Last updated: August 22, 2026
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The short version
The organic garden aisle sells a dozen products that sound interchangeable and are not. Bone meal, blood meal, compost, worm castings, kelp meal and rock phosphate do genuinely different jobs, and buying the wrong one is the most common way gardeners waste money without harming anything.
Two principles cover most of it. Amendments improve soil; fertilisers feed plants, and the distinction matters because they solve different problems. And most established gardens do not need most of what they are sold, particularly phosphorus, which accumulates rather than leaching away.
The single most useful thing on this page costs about the same as one bag of anything: get a soil test. It tells you your pH and your existing nutrient levels, and without those two numbers every purchase is a guess.
Amendments and fertilisers are different things
The words are used interchangeably in shops and they should not be.
A soil amendment changes the physical or biological properties of soil. Compost, leaf mould and coir improve structure, water-holding capacity, drainage and microbial life. They work over seasons and years, and their nutrient contribution is a bonus rather than the point.
A fertiliser supplies specific nutrients to plants. Bone meal, blood meal and fish emulsion deliver nitrogen, phosphorus or potassium in identifiable quantities. They work over weeks to months and do little for soil structure.
Why it matters practically. A garden with compacted clay that drains poorly does not need fertiliser; it needs organic matter. A container of annuals in fresh potting mix has fine structure and will need feeding. Diagnosing which problem you have determines which shelf to stand in front of.
Compost does both, poorly and adequately. It is the best all-round amendment and a weak fertiliser, supplying modest nutrients slowly. For most gardens most of the time, that combination is exactly right.
The three numbers on every bag
N-P-K is nitrogen, phosphorus and potassium, expressed as percentages by weight, always in that order.
Nitrogen drives leafy green growth. It is the nutrient plants use most and the one that leaches out of soil most readily, which is why it needs replacing regularly. Too much produces lush foliage at the expense of flowers and fruit, and makes plants more attractive to aphids.
Phosphorus supports root development, flowering and fruiting. It binds tightly to soil particles and barely moves, which means it accumulates rather than leaching. This is the nutrient most gardens already have too much of.
Potassium supports overall plant function, water regulation and disease resistance. Deficiency is less common than nitrogen deficiency and more common than phosphorus deficiency.
Organic products have low numbers compared with synthetic ones, and that is a feature. A 4-12-0 bone meal releases over months as soil organisms break it down, rather than delivering a soluble pulse that can burn roots and run off into waterways.
Secondary nutrients and micronutrients including calcium, magnesium, sulphur, iron and zinc rarely appear on the front of a bag and are rarely deficient in soil with decent organic matter.
What each amendment actually does
Compost
The default answer for most garden problems, and the one thing nearly every soil benefits from. It improves structure in both directions, loosening clay and helping sand hold water, feeds soil microbial life, and supplies a modest slow release of nutrients. Roughly 5-1-1 to 2-2-2 depending on inputs, which is genuinely low.
Apply two to three inches annually as a top dressing or work it into beds. There is essentially no way to overdo good compost.
Worm castings
Worm-digested organic matter, and the most misunderstood product in the aisle. The nutrient content is low, typically around 1-0-0 to 2-1-1, so anyone buying it as a fertiliser is paying a great deal per unit of nutrient.
What castings genuinely offer is microbial content and humic substances in a concentrated, consistent form. They are excellent in potting mixes and for seedlings, where a small volume goes a long way. They are an expensive way to amend a whole bed. Our organic worm castings guide works through what the evidence supports.
Bone meal
Steamed ground animal bone, typically 3-15-0 or 4-12-0, plus substantial calcium. High phosphorus, slow release, cannot burn roots.
Two conditions must hold for it to do anything. Soil pH must be below about 7, because bone meal is calcium phosphate and needs acidic conditions to dissolve. And your soil must actually be short of phosphorus, which most established gardens are not. Our organic bone meal guide covers both constraints in detail.
Work it into the planting hole rather than top-dressing, since phosphorus moves through soil at a fraction of an inch per year.
Blood meal
Dried blood, around 12-0-0, and the strongest organic nitrogen source. Fast-acting by organic standards and genuinely capable of burning plants if over-applied, which is unusual for an organic input.
Useful for leafy crops and for correcting a real nitrogen deficiency, recognisable as uniform yellowing starting with older leaves. It also deters some mammals by smell and attracts others.
Kelp and seaweed meal
Low N-P-K, valued instead for potassium, trace minerals and naturally occurring plant growth regulators. The evidence for measurable yield benefit is modest, and it is a reasonable supplementary input rather than a corrective one.
Fish emulsion and hydrolysate
Liquid nitrogen-forward feeds, roughly 5-1-1, fast-acting and useful as a mid-season boost. They smell strongly for a day or two and attract animals.
Rock phosphate
Mined mineral phosphate, extremely slow, and dependent on acidic soil in the same way bone meal is. Best understood as a multi-year soil-building investment rather than a seasonal amendment.
Manure
Composted animal manure supplies nutrients and organic matter together and is among the best value inputs by volume. It must be composted; fresh manure burns plants and carries pathogen risk on food crops.
The phosphorus problem
Worth its own section because it is the most consequential misunderstanding in organic gardening.
Phosphorus does not leach. Unlike nitrogen, which washes out and needs replacing, phosphorus binds to soil particles and stays. In a garden receiving compost, manure and a phosphorus-containing fertiliser each year, it accumulates steadily.
Excess phosphorus is not harmless. High levels interfere with iron and zinc uptake, producing deficiency symptoms in plants surrounded by plenty. It also suppresses mycorrhizal fungi, the root symbionts that help plants forage for water and nutrients, which is a genuine loss in a garden built around soil biology.
