Foliar Feeding

Foliar feeding is most effective for micronutrients (iron, zinc, manganese, boron) because crop demand for these elements is small — 100–500 g/ha per season — and can be fully supplied through one or two foliar sprays. Macronutrients (N, P, K) have much larger demand (50–300 kg/ha) that cannot be realistically delivered through foliar applications; foliar N is typically used as a late-season supplement at 5–30 kg/ha, not as a primary nitrogen source.

Absorption depends on formulation, environment, and leaf condition. Nutrients penetrate fastest when leaves are young, humidity is high (60–90%), temperature is moderate (15–25°C), and the solution includes an adjuvant (surfactant) to reduce surface tension. Morning or late-afternoon applications avoid heat stress and maximize stomatal opening. Typical spray volumes are 200–400 L/ha for ground equipment and 40–100 L/ha for aerial. Concentrations must stay below phytotoxic thresholds — urea-based N sprays above 2% concentration can burn leaves, zinc sulfate above 0.5% can cause damage on sensitive crops.

Foliar feeding complements but does not replace soil-based fertility. It is especially valuable for (1) rescue scenarios — a chlorosis-ravaged field can recover within 5–10 days after an iron chelate spray, (2) critical-stage supplementation — boron at flowering to improve seed set, calcium during fruit development to prevent blossom-end rot, (3) saline or alkaline soils where root uptake of micronutrients is suppressed, and (4) foliar-diagnostic workflows where tissue tests identify gaps that can be corrected mid-season. High-value crops (fruit trees, vineyards, greenhouse vegetables) often receive 5–10 foliar applications per season as a planned fertility program.