Foliar fertilizers: what is foliar spray and how do you make it?

Adding composts, organic mulches and various foliar feeds can be used to both restore soils lacking in a healthy soil food web and keep fertility active. Although spring and autumn are the traditional months for adding compost and mulches, a little judicious application over the summer can be just as useful. Foliar feeds throughout the growing season can also do wonders for plants, helping to address deficiency, giving plants a quick pick-me-up and preventing diseases.

For more on mulching head to our guide on how to mulch properly.

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What are foliar fertilizers?

Foliar sprays

Foliar feeds

Home-made compost and organic mulches will feed the soil, lock in moisture and prevent evaporation on hotter days. There are times, however, when you will need to feed the plants directly rather than the soil. This is particularly true of container- or pot-grown plants. There are two ways you can feed: as a soil drench or as foliar feed. Plants take up nutrients far more rapidly through their leaf surface (a matter of minutes) than through their roots and the soil (can take several days). A foliar feed can be a direct injection of energy, compounds and necessary nutrients for an ailing plant or one showing the beginning signs of stress.

The plant’s leaf surface also has its own microbial community; there are many fungi and bacteria that live only on plant leaves and the right feed can also help to boost the good guys in this community (as opposed to the less desirable ones such as mildews).

Foliar feed sprays should be applied during the growing season; depending on the feed they can be used weekly, fortnightly or once a month. They should be applied early morning or early evening, out of direct sunlight. Foliar sprays need to cover roughly 70 per cent of the leaf surface to be effective, ideally both sides of the leaves. Any foliar feed can also be used as a soil drench as part of the feeding regime.

Foliar ferments

Foliar ferments are traditionally made from comfrey or nettles rotted down in water into a liquid feed. Other options for foliar ferments include dandelions (rich in minerals, such as calcium and magnesium, and nutrients, such as potassium), mares’ tail (rich in chlorine, calcium, magnesium and potassium) and chickweed (rich in phosphorous and potassium). Add your plant material to a bucket (around three-quarters full) and cover with water (ideally rain water). After two to three days this can be strained and diluted at a ratio of 1:10 and used as a soil biological inoculant (similar to activated compost tea). After a week the pH drops, the material can be strained the liquid stored or you can let if further decompose (this is when it gets smelly) indefinitely. Strain and use at a ratio of 1:10 (roughly diluted to the colour of weak tea).

How to make compost tea

You can make aerated or activated compost tea using a compost tea brewer with an aerator (often an aquarium pump). This actively pumps oxygen into a mixture of compost and water to create a concentrate of aerobic microbes that are beneficial for the soil food web. You can use it on both the soil and leaf surfaces.

Fermented plant juice also works well

Another alternative is a fermented plant juice. For this you can use either leafy material or fruit (bruised tomatoes work well). Mix your plant material (either leaves or fruit, but not both) with organic brown sugar (roughly three parts plant material to two parts sugar or 1:1 with fruit) in a glass jar and cover the top with more sugar. Weigh down the material with a stone or glass of water to initiate fermentation. Cover with a cloth, ferment for a week or more out of direct sunlight until you have a liquid. Strain the liquid and store in a jar with a tight lid. Dilute to a ratio of 1:500 and as a foliar spray.