Organic Fertilisers: All Your Options and What They’re Best For

What actually counts as an organic fertiliser?

An organic fertiliser comes from a plant, animal or mineral source rather than being chemically manufactured. Compost, manure, bone meal and seaweed feed all fall under this umbrella. What they share is that the nutrients sit bound up in organic material and have to be broken down by bacteria, fungi and other soil life before roots can take them up.

That's the key difference from synthetic fertiliser, which is already water-soluble and gets absorbed right away. Organic fertiliser works more slowly, but it also builds soil biology and structure in a way synthetic fertiliser simply doesn't. If you want the basics on what those three numbers on any fertiliser label actually mean, our guide on NPK fertiliser explained covers it from the ground up.

Nutrient content in organic fertilisers also varies far more than in synthetic ones. A bag of blood meal from one production batch can differ from the next, and homemade compost depends entirely on what went into it. Treat every N-P-K figure in this guide as an approximate benchmark rather than a guaranteed number, and always check the label on any packaged product you buy.

Organic fertiliser also performs best paired with mulch rather than used alone. A layer of mulch holds onto the moisture soil life needs to break nutrients down, while also protecting the soil from drying out or washing away in heavy rain. Our guide on mulching covers how the two work together.

Compost: the most versatile organic fertiliser

If you only pick one organic fertiliser, make it compost. Well-finished compost carries a mild, balanced mix of nitrogen, phosphorus and potassium — often somewhere around an N-P-K of 1-1-1 to 2-1-1.5 depending on what went into it — plus soil biology, humus-building material, and a structure that holds moisture better. It's also nearly impossible to overfeed with, since the nutrients stay bound and release as soil organisms do their work.

A couple of inches (5–8 cm) worked into planting soil, or a thin layer top-dressed onto beds and lawn each spring, covers most garden plants' routine needs. Heavy feeders like tomatoes, cabbage and squash appreciate a thicker layer — 2–3 inches (5–8 cm) worked in before sowing or transplanting.

Making your own compost also means you control exactly what ends up in your soil. If you want to get started, the full process is in our guide on composting for beginners, and a rundown of exactly what can and can't go in the bin is in our guide on what you can compost.

Good to know. Compost that still smells sour or feels warm in the middle isn't finished yet. Unfinished compost can pull nitrogen and oxygen out of the soil instead of adding them, and it risks carrying viable weed seed and pathogens. Wait until it's dark, crumbly and smells like earth before digging it in.

Manure: chicken, cow and horse compared

Manure has been used longer than anything else on this list, for good reason — it's cheap, often free if you know someone who keeps animals, and gives a broad nutrient profile. But the different types vary more than people tend to assume, both in strength and in how soon you can use them.

The single most important rule for any manure is the same: never apply fresh manure directly around the roots of anything you plan to harvest soon. Fresh manure is strong enough to burn root systems, can spread weed seed that never passed through a hot compost, and carries a hygiene risk around vegetables eaten raw. Dig it in during autumn so it has time to break down by spring, or compost it separately for at least a few months.

Pelleted chicken manure from a garden centre has already been heat-treated and dried, which makes it far easier to dose precisely than manure straight from a farm. It's a solid choice when you want manure's nitrogen boost but don't have your own composting set-up.

Common mistake. Wheeling a barrow of fresh horse manure straight into the vegetable bed each spring, meaning to give plants a strong start. Straw and shavings in fresh horse manure pull nitrogen out of the soil as they break down, which often does the opposite of what's intended — plants turn pale instead of growing better.

Nettle feed, green manure and other homemade options

You don't have to buy everything. Two of the most effective organic fertilisers cost nothing and can be built entirely in your own garden, given a little time and patience.

Nettle feed is made by filling a bucket halfway with chopped stinging nettles, topping it up with water, and letting the mix sit and ferment for one to two weeks until it stops bubbling and smells strongly fermented. Strain out the plant matter, then dilute one part nettle feed to ten parts water before applying — undiluted, it's strong enough to damage roots. Nettle feed is rich in nitrogen and acts fast, but it's too low in potassium to carry a fruiting plant through an entire season on its own.

Green manure works differently: you sow a cover crop — usually a mix of legumes like clover or vetch alongside a grass or phacelia — on a bed that would otherwise sit empty, then dig or till it in before it sets seed. The legumes fix nitrogen from the air through bacteria in their roots, and the whole crop then adds organic matter and soil biology as it breaks down. It's a slow method that builds soil across seasons rather than a quick boost for one crop, but it's hard to beat for reviving a tired vegetable bed between plantings.

