One’s a pile of shite, the other’s rotten to the core, but they’re both dead useful in the garden. We explain the difference between manure and compost, and tell you how – and, just as importantly, when – to use them.
When we first started growing our own food, one of the many things we got in a muddle about was compost. Also manure. And, uh, organic matter. Yes, we were very confused. Which was which? Gardening books often talked about “adding organic matter at planting time”. Veg books would say potatoes and runner beans “liked plenty of manure”. Garden centres sold bags of something called seed compost next to other bags labelled “farmyard manure” – both of which, when opened up, seemed to have much the same, brownish, crumbly contents. We had a vague, general idea that this stuff was good for plants, but weren’t really sure why. Or whether there was any difference between them. When in fact the answer was, yes there is. And also, no.
First the no bit. Both compost and manure are essentially just different forms of something called organic matter, or OM for short. OM is a mix of little bits of plant and animal matter, beneficial microbes and humus*. If soil is a factory for growing plants, OM is the factory’s machinery: it works to improve the soil’s texture, allows air and water to move to around roots, stores and slowly releases plant nutrients, and acts as a fertility-enhancing carbon sink.
As they’re both OM, how are compost and manure different? Let’s start with some definitions. First of all, compost. The word, from the French compote, means “mix”, and that’s what compost is: a mix of grass clippings, newspaper, cotton rags, shrub prunings, dead leaves, vegetable and fruit peelings, straw, coffee grounds and general garden rubbish, chopped up, jumbled up, and then left to rot. (Most of what goes into compost will have started life as part of a plant, btw, but pretty much anything that rots can be usefully added to the mix, including egg shells and human hair.) It divides into two kinds
- Garden compost, which you make at home in a big heap, and is usually quite coarse-textured and may contain a lot of unwanted seeds
- Shop-bought compost, which is finer textured, often (boo) mixed with peat, and will have been heat-treated to kill off any weed seeds.
Both kinds of compost smell sweet and earthy, in a good way.
The word manure, meanwhile, comes from the old French verb manouvrer, which originally meant “to cultivate soil”. In modern English, it’s a polite word for poo, so when gardeners talk about manure, they mean sheep, cow, horse, pig or chicken poo. The ready-bagged farmyard manure sold in the UK tends to be a mix of sheep and cow poo, plus their pee-soaked straw bedding, all left to rot right down until it looks like compost and smells only faintly like a giant cow-lavatory.
So, here’s one easy-to-remember difference: compost is mostly made of plant stuff that has rotted. Manure is mostly made of plant stuff that has been eaten, poohed, and then rotted.
How does that difference play out in practice? Well, both manure and compost will contain three essential plant nutrients:
NITROGEN, aka N The first of the three main chemical elements that plants need to stay healthy and crop well, nitrogen, known to chemists and gardeners by the symbol N, helps plants make green, leafy growth. Want lots of salad leaves? You need plenty of N.
POTASSIUM, aka K Helps plants form flowers, then turn them into fruits. Like tomatoes? Give them K.
PHOSPHORUS, aka P Supports both root development and disease resistance. You shouldn’t need to worry about it too much as, unlike nitrogen and potassium which usually decrease after heavy cropping, the amount of P levels in soil tend to stay relatively stable from year to year.
About 1-1.5% of compost and manure will be made up of N, P and K-compounds, so both these kinds of OM can help feed your plants. But here’s the thing – they contain these nutrients in different proportions, or ratios. According to the Royal Horticultural Society Encyclopedia of Gardening (1992), well-rotted manure has an average ratio of 6:1:5 NPK, while you can expect garden compost to have a nutrient ratio of 5:3:8 NPK. Manure has a higher ratio of nitrogen, which makes it useful for boosting leaf and stem growth, while compost has a higher ratio of potassium and phosphorus, making it more useful for boosting fruiting and root formation.
Or in other words, put shit on your salad crops, compost on your roots and fruits.
The time has come
So that’s the what of compost or manure. Now let’s have a think about the when. Because both these sources of OM do different garden jobs depending on the time of year you add them. Luckily, we’ve broken this down for you into a handy list. Aren’t we great? Yes, we are!
To improve soil texture
Lightly dig in roughly one 15l bucket of manure or compost per square metre in October (January for very light, sandy soils).
WHY? Worms will have several months to start converting the organic matter into humus, and although some soluble nutrients will be washed away by winter rains, your aim in this case is to get a more long-term improvement in your soil’s drainage and nutrient-holding ability. Do this every year until your soil is has a firm, but crumbly texture; after that, do it every three years to keep it that way.
To act as a mulch to suppress weed growth
Cover the soil surface with roughly three buckets of compost or manure per square metre in January or February.
WHY? Weed seeds will start germinating the minute the soil temperature pokes its head above freezing, so you need to smother the potential seedlings with a good thick layer early in the year, before the ground starts to warm.
To create a seed bed
Cover soil with three buckets of compost or manure per square metre in late October.
WHY? The frost, wind and rain of winter will help break down any lumps in the surface of the compost or manure, creating a texture good for sowing into the following spring.
To create a no-dig bed from scratch
Cover grassy, weedy areas with a layer of cardboard, topped with six to nine buckets of compost or manure per square metre, in January or February.
WHY? With this luxurious depth of compost/manure, you can sow and plant everything (except probably potatoes) as soon as the growing season starts in March.
To feed crops
Either dig in one bucket of compost or manure per square metre, or cover soil with three buckets of compost or manure per square metre, as soon as the soil starts to warm in March/April.
WHY? Compost/manure that is dug in breaks down quicker, as it has more contact with soil microorganisms – hence you need less of it. And doing it in spring, once those microorganisms warm up, wake up and start feeding on OM, means you get more of the organic compounds being broken down into nutrients that plants can actually access, while also hanging on to the soluble nutrients that might otherwise have been washed away by winter rains.
Got it? Well, no
Just as you thought you were getting on top of this, you’ll meet a gardener who’ll refer to manure as having been “composted”. Don’t despair, all they’re saying is that the manure is properly rotted down: it will be light and crumbly, and the nutrients it contains will be released slowly, by bacterial action in warm soil. Uncomposted manure, on the other hand – the kind you might occasionally get direct fresh from a farm or stable – has a reek of ammonia, is heavy and claggy, and will be full of water-soluble nutrients that will wash into, and out of, soil very quickly. To avoid losing these nutrients, and potentially polluting nearby water-sources with ammonia and other unwelcome nitrogen compounds – only ever apply fresh manure in spring.
Finally, don’t apply manure or compost in late summer. The warmth of the soil means its nutrients will be released quickly into the soil – but the main growing season will be coming to an end, most of the veg plants in your garden will have been harvested, and all that lovely NPK will go to waste. Which, for want of a better word, would be shite.
HUMUS – as opposed to houmous – is a dark, mucous-like substance that is partly a byproduct of worms ingesting organic matter. It is crucial for creating good soil structure and maintaining fertility, but is no good with crudités.
