Foliar Feeding Guide: How to Apply Liquid Fertilizer as Foliar Spray

Plant leaves absorb nutrients through two pathways: stomata (pores in the leaf surface used for gas exchange) and the leaf cuticle (the waxy outer layer of the leaf). Stomata are the primary entry point for most nutrients — they are open during active photosynthesis (daytime) and closed at night. The cuticle is a secondary pathway that is more important for lipid-soluble compounds.

The efficiency of foliar absorption depends on several factors: the size of the nutrient molecule (smaller molecules are absorbed more readily), the form of the nutrient (chelated forms are absorbed more efficiently than ionic forms), the leaf surface characteristics (young leaves with thin cuticles absorb nutrients more readily than mature leaves), and environmental conditions (temperature, humidity, and light intensity all affect absorption rate).

The timing of foliar applications has a major impact on their effectiveness. The best time to apply foliar fertilizer is during early morning (before 9 AM) or late afternoon (after 4 PM) when temperatures are moderate, humidity is higher, and stomata are open for gas exchange. Avoid applying during the heat of the day (10 AM - 3 PM) when stomata close to reduce water loss and leaf temperatures are high enough to cause spray burn.

Avoid applying foliar fertilizer immediately before rain, as the spray will be washed off before it can be absorbed. Also avoid applying when wind speeds exceed 15 km/h, as this reduces spray coverage and increases drift. The ideal conditions for foliar application are: temperature 15-25°C, relative humidity 60-80%, low wind, and no rain forecast for 4-6 hours after application.

Not all nutrients are equally effective as foliar sprays. The nutrients that respond best to foliar application are those that are relatively immobile in the plant (calcium, boron, iron, manganese, zinc, copper) and those that need to be delivered rapidly to specific tissues (potassium and magnesium during fruit development). Nitrogen, phosphorus, and potassium can be applied as foliar sprays but are generally more efficiently delivered through the root zone.

Calcium is particularly important as a foliar application because it moves through the plant via the transpiration stream — applying it directly to the fruit or young leaves bypasses the root-to-shoot transport limitation that causes localized calcium deficiency. Boron is most effectively applied as a foliar spray at flowering because it needs to be present in the flower at the exact time of pollination.

The key to effective foliar fertilization is using the correct dilution rate. Too concentrated, and the spray will cause leaf burn. Too dilute, and the application will be ineffective. As a general rule, foliar fertilizer solutions should be diluted to 0.1-0.5% (1-5 g/L or 1-5 mL/L) for most nutrients. Urea can be applied at up to 2% (20 g/L) without burn risk on most crops.

GreenGrow's liquid fertilizers are formulated for foliar application at 1:500-800 dilution (1.25-2 mL/L). At this dilution, a 200-liter spray tank treats approximately 1 hectare of crop. Apply until the leaves are thoroughly wetted but not dripping. Adding a surfactant (wetting agent) at 0.05-0.1% improves spray coverage and absorption on waxy-leaved crops like citrus and mango.

Foliar sprays and soil fertigation are complementary — not competing — methods of crop nutrition. The most effective programs use both methods in combination, with soil fertigation providing the bulk of macronutrient supply and foliar sprays delivering targeted micronutrient corrections and biostimulant applications at critical growth stages.

A typical integrated program might use drip fertigation for weekly NPK applications throughout the season, combined with foliar sprays of calcium and boron at pre-flowering, zinc and manganese at vegetative growth, and seaweed extract at flowering and fruit set. This approach delivers the efficiency advantages of drip fertigation for macronutrients while using foliar sprays for the precise, rapid delivery of micronutrients and biostimulants where they are needed most.