Applying foliar nutrients to manage a deficiency or routinely using it as part of crop nutrition program is a common practice in horticulture. But how nutrients are taken up by the leaf still poorly understood. A recent review1 may help in understanding why they may be effective one time and not as effective at other times.

Unlike roots, foliar nutrient uptake is not selective. It is a passive process and depends on:

  1. the leaf surface permeability, and
  2. the nutrient concentration gradient between the leaf surface and the leaf interior.

For any plant both of these parameters not constant. They are affected by their environment. Relative humidity is one of the most important influencing factors.

How relative humidity affects leaf permeability

Leaf cuticle permeability to nutrients is poor. It is hydrophobic; meaning it repels water and does not allow it to easily enter the leaf. But still, dissolved mineral nutrients are observed to be taken up through the leaf surface. Current research proposes two pathways: aqueous pores in the cuticle and stomata.

Aqueous cuticular polar pores: It is proposed that these pores allow for water to cluster and be absorbed by the cuticle. The clusters may form an aqueous bridge allowing dissolved nutrients to move through the cuticle.

High and increasing relative humidity allows for these aqueous pore bridges to form. This increases leaf permeability and consequently nutrient uptake increases.

Getting nutrients through the stomata: The role of stomata in foliar nutrient uptake has long been debated. Recent studies using nano-sized particles demonstrated that the particles enter the leaf by the stomata. However, it is by diffusion on the surface of stomata pores and not by mass flow.

Research has also observed that the percentage of stomata involved in foliar uptake is generally quite small. But at the same time, the total amount of nutrient taken up through the stomata can be substantial.

How relative humidity affects nutrient concentration gradient

A nutrient is dissolved in water and applied to the leaf surface at a certain concentration. As the water evaporates the nutrient concentration increases. With a higher nutrient concentration on the leaf surface and a lower concentration on the leaf inside, the concentration gradient increases. This drives the nutrient into the leaf through the aqueous pores and stomata.

However, with increasing relative humidity the water evaporates slowly. The applied nutrient concentration decreases and remains in solution longer. There is not concentration gradient between the leaf surface and its interior. Consequently the amount of nutrient moving into the leaf is reduced.

Whether or not the applied solution dries out completely or remains in a liquid in equilibrium with the atmospheric RH depends on the salt and how well it absorbs or adsorbs water from its surrounding environment.

1 Eichert, T. 2013. Foliar Nutrient Uptake - of Myths and Legends. Proc. VII International Symposium on Mineral Fruit Nutrition of Fruit Crops, Acta Hort. 984. 69-76.