On nearly every kitchen windowsill there is a jar like this. Coffee grounds from the morning, half dried out, next to a bowl of crushed eggshells, and somewhere a banana skin waiting for its moment. You do not throw that away. That is fertiliser, surely.
Partly it is. Partly it is kitchen waste that we have talked ourselves into keeping, and in one case it is something that has no business being in a bed at all. The difference can be worked out with a pencil, and that is exactly what this article does.
One at a time, then: what is actually in these leftovers, which of the promises survive a proper trial, and what you should do with the scraps instead.
At a glance
- Coffee grounds are close to pH neutral once brewed. Usually pH 6 to 7. The acidity stays in the cup. For acidifying soil for blueberries they are useless.
- Eggshells only work as flour. Coarse pieces just sit in the soil doing nothing. Only ground to dust do they release any lime.
- Neither works as a slug barrier. Slugs glide over broken glass on their own mucus, and trials show no reduction in feeding damage.
- Banana skins belong on the compost heap, not in the planting hole. Buried, they rot without air instead of releasing potassium.
- Wood ash carries by far the greatest risk. It pushes the pH up sharply and concentrates heavy metals. Around ericaceous plants, leave it out entirely.
Coffee grounds: plenty of truth, one stubborn error
What holds up: coffee grounds contain around 2 % nitrogen. Phosphorus and potassium are barely worth mentioning, with US extension service analyses reporting roughly 0.06 % phosphorus and 0.6 % potassium. The carbon to nitrogen ratio of about 20:1 is close to ideal for composting. So they are a useful compost ingredient and a weak supplementary feed, but not a complete fertiliser: the nitrogen is locked too tightly into the organic matter.
The error persists anyway, and has done for decades: coffee grounds do not acidify soil. The roasted bean is full of acids, that much is true. But those compounds are water soluble and travel into the cup during brewing. What stays behind in the filter usually sits between pH 6 and 7, a little above or below depending on the roast and the brewing method.
For blueberries (Vaccinium corymbosum), rhododendron, azaleas or lingonberries that does nothing at all. They want pH 4.0 to 5.0, and you only get there with an acidic growing medium or with sulphur, as described in the article on acidic soil for berries. Your blue hydrangea will stay pink on a diet of coffee grounds too.
Why a thick layer does the opposite of what you want
Coffee grounds consist of very fine particles, all much the same size. Spread on damp, they stick together into a layer that repels water like a felt hat once it dries. Underneath, the soil stays dry, and the water you pour on simply runs off the top.
If the layer stays wet, it goes mouldy. White fungal growth does the plant no harm, but it is a clear sign that too much has gone down in one place at once.
There is one more effect that few people have on their radar: caffeine and polyphenols inhibit germination. On a freshly sown bed of carrots or lettuce, coffee grounds are a bad idea. Around established plants they make no difference.
Dry the grounds before they go into the jar
Damp coffee grounds go mouldy in a closed jar within days. Tip them onto a baking tray, leave them out in the air for a day, and only then put them away.
On the bed, spread a whisper thin layer only
One to two millimetres, no more, then hoe it in shallowly or cover it with compost. Mixed into soil, the grounds can no longer mat together. If a dog or cat has the run of your beds, work the grounds in straight away and never leave them lying open: caffeine is toxic to both.
Better still, put them straight on the compost
There they count as wet, nitrogen rich material and need coarse structure as a counterweight: leaves, shreddings, crumpled newspaper. They should never make up more than a fifth of the total. Unbleached paper filters can go in with them.
Go easy with pot plants
In containers, a heaped teaspoon per litre of compost is plenty, worked into the top layer, two or three times a year.
Coffee grounds against slugs
This tip survives because one trial once showed that a caffeine solution of one to two per cent kills slugs. A ring of damp grounds never comes anywhere near that concentration, and the animals crawl straight through it. What actually helps is covered in the article on slugs.
Eggshells: almost pure lime, in slow motion
An eggshell is roughly 95 % calcium carbonate, the same substance as the garden centre lime you buy in a bag. Chemically the folk remedy checks out. The problem is the arithmetic.
A hen's eggshell weighs about 5 to 6 g. Liming rates in a vegetable garden run to 100 to 200 g per square metre and then last two to three years. For a bed of 10 m² you would need the shells of 200 to 400 eggs. A family collects that in one to two years, for a single bed.
