Vegetable growers sometimes spray liquid fertiliser on the leaves of their plants and this is called foliar feeding. Foliar feeds work well when applied in the form of a fine mist.
Generally we acknowledge that plant roots are responsible for the absorption of minerals. However leaves are also capable of absorption.
Botanists point to examples of foliar feeding which take place in forests. The tall trees prevent plants and shrubs below them from receiving much sunlight. This means that little photosynthesis takes place in these plants. So how do they produce complex sugars for plant growth, root growth and root exudates? The plants are able to absorb amino acids, minerals, and even organic humus fractions through their leaves. Fine dust particles, dead insects, bird droppings, algae, and mosses decay drop down from the canopy above and are processed by the plant leaves.
How does foliar feeding take place?
There are three ways in which leaves take in amino acids and minerals: through the leaf hairs, through the wax layer on the leaf and by absorption and release via cells.
Botanists tell us that we can find leaf hairs or trichomes in the epidermis or outer layer of a leaf. Typically they are unicellular, i.e. made up of one cell. Positively charged minerals and amino acids are usually well absorbed through these bulges.
The wax layer or cuticles on the leaf can absorb foliar fertilisers in very small amounts.
Absorption and release via cells is called endocytosis. This is a process by which cells absorb external material by engulfing it with the cell membrane. The minerals are transported to so-called storage cells (sink cells), which absorb and store the minerals for later use.
When to apply foliar feeding
Spraying liquid fertiliser will help the initial vigorous growth of a plant. Later it encourages fruiting and ripening.
It is particularly effective for plants that are showing signs of stress. This could be due to excessive heat, drought or problems with the fertility of the soil. A stressed plant closes its stomata, which affects the process of photosynthesis. This makes the plant more vulnerable to pests and disease. A supplementary foliar spray will help them to produce complex sugars quickly.
They are also good for treating deficiencies in iron, zinc, magnesium, and more. However, foliar fertilisation does not replace good soil fertility management.
It is best to spray early in the morning or in the late afternoon.
Types of foliar fertiliser
Foliar sprays are based on a variety of different sources.
If you are buying a liquid fertiliser, make sure that it is suitable for foliar applications. Follow the instructions carefully, as it is easy to burn the leaves if the mixture is too strong. Nourish, made by Talborne Organics, is a popular organic foliar fertiliser.
Smallholders who make their own sprays use kelp, compost tea, weed tea, herbal tea, comfrey tea and fish emulsion.
Additional information supplied by Talborne Organics.
This is part five of a series of articles on seeds and plant nutrition. For more, click here.