Garden Notebook

Plants, gardening and good tools, between passion and practice

Homemade fertilisers: what really works, and what is just folklore

Daniel Platteau Articles gardening tips

A culture of recycling, between peasant knowledge and optimistic tinkering

Let us start by doing this practice justice. Making your own fertilisers is not a modern fad born with social media: it is what gardeners did for centuries, for lack of an alternative. Nettle tea was being made on farms long before the chemical industry existed, and 19th-century agricultural treatises already described plant macerations in detail. This continuity deserves respect.

But it has also had a less happy side effect. Because these recipes are passed on by word of mouth, from neighbour to neighbour and then from forum to forum, they have picked up a thick layer of beliefs that were never checked. The reasoning behind them is often the same, and it is appealing: "this waste contains such-and-such an element, so adding it to the soil will feed my plant". The problem is that chemistry does not work that way. Between containing a nutrient and making it available to a root there is sometimes a gap of several years, and sometimes an insurmountable obstacle.

The sorting that follows is not meant to discredit homemade gardening — quite the opposite: several of these preparations are excellent. It aims to separate what rests on agronomic measurements from what rests on misplaced intuition, so that the time you spend on these preparations is time well spent.

Nettle tea, the gardener's nitrogen

Of all homemade preparations, this is probably the best documented, and its reputation is deserved. The stinging nettle is a nitrophilous plant: it thrives in nitrogen-saturated soils and accumulates nitrogen in its tissues in remarkable concentrations. When it ferments in water, anaerobic bacteria break down these tissues and release nitrogen in soluble forms — mainly ammonium — that roots can absorb almost immediately. The liquid also contains potassium, magnesium, iron and various trace elements, in proportions that vary enormously with the harvest date, the soil it grew in and the length of fermentation.

That is precisely the main limitation of nettle tea: its variability. Published analyses give nitrogen contents that can differ by a factor of five between two batches. So you never dose nettle tea the way you dose a shop-bought fertiliser, to the gram; you use it with a comfortable safety margin. The consensus dilution is one part tea to ten parts water for watering at the base, and one to twenty for foliar spraying — the latter acting faster but more superficially.

Making it requires little equipment and a lot of olfactory patience. Allow about one kilogram of fresh nettles, roughly chopped, for ten litres of water — rainwater by preference, as the chlorine in tap water hinders fermentation. The container should be plastic or wood, never metal, which would react with the acidic liquid. Stir every day and watch the foam: as long as it forms abundantly, fermentation is active. When it stops, usually after one to two weeks depending on the temperature, the tea is ready. Strain it and store it away from light in a closed container, where it will keep for several months.

One point needs clarifying, as there is much confusion about it. Nettle tea is not an insecticide, despite what you often read. What we observe is that a well-fed plant withstands attacks better, and that some sprays seem to temporarily disrupt aphids settling. But it kills nothing, and relying on it to deal with an established infestation leads to disappointment. The confusion may come from the fact that a short maceration of nettle — twenty-four hours, unfermented — has shown some repellent effect in trials, which is quite a different use from fermented tea.

A final precaution follows directly from its nitrogen content: this tea pushes leafy growth. It is excellent in spring, on leafy vegetables, young plants that are growing, a lawn getting started. It becomes counterproductive on tomatoes in full fruit, where it will produce a fine green plant at the expense of the fruit, and on any plant you want to see flower.

Comfrey, the answer to flowering and fruiting needs

If nettle tea is the spring fertiliser, comfrey tea is the summer one. The difference lies in the plant's biology. Common comfrey, and even more so the cultivar Bocking 14 selected in the 1950s by the British gardener Lawrence Hills, develops a taproot system that goes down more than one and a half metres. At that depth it draws on minerals out of reach of ordinary crops, in particular potassium, which it concentrates heavily in its leaves.

This difference in profile shows in the analyses. Where nettle tea has a relatively balanced nitrogen-to-potassium ratio, comfrey tea leans clearly towards potassium, the element of flowering, fruit filling and tuber formation. That is why it is traditionally reserved for tomatoes after fruit set, squashes, potatoes, strawberries and flowering plants.

It is made in exactly the same way as nettle tea, with the same dilutions. One variant is worth mentioning for gardeners in a hurry or those put off by the smell: concentrated comfrey liquid, obtained simply by packing leaves into a container with a hole in the bottom, without adding water, and collecting the black, viscous liquid that drips out after a few weeks. This concentrate is used diluted one to twenty and smells much less strongly.

The cultivar Bocking 14 has one decisive practical advantage for anyone who wants to grow it: it is sterile and produces no viable seed. Wild comfrey, on the other hand, self-seeds and quickly becomes difficult to eradicate, its root regrowing from the smallest fragment left in the ground. Choose your plant carefully, and a permanent spot.

Coffee grounds, or the story of a stubborn misunderstanding

Here is the preparation that best illustrates the gap between intuition and measurement. The dominant belief is that coffee grounds acidify the soil, making them an ally of hydrangeas, camellias and blueberries. This is false, and it has been measured several times.

Coffee as a drink is indeed acidic, around pH 5. But that acidity is very largely water-soluble: it goes into the cup. The grounds left in the filter have a pH close to neutral, generally between 6.5 and 6.8 — sometimes even slightly alkaline after composting. Pouring grounds at the foot of a hydrangea to turn its flowers blue therefore has no effect on pH. Gardeners who see an improvement are actually seeing the effect of the added organic matter, not acidification.

A second misunderstanding concerns its fertilising value. Grounds contain about 2% nitrogen, which seems encouraging, but this nitrogen is locked in complex molecules that are released only very slowly, at the pace of microbial decomposition. Worse: fresh grounds have a carbon-to-nitrogen ratio that drives soil micro-organisms to tie up the available nitrogen to digest this material, temporarily causing nitrogen starvation at the plants' expense. You then see exactly the opposite of the intended effect.

