Foliar feeding

Leaves and roots

Foliar feeding means spraying fertilizer dissolved in water onto the leaves. University of Missouri Extension points out that leaves are not built to take up nutrients. Roots are. By one estimate, only 15 to 20% of what lands on a leaf gets in, and what does get in moves around the plant less well than nutrients taken up by the roots.

Missouri explains that nutrients mostly enter through tiny pores in the leaf's waxy surface. Those pores carry a negative charge, so they let in positively charged ions like calcium, magnesium, potassium and ammonium more easily. Phosphate, sulfate and nitrate are negatively charged and get in less easily.

Getting in is only half of it. Once inside, small molecules like urea, potassium and ammonium move on to other parts of the plant. Calcium, iron, manganese, zinc and copper mostly stay close to where they entered. A spray on old leaves won't do much for new growth.

Best uses

Missouri finds foliar feeding of micronutrients much more effective than of macronutrients. Its list, drawn from University of Delaware vegetable work:

Missouri's bottom line is that foliar feeding is usually not the cheapest way to feed a plant. Where it earns its place is fixing a specific deficiency, quickly.

Calcium

Blossom-end rot in tomatoes and peppers, and tipburn in lettuce, are both calcium shortages in one part of the plant. Often there is enough calcium in the root zone. The problem is getting it to the right place.

Calcium moves through the plant almost only in the water flowing from the roots to the leaves. A 1993 study by Adams and Ho in Plant and Soil found that tomato calcium uptake rose with sunlight and root temperature, and fell as root-zone salinity rose. Humidity changed where the calcium went:

Adams and Ho 1993, Table 1. Fruit 20 days after flowering, night VPD held at 0.8 kPa. Calcium per fruit rose from 0.89 to 1.13 mg.

When the leaves transpire hard, at a high vapor pressure deficit (VPD), they pull the water, and the calcium with it, toward themselves. Fruit transpires little, so it gets less. More humid days sent more calcium to the fruit and less to the leaves. VPD and transpiration come first, then the spray. The VPD guide and the VPD chart cover reading and setting it.

Calcium sprays can help, within limits:

• Tomatoes: Purdue Extension reports calcium chloride sprays reduced blossom-end rot in greenhouses but not in field tomatoes, and calls them no substitute for good soil management. Since calcium doesn't move from the leaf to the fruit, Delaware's advice, via Missouri, is to spray when the fruits are very small.
• Lettuce: a 2020 HortScience trial on hydroponic lettuce sprayed calcium chloride twice a week at 400 or 800 ppm calcium. Both controlled tipburn, with only a minor effect on yield.

Iron

Yellowing between the veins of new leaves is a sign of iron chlorosis. A 2026 e-GRO Alert explains that the usual cause is a high root-zone pH or alkaline water, which ties up iron the roots could otherwise use. A spray greens the leaves faster, but it doesn't fix the pH, so the symptoms come back. e-GRO calls foliar iron a short-term fix that goes with correcting the root zone, not a replacement for it.

e-GRO's rules for iron sprays:

• Use a chelate such as Fe-DTPA. Never spray iron sulfate on leaves.
• Spray a small test group first and wait 3 days for any injury.
• Cover both sides of the leaves. Iron gets in mainly through the undersides.
• Don't rinse it off right away. The chelate needs time on the leaf.

Spray strength

Foliar rates are often given as ppm of the nutrient. To turn that into grams of product, divide by the share of that nutrient in the product. Calcium chloride flake at 77% is about 27.8% calcium, so 400 ppm calcium takes 400 ÷ 0.278, or about 1,440 mg (1.44 g) of flake per liter.

Try it

Turn a target ppm into grams of product

Product CaCl₂ 77%Ca nitrateEpsom saltFe DTPA 10% Target ppm Batch (gal)

Per liter

1.44 g

Per batch

5.4 g

400 ppm calcium. The lettuce trial's range was 400 to 800.

Spraying

Missouri's advice on how to spray:

• Spray when the air is fairly cool, around 72°F. In summer that means early morning or evening.
• Don't spray until droplets form on the leaves. That encourages burn.
• Weaker sprays more often beat stronger sprays less often.
• Try any new spray on a few plants first.

e-GRO adds that early morning, with cooler air and lower light, reduces the chance of injury, and that good air movement helps the leaves dry. A surfactant labeled for fertilizer sprays can improve coverage.

Sources

University extension and peer-reviewed research.

• Trinklein, D., University of Missouri IPM, "Foliar Feeding Revisited" ↗, Missouri Produce Growers Bulletin (2019). Uptake through the leaf, which nutrients move, the Delaware list and how to spray.
• Adams, P. and Ho, L.C., "Effects of environment on the uptake and distribution of calcium in tomato and on the incidence of blossom-end rot" ↗, Plant and Soil 154 (1993). Calcium, humidity, sunlight, root temperature and salinity.
• Latin, R.X., Purdue Extension, "Blossom End Rot of Tomato Fruit," BP-13-W ↗. The calcium chloride spray for greenhouse tomatoes.
• Samarakoon, U., Palmer, J., Ling, P. and Altland, J., "Effects of Electrical Conductivity, pH, and Foliar Application of Calcium Chloride on Yield and Tipburn of Lactuca sativa" ↗, HortScience 55 (2020). Calcium sprays for lettuce tipburn.
• Owen, W.G., "Ironing Out Chlorosis: Selecting and Applying Iron Chelates Correctly" ↗, e-GRO Alert 15(9) (2026). Iron chlorosis and iron sprays.