How leaves absorb nutrients in foliar fertilization

Leaves can absorb nutrients directly from solutions sprayed onto their surface, and foliar fertilization is built on this ability. It is a tool for fast, targeted nutrition that complements rather than replaces soil fertilization.

How nutrients enter the leaf

The outer surface of the leaf is covered by the cuticle, a waxy, hydrophobic layer that acts as a barrier both against water loss and against the entry of dissolved substances. Hydrophilic nutrients pass through slowly, via microscopic pores in the cuticle and, to a lesser extent, through the stomata. Because the fate of foliar-applied nutrients remains uncertain, from the leaf surface to the plant's metabolism, the form and chemical state of the nutrient largely determine whether it will enter and be used.

What determines efficiency

The efficiency of a foliar application varies widely and depends on the composition of the product, the environmental conditions, and the plant itself. Wetting and sticking agents lower surface tension and improve the coverage and retention of the solution on the leaf. Humidity and temperature matter, since cool, humid hours slow drying and leave more time for absorption. For this reason the effectiveness of each application is judged by the combination of timing, fertilizer type, and soil nutrient availability, not by any single factor.

Nutrients pass through the cuticle and the stomata of the leaf.

Which nutrients work as foliar sprays and which do not

Foliar application is particularly effective for rapidly correcting micronutrient deficiencies, such as iron, zinc, boron, and manganese, which are required in small amounts. Nitrogen and potassium work as a supplement at high-demand stages, without replacing the base fertilization. By contrast, some nutrients do not remobilise easily within the plant once bound to tissues, with calcium as the classic example, so the spray is directed straight at the organ we want to strengthen.

When it beats soil fertilization

Foliar fertilization gains a real advantage under specific conditions. When crop demand exceeds the root's capacity to absorb, or when soil conditions restrict uptake, spraying the foliage bypasses the obstacle. In the calcareous, alkaline soils common around the Mediterranean, iron and other micronutrients are fixed at the high pH and become unavailable, causing the characteristic chlorosis. There, foliar correction gives a fast result, while addressing it through the soil would be slow and ineffective.

leaves with iron chlorosis (interveinal) before and after correction.png

In alkaline soils, iron deficiency is corrected faster by foliar spraying.

Practical application under Mediterranean conditions

The spray is applied during the cool hours, early morning or late afternoon, with full wetting of the foliage and at doses that do not cause scorching. The nutrient form is chosen to be readily available, while wetting agents improve coverage on hard or waxy leaves. For nutrients that move poorly, applications are repeated at the appropriate stages. In every case, foliar fertilization fits within a programme based on soil fertilization and analysis.

Applying during the cool hours increases absorption time..png

Applying during the cool hours increases absorption time.

How the main nutrients work and how to choose a fertilizer by crop is covered in our guide on compound NPK fertilizers. Relevant products are available in the category of liquid and foliar fertilizers.