The second catch is time. Calcium carbonate only dissolves at the surface, and coarse pieces have almost none. Hoe in half shells and you will find them barely changed five years later. Only as a dust fine flour does the lime go into solution within one season.
The barrier that is not one
The idea sounds plausible: sharp edges, soft slug belly, problem solved. Except that slugs glide along on a thick film of mucus, which carries them over broken glass without trouble.
The Royal Horticultural Society tested this in 2018 on 108 lettuces in containers and raised beds. Behind the eggshell barrier, feeding damage was exactly as high as without it. Unrinsed shells smell of egg white and if anything attract slugs.
Blossom end rot on tomatoes cannot be fixed with eggshells either. The calcium shortage does not arise in the soil but in transport: when watering is erratic, the calcium never reaches the tip of the fruit, even though there is plenty in the bed. Watering evenly helps, adding lime does not. The details are in the article on blossom end rot.
If you do want to use your shells, do it like this
Rinse the shells, dry them in the residual heat of the oven, collect them, and once a year run them through an old coffee grinder. Before that, though, measure your pH with test strips or a lab sample. Shell flour is lime, and lime only belongs on soils below roughly pH 6.5 on sandy ground or pH 7.0 on heavy ground. If the value fits, the flour goes where lime is wanted: under brassicas, celeriac and spinach. On soil that is already chalky it achieves nothing and can trigger the same yellow leaves as over liming. On acidic berry beds, never.
Banana skins: yes to potassium, no to a hole in the bed
Banana skins are the darling of every rose guide, and the nutrient behind the claim is genuinely there. In the dry matter, potassium is by far the most abundant mineral, and potassium drives flowering, fruit set and frost hardiness. Roses and fruiting vegetables can put it to good use.
The catch is the water. A fresh skin weighs around 40 g and is about 85 % water. What remains is roughly 6 g of dry matter, and within that considerably less than a gram of potassium. Two skins a week are a contribution, not a feeding programme.
Then there is the method. Laying whole skins in the planting hole is the worst option going: below ground there is no oxygen, so the skin putrefies rather than rots, producing butyric acid and other compounds that fine roots dislike. Left on the surface it draws fruit flies within hours. Both problems disappear if you chop the skin up and put it on the compost. One point the rose guides leave out: the fungicides used on conventional bananas after harvest to stop them going mouldy sit exactly where you are about to put them, namely in the skin. For the compost heap, then, use organic skins or give them a hot wash first.
Kitchen and household scraps, the short verdict
- Coffee groundsUsable. Thinly on the bed or onto the compost, dried first. Does not acidify, does not stop slugs.
- EggshellsOnly ground. As flour a slow lime feed, as a coarse shard useless. No effect on slugs.
- Banana skinsGood, but composted. The potassium is real, the quantity is small. Never bury them whole.
- Wood ashWith great care, or not at all. Pushes the pH up sharply, concentrates heavy metals.
- Tea leavesYes, but without the bag. The fabric often contains plastic, and the string and staple certainly do.
Wood ash: the candidate with the greatest potential to do harm
On paper wood ash has respectable numbers: roughly 3 to 6 % potassium, 2 to 3 % phosphate and a great deal of calcium. A wood burner produces plenty of it over winter, and the idea of returning that to the garden is an old one.
The trouble is that burning brings two effects with it. The first is pH: ash sits well into the alkaline range and acts like a heavy dose of lime. On the naturally chalky soils across much of southern Germany, Austria and Switzerland, the pH tips beyond the range in which iron and manganese stay available. Yellow leaves with green veins are the result.
The second effect is the more serious one. Over decades a tree takes up lead and cadmium from the air and the soil, and when the wood burns those heavy metals stay behind, concentrated many times over. On top of that comes chromium, part of which forms in the fire as readily soluble and highly toxic chromium(VI). The Bavarian environment agency therefore advises against putting wood ash on the garden or the compost heap at all, even from untreated timber. In Switzerland, ash from domestic stoves goes into the general waste and explicitly not into green waste collection.
What must never go on the garden
Ash from varnished or treated timber, from chipboard, coal, briquettes or firelighters belongs in the general waste, not in the food waste bin and not on the compost. If you decide to use ash from clean split logs despite all of the above: no more than 30 g per square metre per year, which depending on how dense the ash is comes to only about 50 to 100 ml, and never around ericaceous plants. Leafy vegetables such as chard and kale accumulate cadmium strongly, so leave ash out there completely.