If you'd rather build soil structure and biology with worms instead of a cover crop, there's a related option in our guide on worm composting at home. Another homemade option that's gained ground in recent years, bokashi, ferments kitchen waste with the help of microorganisms before it goes into the soil — the process is covered step by step in our guide on bokashi composting.

Blood meal, bone meal and other animal-based concentrates

These products are far more concentrated than compost and manure, so they're used in smaller amounts to solve a specific need rather than as a general feed for an entire bed.

Blood meal is almost pure nitrogen and acts unusually fast for a dry concentrate, which makes it a good first choice when a plant has clearly yellowed from nitrogen shortage and needs help within days rather than weeks. Overdo it, though, and it can burn roots just like a strong synthetic fertiliser — follow the label rate even though it's "just" organic.

Bone meal, by contrast, is phosphorus-heavy and works much more slowly, often over weeks to months, which makes it best mixed into the planting hole rather than sprinkled on the surface when you need results soon. Phosphorus from bone meal also becomes harder for roots to access in alkaline, high-pH soil, so the effect can be weaker than expected if your soil already leans toward the basic side.

Shell meal, made from crushed shrimp or crab shells, supplies calcium and chitin. The chitin can help suppress populations of certain soil pests, such as nematodes, because breaking it down favours microorganisms that feed on chitin-containing organisms. It isn't a fast or guaranteed pest treatment, though — more a long-term addition to soil that's already biologically active.

A couple of practical points are worth knowing about these dry concentrates. Blood meal and bone meal smell strongly of protein once damp, which can draw dogs, cats and wildlife to dig around the bed for days afterward — bury the concentrate an inch or two (2–5 cm) deep rather than just scattering it on the surface. Keep opened bags dry and tightly sealed; moisture makes the powder clump and can lead to mould, which ruins the nutrient value before you've had a chance to use it.

Seaweed feed, fish emulsion and other marine options

The ocean supplies a couple of the most widely used organic fertilisers, and they play different roles. Seaweed feed, often made from kelp, is especially rich in potassium and also carries natural growth compounds that appear to strengthen roots and make plants more resilient to stress like drought and cold. It's mostly used as a diluted watering feed or foliar spray, rarely as a main nutrient source.

Fish emulsion — a liquid product made from fish-processing by-products — carries more nitrogen and phosphorus than seaweed feed, but strength varies a lot between brands. Smell is the most common complaint; it fades within a couple of days but can be noticeable right after application, which makes fish emulsion a poor fit for houseplants indoors.

Both feed a nitrogen or potassium boost, but seaweed and fish products are especially useful on plants that benefit from potassium and trace minerals during flowering and fruiting — tomatoes, berry bushes and roses are among the most common examples. Apply either every two to four weeks through the growing season, noticeably less often than nettle feed. If you're spraying either onto leaves, do it early morning or evening, never in strong midday sun, since droplets on foliage can act like tiny magnifying glasses and scorch the leaf tissue.

Do coffee grounds and eggshells actually work as fertiliser?

These circulate in nearly every gardening forum, and the truth is mixed: a few give a small but real benefit, others do more good a different way than people assume.

Coffee grounds simply don't carry enough nitrogen, phosphorus or potassium to function as fertiliser in any meaningful sense. They do more good in the compost bin, where they add organic matter and attract earthworms, than scattered directly around a plant you're hoping will grow better because of them.

Eggshells solve the same problem bone meal does — calcium shortage — but at a far slower pace. Crushed eggshells dug into soil can take several years to break down enough to release meaningful calcium, which makes them useless against an acute calcium deficiency in the same season. Grinding them to a fine powder speeds things up somewhat, but still nowhere near as fast as bone meal or calcium nitrate.

Wood ash is actually effective, unlike the other kitchen tips on this list — but for a reason that's often overlooked: it raises soil pH significantly at the same time as it adds potassium. Use it sparingly, never on plants that prefer acidic soil, and never in amounts heavier than a light dusting per square yard (about a square metre) per year.

Which organic fertiliser suits which plant?

The right choice mostly comes down to how fast you need results and how sensitive the plant is to a strong dose.

One pattern worth noticing: the more sensitive or confined a plant's root system — containers, new transplants, young seedlings — the more it pays to choose a mild, slow or well-diluted fertiliser. Out in open ground, with more soil volume and natural drainage from rain, soil generally tolerates stronger, more concentrated options.

Timing by growing zone

Organic fertiliser generally needs more warmth and moisture for soil life to break nutrients down, which makes timing more climate-dependent than it is for fast-dissolving synthetic fertiliser.

