Do We Actually Know What We're Feeding Our Plants?

A gardener asked me a question that I suspect many of us could have asked.

She had been gardening for more than twenty years. She made her own compost, used horse manure, seaweed feed and homemade comfrey feed, grew fruit and vegetables and looked after an orchard. She certainly wasn't inexperienced.

Yet she admitted she'd never entirely understood which feeds were high in nitrogen or potassium, when she ought to use them, or what they were actually doing.

I think plant feeding is one of those parts of gardening we've somehow managed to make far more complicated than it needs to be.

Walk into a garden centre and you'll find shelves full of fertilisers. Rose feed, tomato feed, citrus feed, ericaceous feed, general-purpose feed, seaweed extract, fish, blood and bone, bonemeal, chicken manure pellets, slow-release granules — and the packaging usually carries a photograph of some flower or fruit at a size it will never actually reach in your garden.

It's quite easy to come away believing that every plant needs feeding with something.

Most of them probably don't. Established trees, shrubs and perennials growing in reasonable garden soil can often manage perfectly well without regular fertiliser. Plants have, after all, been growing for rather longer than we've been buying bottles of liquid feed.

That doesn't mean feeding is pointless, mind. Plants in containers are dependent on a comparatively small amount of compost, and the nutrients in it will eventually be used up or washed away. Fruit and vegetables can be demanding because we're asking them to produce a crop. Bedding plants, roses and other repeat-flowering plants may benefit too.

But maybe we should start further back than that.

What does feeding a plant actually mean? Plants don't eat fertiliser the way we eat a meal — they manufacture much of what they need using light, water and carbon dioxide. What we call plant food supplies the mineral nutrients that let those processes happen properly.

That might sound like splitting hairs, but I think it matters. Fertiliser isn't a tonic that makes a plant generally healthier. It supplies particular nutrients for particular jobs, and if those nutrients are already there — or something's stopping the plant using them — adding more may do very little.

The Three Letters on the Packet

Most fertiliser packaging carries three letters: N, P and K.

Nitrogen is closely tied to green, leafy growth — it's essential for chlorophyll and proteins, so a plant short of it may grow poorly and develop pale or yellowing leaves. That's why feeds for lawns and leafy vegetables tend to carry a relatively high proportion of it. Leaves are exactly what we're after there.

More nitrogen isn't automatically better, though. Too much can produce soft, lush growth that's more vulnerable to pests, disease and cold, and it can push leaf growth at the expense of flowers. A tomato plant with magnificent foliage and hardly any fruit probably doesn't need more encouragement in that direction.

Phosphorus is involved in energy transfer within the plant and contributes to root development, though it gets described rather too neatly sometimes as "the nutrient for roots," as if the three main nutrients ran separate departments. They don't — plant nutrition is far more interconnected than the shorthand suggests. Phosphorus is also a decent example of why fertilising without understanding your soil can be wasteful: many cultivated garden soils already have plenty of it, and repeated applications just lead to it building up unnecessarily. A struggling plant doesn't need bonemeal simply because we'd like its roots to try harder.

Then there's potassium — traditionally potash — which is bound up in water regulation and a lot of what supports flowering, fruiting and a plant's general ability to cope with stress, which is why tomato feeds carry proportionally more of it than general-purpose ones. Not because tomatoes have some mysterious extra requirement no other plant shares. It's just formulated for plants that are flowering and fruiting. Comfrey liquid gets used for much the same reason — it's a reasonable source of potassium, though obviously a homemade feed won't come with the precise analysis a shop-bought one does. Its strength depends entirely on how it was made, how much comfrey went in and how long it sat. That uncertainty doesn't make it useless. It just means treating it as roughly right rather than laboratory-precise.

N, P and K Are Not Three Magic Buttons

The familiar shorthand — nitrogen for leaves, phosphorus for roots, potassium for flowers — is a useful starting point. It becomes misleading the moment we start treating those nutrients as three buttons we can press to make a plant perform.

Plants need other elements too — magnesium, calcium, sulphur, a range of micronutrients in smaller amounts. More importantly, nutrients have to actually be available to the roots. They can be sitting there in the ground and the plant still can't get at them.

Soil pH has a lot to do with this. Very acidic or alkaline conditions can lock particular nutrients up, making them hard for the plant to access. The RHS notes that the major nutrients tend to be most available around pH 6.5, though naturally plants vary in what they prefer — and understanding your soil's pH can do more for you than reaching for another bag of fertiliser. Understanding soil pH is worth a look before you buy anything.

Water matters too. Nutrients move through soil moisture towards the roots, so a plant sitting in bone-dry ground can't make much use of even a generous scattering of feed. A waterlogged plant may have damaged, oxygen-starved roots and be just as unable to take anything up. In both cases the leaves can turn yellow and growth can slow.

Neither plant is necessarily hungry.

A Product Name Is Not an Analysis

Seaweed feed is a good example of another complication. Some seaweed products contain fairly small quantities of nutrients and are really sold as tonics or sources of trace elements. Others have nitrogen, phosphorus and potassium added and are complete fertilisers in their own right. So "I feed with seaweed" doesn't actually tell you much about what's being applied.