Tea bags: yes to the leaves, no to the wrapper
Tea leaves are harmless compost material: a little nitrogen, quick to break down. The notion that tea acidifies soil belongs in the same drawer as the coffee myth. The tannins are too weak and there are too few of them.
The problem is the packaging. To keep bags from tearing in boiling water and to allow them to be sealed by machine, many filter papers are reinforced with polypropylene, especially the stringless ones with a glued seam. That plastic breaks down neither on a garden heap nor in municipal food waste, and PLA pyramid bags need industrial temperatures. So: cut them open, leaves onto the compost, wrapper into the general waste.
What actually works
The honest answer is unspectacular: all of these leftovers are good compost ingredients and poor direct fertilisers. The heap mixes and buffers until the nutrients are free and the side effects are gone. Matted coffee grounds and a putrid banana skin end up as the same crumbly material, and that you can spread on your beds without a second thought.
Start collecting now, at the end of July in CW 31, turn the heap in autumn, and by spring in CW 12 to 16 you will have coarse fresh compost for mulching and for hungry crops. Mature, sieved compost generally takes nine to twelve months, so it will not be ready until the following summer. How to build the heap is covered in the guide to making your own compost, and the usual things that slow it down in the article on speeding compost up.
For targeted feeding, reach for material where the quantities add up: horn meal for nitrogen, mature compost as a base, and a liquid feed made from comfrey for potassium during fruiting. What suits which moment is set out in the overview of feeding organically without chemicals.
Kitchen scraps are compost ingredients, not fertilisers. What takes the detour via the heap is what actually arrives in the bed.
Gartenkern rule of thumb
The same goes for the rest of the kitchen: peelings from potatoes, leaves from rhubarb and crowns from strawberries all go on the heap chopped small. Cooked food and anything from an animal stays out, because it brings rats.
If you write down when which scrap went where, by the second and third year you will be able to see what your soil actually got out of it. The Gartenkern garden journal is where you record that, and where you can set turning the compost as a recurring task. After a few seasons you will know what was a genuine remedy and what was merely a belief.
Common questions
Do coffee grounds make soil more acidic?
No. After brewing, coffee grounds usually sit between pH 6 and 7, which is practically neutral. The acidic compounds in the bean are water soluble and end up in your cup. For blueberries, rhododendron or blue hydrangeas you need an acidic growing medium or sulphur.
Do eggshells keep slugs off a bed?
No. Slugs produce mucus that lets them crawl over broken glass without injuring themselves. In the Royal Horticultural Society barrier trial of 2018, using 108 lettuces, feeding damage behind eggshells was just as high as without them.
Should I bury banana skins under my roses?
Better not. Below ground there is no oxygen, so the skin putrefies rather than rots and forms compounds that damage fine roots. Chop it small and put it on the compost, and the potassium will reach the rose a few months later.
Can wood ash from my stove go on the garden?
Only with restrictions, and many authorities advise against it entirely. Burning concentrates lead, cadmium and chromium in the ash, and the strongly alkaline pH can over lime your soil. In Switzerland, ash from domestic stoves goes into the general waste. If you do use it at all: only ash from untreated split logs, no more than 30 g per square metre per year, which is about 50 to 100 ml, and never around ericaceous plants or leafy vegetables.
Can I throw whole tea bags on the compost?
Only the leaves. Many filter papers are reinforced with polypropylene so the seam holds in boiling water, and PLA pyramid bags need industrial temperatures to break down. Cut the bag open, tip the leaves onto the heap, and put the rest in the general waste.
Sources and further reading
- Making your own compost · structure and layering
- Speeding compost up · why a heap stalls
- Feeding organically without chemicals · horn meal, manure and liquid feeds
- Recognising and dealing with slugs · what really works
- Blossom end rot · why lime does not solve it
- Acidic soil for berries · getting to pH 4.0 to 5.0
- Home remedies against pests · what the kitchen tricks are worth
- Royal Horticultural Society · slug barrier trial, 2018
- Oregon State University Extension · nutrient analysis of coffee grounds
- Bavarian Environment Agency · disposal of wood ash
- Federal Office for the Environment (Switzerland) · ash from domestic stoves
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