Cold soil slows the breakdown of any organic material, whatever the fertiliser type. Feed early in spring while the soil is still cold, and expect the effect to take longer to show than it would in midsummer. It's better to wait a week or two for the soil to warm up than to double the dose thinking the cold is the problem.

How to dose and combine without overfeeding

Overfeeding happens at least as often with organic options as with synthetic fertiliser, even though a lot of people assume "natural" makes it harder to get wrong.

1. Test the soil first. A simple soil test shows pH and basic nutrient status, and reveals whether you actually need to feed at all or the soil already has what the plant needs.
2. Build with compost or manure as the base. Work it into the soil at planting, or apply a thin layer each spring — that covers most of the ongoing need for most plants.
3. Aim concentrates at a specific problem. Treat blood meal, bone meal and seaweed feed as targeted add-ons, not something you scatter across the whole garden as routine.
4. Follow the label rate, not a guess. The concentration is calculated to be gentle at the right amount — doubling a dose rarely solves a problem faster, it just creates a new one.
5. Give it time between feedings. Let the breakdown process work before deciding whether more is needed, especially with slow options like bone meal and green manure.
6. Stop well before the season ends. Late feeding pushes out soft new growth that never has time to mature or harden off before cold weather arrives.

Combining several organic fertilisers is rarely a problem, unlike mixing strong synthetic concentrates. Compost as a base, a nitrogen concentrate when needed, and seaweed feed heading into flowering or fruiting is a perfectly reasonable combination for most gardens. Just avoid stacking several strong concentrates — blood meal and fresh chicken manure, for example — at the same time, since their combined effect gets harder to predict.

Common mistakes

• Applying fresh manure straight onto roots. Burns roots, spreads weed seed, and carries a hygiene risk near vegetables eaten raw.
• Assuming "organic" means "impossible to overfeed." Concentrates like blood meal and bone meal can damage roots in too large an amount, just like synthetic fertiliser.
• Expecting fast results from slow options. Bone meal and green manure work over weeks to years, not days — pair them with a faster option if the problem is urgent.
• Spreading wood ash heavily on acid-loving plants. Raises pH enough that blueberries, rhododendrons and other acid-soil plants suffer instead of benefiting.
• Treating kitchen scraps as full-strength fertiliser. Coffee grounds and eggshells provide too little, too slowly, to replace proper feeding.
• Feeding bone-dry soil. Liquid concentrates applied to dry soil can burn roots whether they're organic or not. Water lightly first.

Frequently Asked Questions

Is organic fertiliser better than synthetic fertiliser?
It depends what you're prioritising. Organic fertiliser builds soil biology and structure over time and is harder to accidentally overfeed with, but it works more slowly and often needs a larger amount for the same effect. Synthetic fertiliser gives faster, more precisely controlled nutrition but does nothing for the soil's long-term health. Many gardens do best with a combination: organic as the base, synthetic or a fast-acting concentrate for acute needs.

Can you overfeed with organic fertiliser?
Yes, especially with concentrated products like blood meal, bone meal and fish emulsion. The main difference from synthetic fertiliser is that the effect shows up more slowly, which makes it easier to miss that you've already overfed before symptoms appear. Always follow the label rate even when the product is organic.

How long should manure be composted before use?
Plan on at least a few months for chicken and horse manure, and ideally a full season for horse manure mixed with straw or shavings. It's ready when it no longer smells strongly of ammonia, feels cool rather than warm, and has a dark, crumbly texture with no obvious trace of the original material.

Which organic fertiliser works best for houseplants?
A diluted liquid feed, such as seaweed feed or well-strained compost tea, at a lighter dose than you'd use outdoors. A pot's limited soil volume means both nutrient shortage and overfeeding show up faster, so small, frequent doses work better than rare, strong ones.

Is nettle feed as effective as a store-bought liquid fertiliser?
It's effective for nitrogen and acts fast, but it carries too little potassium and phosphorus to carry a fruiting plant through an entire season on its own. Use nettle feed as a supplementary nitrogen boost, ideally early in the season, rather than a plant's only feed.

Do you need to test soil pH before feeding organically?
It isn't required, but it makes dosing considerably safer, especially with bone meal and wood ash, both of which are affected by or affect pH. A simple soil test costs less than a bag of fertiliser and reveals whether your soil is already off balance before you add something that makes it worse.

Can you mix several organic fertilisers at the same time?
Yes, that's rarely a problem since the nutrients stay bound in organic material and release gradually. Compost as a base plus a targeted concentrate when needed is a common, workable combination. Just avoid stacking several strong concentrates, like blood meal and fresh chicken manure, at once.

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