An introduction to foliar nutrition

Foliar fertilisers are used both as part of plant nutrition programmes and to correct deficiencies of particular nutrients.

Soil is the primary focus of programmes supplying plants with macro- and micronutrients, and it is their main nutrient source. Where various factors prevent sufficient nutrients from reaching the above-ground parts through the xylem, foliar feeding can provide temporary, supplementary or corrective nutrition, but only under suitable environmental conditions.

Unlike uptake through roots, foliar nutrient uptake is a passive process. It depends on the leaf surface, or epidermis, and the concentration gradient between that surface and the parenchyma, the internal leaf tissue between the upper and lower epidermis.

Passive transport is the simplest form of movement across a cell’s plasma membrane. It requires no cellular energy because substances diffuse across the membrane along their concentration gradients.

A concentration gradient is a difference in a substance’s concentration between different areas. Substances diffuse down their gradients, from higher to lower concentrations.

The leaf surface is a natural barrier. Its waxy cuticle repels water and dissolved micronutrients, limiting their movement into the leaf.

Foliar micronutrient uptake depends on:

– The plant species.

– The nutrients, their chemical forms and their mobility in the phloem.

– Spray coverage of the leaf and subsequent uptake.

– The leaf’s condition and structure, which affect that coverage and uptake.

– Environmental conditions.

– Plant requirements and the amount of nutrients available in the soil.

Relative humidity is particularly important because it affects nutrient permeability through the cuticle. It also influences how quickly the spray solution evaporates.

For leaves and other green plant parts to absorb sprayed nutrients effectively, relative humidity should be between 60-80%.

Another important factor is nutrient mobility in the phloem. Once nutrients enter a leaf, the phloem is their only means of onward transport. Some elements, particularly calcium, manganese and boron, have limited phloem mobility, depending on the plant species.

These nutrients must therefore be sprayed onto the green tissues that need to absorb them. Where the entire plant needs treatment, the spray solution must cover the whole tree.

The amount of nutrients that can be applied through leaves is limited by the risk of toxicity from overdosing. Apply low concentrations carefully, guided by analysis and good agricultural practice.

Agronomist’s recommendations:

For leaf analysis to establish precise macro- and micronutrient levels, leaves should have reached full size, when their nutrient concentrations stabilise.

Because leaves do not reach full size until early summer, low-concentration foliar applications of various macro- and micronutrients during spring can help prevent deficiencies, following good agronomic practice.

Adding spray adjuvants and pH adjusters to the tank is advisable. Avoid applications in intense, scorching sunlight.

We recommend foliar nutrition products from the Spanish companies Nufol, MASSO and Fertinagro.

Article prepared by:

Agronomist: Davit Dabrundashvili

Source: https://www.publications.gov.on.ca/store/20170501121/Free_Download_Files/300751.pdf