How effective are foliar applied nutrients in dry weather?

Christoph Kessel, Soil Fertility Specialist (Horticulture), OMAFRA 

Many parts of Ontario have seen extremely warm weather and below average rainfall this season. With this weather the soils are drier as well.  These extended periods of dry soils may lead to reduced plant nutrient uptake since plant roots can only take up nutrients that are in soil solution.

It would seem then that foliar applied nutrients during these hotter, dry periods may look like an opportunity to compensate for limited soil nutrient uptake. However, these same weather conditions affect:

• leaf cuticle chemical composition
• the passive uptake through leaf stomata and cuticle, and
• the drying of foliar applied nutrients.

These factors combined will limit the effectiveness of foliar applied nutrients.

The leaf surface is a natural barrier: Foliar fertilization is against nature.  The leaf surface is a barrier. The nutrients we want to get into the leaf are dissolved in water.  But the leaf surface is covered in a waxy cuticle that repels water restricting nutrient movement into the leaf.  The chemical composition of the cuticle reflects the environmental and plant’s physiological conditions during development.  Long periods of hot dry weather during leaf development will affect cuticle thickness.

Uptake through the leaf:  Nutrient uptake into the leaf is a passive process.   It depends on the leaf surface permeability and the nutrient concentration gradient between the leaf surface and the leaf interior.  While not clearly understood, this passive uptake appears to take place through the leaf stomata and aqueous cuticular polar pores.

In response to the drier soil conditions, stomata close to reduce water loss, reducing the diffusion of nutrients into the stomata.

Aqueous cuticular polar pores allow for water molecules to cluster together and be absorbed by the cuticle.  The clusters form an “aqueous bridge” between the outer layer of the cuticle and the leaf epidermis, permitting dissolved nutrients to move through the cuticle.  However, during hot, dry weather conditions, these aqueous bridges do not form, limiting passive uptake through the cuticle.

Drying of the product on the leaf:   Relative humidity (RH) affects the evaporation rate of the applied nutrient solutions. How quickly or slowly it evaporates affects solute concentration on the leaf surface.  This in turn influences the driving force of penetration and uptake rates.   At low RH, the solutions evaporate quickly and uptake is low.  If the RH is too high, the nutrients remain in solution longer and more dilute, and uptake is low.

Because of the RH’s combined effects on the concentration gradient and the permeability of the leaf surface, maximum penetration rates can be expected at an intermediate RH.

Foliar applied nutrients can be a useful tool to compensate for nutrient deficiencies.  However, during extended drier, hotter weather conditions, their effectiveness can be limited by changes in cuticle chemical composition, the passive uptake through leaf stomata and aqueous pores, and the drying of foliar applied nutrients.