How to make fertilizer

by Emma Cooper

credit: Emma Cooper

We expect a lot from our gardens, and in vegetable patches or cutting gardens, we take a lot out. To keep beds and borders blooming, we need to feed the plants or the soil but there’s no need to resort to commercial fertilisers – you can easily make your own.

Fertilisers are liquid or solid, and the main difference is the speed at which they act. Solid fertilisers tend to be slow-release, feeding plants over a long period. Liquid fertilisers give plants a quick fix but can wash out of the soil. They should only be applied to soil or compost that’s already damp.

Commerical fertilisers have an N:P:K ratio, which tells you the proportions of the three major plant nutrients: nitrogen, phosphorus and potassium that they contain. Magnesium, calcium and sulphur are needed too. There are also micronutrients, the plant equivalent of vitamins, needed in smaller amounts, and at different stages of growth.

It’s important to use the right fertiliser for the plant, its stage of growth and the time of year. Using the wrong one wastes money, and can cause environmental problems. Over-feeding plants can make them sappy and vulnerable to pests and diseases; it can even stop them from flowering and producing fruit.


The ideal solid fertiliser for your garden is homemade compost (there are plenty of composting options for urban gardeners: see here for more details). Compost contains a good mix of nutrients, and improves the structure of the soil, making it easier for plant roots to access the air, water and food that they need.

Compost can be applied throughout the garden, either dug in to the soil or applied as a mulch. You can also add it to your container mixes, where it will slowly release nutrients.

Worm compost bins also produce run-off that can be watered down to the colour of weak tea and used as a plant feed. It’s ideal for containers, or any plant needing a quick boost, as it supplies a good range of nutrients.


credit: Emma Cooper


Nettles can be turned into a high nitrogen liquid feed by rotting them down in a bucket of water. Nitrogen is the N in NPK which boosts green leafy growth and is an important component of plant cells. In the veg patch it’s an ideal feed for quick-growing salads and leafy veg as well as blackcurrants and sweetcorn.

Don’t be too quick to use it on flowering plants though, as it might encourage leaves instead of flowers. And don’t apply high nitrogen feed in the autumn because it promotes soft, sappy growth that won’t survive winter weather.

To make a nettle feed, harvest enough leaves and stems to fill a lidded bucket, and scrunch them up to bruise them slightly. Weigh them down with a brick (or similar) and cover them with water. Pop the lid on your bucket, and leave the mixture to stew for about a month. Once your mixture is brown (and good and smelly!), it’s ready for use. Strain out the remains of the nettles, and dilute your nettle feed to the colour of weak tea (about 1:10) for use.


Comfrey is another plant you can rot down into a liquid feed using the procedure outlined above, but this time it’s high in potash (or potassium) -the K in NPK which promotes flowers and fruits as well as helping along processes in plant cells such as the movement of sugars. Comfrey is a common wild plant, but you can also have a patch of the ‘Bocking 14’ variety in the garden, which is easy to keep under control.

Tomatoes, peppers and other fruiting vegetables all benefit from a high potash feed when they are flowering and fruiting. It’s also good for strawberries, other soft fruit, and rhubarb.

In the flower garden, use comfrey feed on agapanthus, flowering plants in pots, and fuchsias. Potash also promotes hardiness, so you can apply it to all plants during the run-up to winter.


If you’re by the coast then you could collect seaweed, although check with the local council before you do this and remember to check the tide times and stay safe. Stick to ‘beach cast’ seaweed, i.e. strands that have been detached and stranded on the beach. Dry seaweed is much lighter to carry than fresh, and if you can collect it after rain, your haul will be less salty.

Seaweed can be a good source of micronutrients, and makes a great plant tonic (follow the above instructions for making liquid feeds), but is also used as a mulch for general feeding, and can be added to the compost heap.

Green manures

Green manures are plants grown to benefit the soil. There are many varieties, and which one you choose depends on when you want to sow it, how long you want it to grow for, and what you want it to do. Green manures have a variety of roles: some simply add organic matter while nitrogen-fixing ones add extra nitrogen to the soil. Green manures also make phosphorous – the P in NPK – available to plants – mustards and legumes are particularly useful in this respect. Phosphorous promotes root growth and development; it helps stimulate early maturity of fruits and seeds, encourages flowering and seed production and enhances winter hardiness.

Good choices for small spaces include trefoil, which is a low-growing, nitrogen-fixing plant. Used to undersow brassicas, it can also help to protect against cabbage white butterflies. Trefoil can be left in place until you harvest your crops, and then dug in. Sow seeds from March to August.

Phacelia tanacetifolia is a metre tall, with pretty purple flowers that attract beneficial insects, but you should dig it in (or cut it down and compost it) before it seeds. Sow from March until September.


credit: Emma Cooper

Mustards are fast-growing, leafy plants that contribute a lot of organic matter very quickly. Fenugreek can do the same, but has much smaller leaves. Both can be sown from March until August.

Other ideas

Human urine makes a great nitrogen-rich liquid feed, watered down 10:1 for mature plants and more for seedlings. Use it fresh, as once it starts to smell you’re losing nitrogen as ammonia gas. Urine can be salty, so don’t overdo it, especially on container plants.

Meat eaters can make their own bonemeal. Clean bones and hang them to dry (or bury them in the compost for a while). Once they’re brittle, they can be smashed with a hammer (wear suitable protective gear) and used as a slow-release source of phosphorus.

Chicken manure pellets can be turned into manure tea by stirring them into water (as can sheep poo, if you can find a local source). Apply it freshly made to plants that appreciate a lot of nitrogren.

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