On top of that comes a very real physical problem, visible in any garden where coffee pots are habitually emptied in the same spot. Coffee particles, very fine and uniform in size, pack into an impermeable crust as soon as they dry. This crust repels water instead of absorbing it and suffocates the soil beneath. In pots, where the volume is small, the effect is even faster and more damaging.

Does that mean you should throw away your grounds? Certainly not — you just need to send them somewhere else. In the compost, coffee grounds are an excellent ingredient, popular with earthworms, adding nitrogen to the mix and improving the structure of the finished compost. The rule is to treat them as a green material, to be balanced with brown materials, and not to exceed about a fifth of the total volume. If spread directly on the garden, they should go on in a very thin layer, less than a centimetre, immediately mixed into the soil or worked into a coarser mulch that prevents a crust forming.

As for their use as a slug repellent, tests under controlled conditions have not confirmed the effect. A concentrated caffeine extract does kill slugs and snails, but the concentration in used grounds is nowhere near enough, and in any case such a use would fall under pesticide regulations.

Eggshells and the calcium illusion

The reasoning seems watertight: blossom-end rot in tomatoes is a calcium problem, eggshells are 95% calcium carbonate, so crushing shells into the bottom of the planting hole should solve the problem. Two errors hide in this apparently logical chain.

The first is the most important: blossom-end rot almost never comes from a lack of calcium in the soil, but from an interruption in its transport within the plant, caused by irregular watering. Adding calcium to a soil that already has enough changes nothing.

The second error is more interesting, because it concerns the release rate. The calcium carbonate in a shell only becomes available after being dissolved by soil acids, and this dissolution depends directly on the size of the fragments. University work has shown that shells simply crushed by hand can still be identifiable in the soil after several years. To get release within a season, you would have to grind them into an extremely fine powder, like flour, which requires a powerful grinder and a great deal of time for a marginal benefit.

Here again, the right destination exists: the compost bin, where the shells will eventually dissolve at their own slow pace, contributing to the compost's mineral balance. And to really prevent blossom-end rot, the answer comes down to two things that have nothing to do with calcium: regular watering and mulching.

Cooking water, between common sense and a false good idea

This practice is based on a sound observation. When you boil vegetables, some of their minerals — potassium, magnesium, calcium, a few trace elements — do pass into the water. Collecting it rather than pouring it down the sink is common sense, and it does provide something, albeit in modest amounts.

Three conditions must be met, however, and the first trips up most attempts. The water must be completely cool: pouring a hot liquid on roots scalds them, no question. It must not be salted, which rules out pasta water, salted rice water and most vegetables boiled in salted water — sodium builds up in the soil and becomes toxic, especially in pots where nothing washes it out. Finally, it must contain no fat, stock or seasoning, otherwise it will mainly feed moulds and attract pests.

Egg-boiling water, often cited as rich in calcium, shows how modest the amounts are: the shell releases only a tiny fraction of its carbonate in a few minutes of boiling. It is not harmful, but do not expect any noticeable effect.

Unsalted cooking water from potatoes, rice or green vegetables is therefore an honest top-up, to be used freshly cooled and not stored — a starchy liquid ferments quickly and starts to smell bad within a few days.

The rest of the folklore, put to the test

Several other recipes keep circulating and are worth putting in context.

Wood ash is a nuanced case: it really is rich in potassium and calcium, which makes it a valid fertiliser. But it is also strongly alkalising, so much so that it was historically used as a substitute for lime. One handful per square metre per year, never more, only on acid or neutral soil, and never around ericaceous plants. Ash from treated or painted wood, or from barbecue charcoal, must be avoided completely: it contains heavy metals.

Banana peels are indeed rich in potassium, but the common practice of burying them whole at the foot of a rose works poorly: they decompose slowly, attract rodents and can go mouldy. Finely chopped and added to the compost, they do their job much better. As for "banana water", the soaking of peels in water that is so popular online, it mainly releases a little sugar and easily sets off unwanted fermentation in pots.

Diluted milk comes up regularly as an all-purpose fertiliser or fungicide. A few studies have shown partial effectiveness against powdery mildew as a spray, which is a documented use. As a fertiliser, however, it is of no interest, and its smell as it decomposes makes it hard to recommend for home use.

Aquarium water, finally, is one of the few homemade recipes that can be recommended without reservation — provided it comes from an untreated freshwater tank. Loaded with fish waste, it contains nitrates that plants can use directly and is applied neat, without dilution, when you change the water. It is a case where one system's waste becomes, unchanged, another's resource.

What to take away from this sorting

One thread runs through all these observations, and it is worth stating clearly: the preparations that work are those that put the material through a biological transformation before it reaches the plant. Nettle tea works because fermentation has already digested the plant tissue and released the minerals in soluble form. Compost works for the same reason. Conversely, the disappointing recipes are those that place raw material directly at the foot of a plant — an intact shell, a whole banana peel, a thick layer of fresh grounds — hoping that nature will quickly do the rest. It does, but at its own pace, which is measured in seasons rather than weeks, and sometimes creating problems along the way.

The practical corollary is simple and slightly disappointing for recipe lovers: the compost bin remains the best tool for making use of household waste, far ahead of all the ingenious preparations. Coffee grounds, eggshells and fruit peels find their natural place there and give their full value, transformed by microbial work that nothing can replace.

That leaves nettle tea in spring and comfrey tea in summer, two genuinely effective, free preparations whose mastery brings a satisfaction no bought bottle will ever give. They are enough to cover most of a vegetable garden's needs. The rest is well-meaning recycling — useful for the compost, with no direct effect on the plant.

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