediate burn.

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Step-by-Step Mixing Instructions

1. Check Water Quality: Use clean water. High-pH or very hard water can reduce nutrient solubility.
2. Measure Accurately: Use a digital scale. Guessing "a tablespoon" is how leaf burn happens.
3. Dissolve Fully: Urea is highly soluble. Add it to a small amount of warm water first to create a concentrate, then dilute to final volume.
4. Add a Surfactant (Optional but Recommended): A tiny drop of dish soap or a professional "spreader-sticker" breaks the surface tension of the water. This prevents the spray from forming beads and rolling off the waxy leaf surface.
5. Test a Small Area: Before spraying your whole field or greenhouse, test the mix on 2-3 leaves and wait 24 hours.

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Avoiding Biuret Toxicity

Biuret is a chemical byproduct sometimes found in commercial urea. While harmless in the soil, biuret is Toxic to Leaves.

When buying urea for foliar use, look for "Low Biuret" Urea (typically less than 0.5% biuret content). Standard agricultural urea can sometimes have 1-2% biuret, which is fine for soil but risky for leaves.

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Summary

Foliar spraying is a powerful tool for the modern grower. By keeping your concentrations between 5g and 10g per liter and spraying during the cool parts of the day, you can see a visible greening effect within 48 hours.

Want to calculate a complex mix of Urea, Potassium Nitrate, and Chelated Iron? Use our calculator to ensure your total salt concentration remains in the safe zone.

Try the Foliar Rate Calculator →

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Related Reading

• What is NPK?
• PPM to Grams Conversion
• Fertilizer Compatibility Chart