The same goes for a lot of familiar names. Rose feed isn't only useful for roses, and tomato feed isn't restricted to tomatoes — the name on the front tells you how the manufacturer expects it to be used, but the nutrient analysis on the back tells you what's actually in it. Those NPK figures are worth reading. They let you compare products with completely different names that supply very similar nutrients, and they'll sometimes reveal that two feeds you're using together are just duplicating each other.

If you're already using a fertiliser-enriched compost, adding slow-release granules and giving regular liquid feeds on top, you're probably not helping the plant three times over. You're just feeding it three times.

The directions matter as well — a stronger mix doesn't give faster or better results. Excess soluble fertiliser can damage roots, particularly in dry compost, and whatever the plant can't use tends to get washed away anyway.

Feeding the Plant and Improving the Soil

Several gardeners raised another distinction I think is worth sitting with: feeding a plant isn't quite the same as improving the soil.

Garden compost, leaf mould and well-rotted manure add organic matter. Nutrients get released as they break down, sure, but their real value lies in what they do to the ground itself — improving structure, helping sandy soils hold moisture, making heavy soils more workable, giving soil organisms something to work with. The RHS describes organic matter as contributing to both structure and fertility, and that's a much slower, broader process than giving a container tomato its weekly liquid feed.

I tend to think of good compost as feeding the soil system — though even that phrase shouldn't lead anyone to assume every brown, compost-like product does the same job. Potting compost is a growing medium for containers. Soil improver is meant to go into or over existing soil. Mulch sits on the surface, mainly to conserve moisture and suppress weeds while slowly improving what's underneath. One product can sometimes do more than one of those jobs, but the words on the bag aren't meaningless.

Leftover fertilised compost from containers isn't necessarily good mulch for a border — particularly late in the season, when you probably don't want to be encouraging soft new growth. And something sold as soil improver might be too coarse or too rich to use on its own in a raised bed. If it smells strongly of ammonia, I'd leave it be for a while longer before going anywhere near it with a trowel.

Natural Does Not Mean Unlimited

Homemade compost, manure and plant-based feeds are excellent resources — they let us recycle material, improve the ground and rely less on things that were manufactured and shipped in. But natural doesn't mean it can't be overdone.

Horse manure varies enormously depending on storage and bedding. Chicken manure is comparatively rich and wants rather more restraint than leaf mould does. Wood ash supplies potassium but also raises soil pH, so it's not something to keep scattering around acid-loving plants or onto soil that's already alkaline. Comfrey liquid is genuinely useful around fruiting plants; a feed made from mixed weeds gives you a broader but far less predictable mix. Neither needs pouring around everything in sight just because it cost nothing to make.

The same question applies whether the fertiliser came from a factory, a stable or a bucket behind the shed: what am I actually trying to achieve here? Extra nitrogen for a hungry, leafy vegetable, maybe. A higher-potassium feed for a container plant that's flowering and fruiting hard. Or, if the soil itself is thin and dries out fast, organic matter is probably doing more for you than another bottle of liquid feed ever will.

When an Unhappy Plant Isn't Hungry

I suspect one of the most common feeding mistakes starts from a good place. We see a plant looking unhappy and want to help.

The leaves are yellow, growth's poor, it's barely flowered — something must be missing, so we reach for a feed. But those same symptoms can come from the plant being too dry, too wet, too shaded, too exposed, its roots damaged or restricted or disturbed by pests, the pH wrong for it, or a difficult season it's still recovering from. It might simply be in the wrong spot altogether.

Adding fertiliser can occasionally make things worse. Pushing fresh top growth on a plant whose roots are already struggling puts more demand on exactly the part that can't cope. Feeding dry soil can damage roots outright. High-nitrogen fertiliser applied late in the season can produce tender growth right as the plant should be slowing down for winter.

Before I feed anything that's struggling, I'll look at moisture, drainage, roots, light, the general growing conditions first. If the same problem's turning up across several plants, a soil test will usually tell you more than another bottle bought on a hunch.

Perhaps Feeding Should Begin With Restraint

After more than twenty-five years of doing this, I probably feed fewer things indiscriminately than I once did. I pay more attention to the ground now than I used to, and I've come to feed mainly where there's a clear reason — pots, because their resources are genuinely limited, or productive plants that are actively growing and being asked to do a lot. I no longer assume everything in a border needs a seasonal dose of something just because the calendar says it's that time of year.

There's an environmental angle to this too, I think, alongside the gardening one. Fertilisers cost something to manufacture and transport, and whatever our plants don't use doesn't simply vanish — it goes somewhere. Using what the garden itself produces, sourcing organic matter locally, applying anything commercial only where it's genuinely doing a job, all of that probably saves money as much as anything else.

I don't think the answer is to stop feeding plants. It's more that I'd like to know what I'm actually supplying before I supply it, and why, and whether the plant's even in a position to use it.

I'm not sure we ask that question nearly enough. I certainly didn't for a long time.

Happy Gardening.
Marcus

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