There is an environmental dimension. Phosphorus running into waterways drives algal blooms, and several jurisdictions restrict phosphorus in lawn fertiliser for that reason.
The practical consequence. Bone meal and rock phosphate are corrective amendments for a diagnosed shortfall, not annual rituals. If your soil test says phosphorus is high or optimum, adding more makes the picture worse.
Soil texture, and why it governs everything else
Before nutrients, before pH, there is the physical question of what your soil is made of. It cannot be changed quickly and it determines how everything else behaves.
Sand, silt and clay are the three mineral particle sizes, and their proportions define soil texture. Sand particles are large with big gaps between them, clay particles are microscopic and pack tightly, silt sits between.
Sandy soil drains fast and holds little. Water runs through, taking dissolved nutrients with it, which means sandy gardens need more frequent watering and more frequent feeding. Nitrogen in particular leaches quickly.
Clay soil holds water and nutrients well and drains badly. It compacts under foot traffic, becomes waterlogged in winter, and sets hard in summer. It is also frequently the most fertile soil once its structure is improved, because those tiny particles hold nutrients that sand cannot.
Loam is the balanced mixture everyone wants and few people start with.
The jar test tells you what you have. Fill a straight-sided jar a third full with soil, top up with water, shake hard and leave it for a day. Sand settles within a minute, silt within a few hours, clay over a day or more, and the visible layers give you rough proportions.
You cannot change texture, only structure. Adding sand to clay produces something closer to concrete than to loam, which is a well-known and expensive mistake. What actually works is organic matter, which binds clay particles into larger crumbs that drain better, and which helps sand hold water. The answer to both problems is the same, and it is compost.
Raised beds sidestep the question where the underlying soil is genuinely unworkable, which is often the cheaper route than trying to transform heavy clay across a whole plot.
Reading a soil test
The cheapest and highest-value thing in this whole category.
Where to get one. County extension services in the US offer inexpensive tests, and garden centres sell home kits. Extension tests are more accurate and worth the wait.
pH is the first line to read. Below 6.0 and above 7.5, nutrient availability drops sharply regardless of what is present. Lime raises pH and elemental sulphur lowers it, both slowly over months, which is why correcting pH is a season-ahead job.
Nutrient levels usually come as categories rather than raw numbers: low, medium, optimum, high. Anything at optimum or high needs nothing added.
Organic matter percentage is the other number that matters. Below about 3% suggests the soil would benefit substantially from compost. Above 5% is good.
What a test does not tell you is nitrogen with any reliability, because nitrogen levels shift constantly with temperature, moisture and microbial activity. Nitrogen is managed by observation and by adding organic matter rather than by testing.
Diagnosing by symptom, carefully
Visual diagnosis is unreliable and worth attempting only alongside a test.
Uniform yellowing of older leaves first suggests nitrogen deficiency, since nitrogen is mobile and plants move it to new growth.
Yellowing between the veins of new leaves suggests iron, which is immobile, and is frequently caused by high pH locking iron up rather than by absence of iron.
Stunted growth with unusually dark leaves, sometimes with purple tints on stems and leaf undersides, suggests phosphorus. It also occurs temporarily in cold soil, because phosphorus uptake slows at low temperatures, and that version resolves as the soil warms without any amendment.
Browning leaf margins can indicate potassium deficiency and can equally indicate salt buildup, drought stress or over-fertilising.
Blossom end rot on tomatoes and peppers is calcium-related and almost always caused by inconsistent watering preventing calcium reaching the fruit, rather than by calcium absent from soil. Mulching and even watering fix it more reliably than any amendment, as our organic tomato fertilizer guide explains.
The honest caveat: most of these symptoms overlap with watering problems, root damage, pests and disease. Deficiency is diagnosed far more often than it occurs.
What most gardens actually need
New beds on poor soil need organic matter above all. Compost, in quantity, worked in and topped up annually.
Established vegetable beds generally need nitrogen and organic matter. Most other nutrients accumulate from years of compost.
Containers need regular feeding because there is no soil reserve to draw on and watering flushes nutrients out. This is the one case where a balanced liquid fertiliser earns its place routinely.
Lawns need nitrogen and very little else, which is why phosphorus-free lawn feed is now standard and in places mandatory.
Fruiting plants want lower nitrogen relative to potassium once established, since excess nitrogen produces leaves at the expense of fruit.
Perennial borders need remarkably little. An annual compost mulch is usually the whole programme.
Our beginners guide to organic gardening covers building a garden from this starting point.
Amendments that are mostly marketing
A few products in this aisle deserve more scepticism than they get, and saying so is more useful than another recommendation.
Epsom salts are magnesium sulphate, widely recommended for tomatoes and roses. Magnesium deficiency is genuinely uncommon in most soils, and where it is not deficient, adding magnesium can interfere with calcium uptake. It is a corrective for a diagnosed shortfall, not a routine tonic.
Eggshells are calcium carbonate and break down extremely slowly, over years rather than a season. Crushed shells added to a planting hole do essentially nothing for the plant growing there this summer. They are fine in the compost heap.
Coffee grounds are not meaningfully acidic once brewed, despite the persistent claim. They are a reasonable nitrogen-containing compost input and a poor soil acidifier.
Compost tea has an enthusiastic following and thin evidence for measurable benefit over simply applying compost. Poorly made tea can also multiply pathogens, which is a real concern on food crops.
Mycorrhizal inoculants are genuinely interesting and frequently unnecessary. The fungi are already present in most soils that have not been sterilised or heavily cultivated, and high phosphorus levels suppress them regardless of how many you add. Correcting phosphorus excess helps more than inoculating.
Biochar has real research behind it for long-term soil carbon and structure, and it needs charging with nutrients before use. Applied raw it initially adsorbs nutrients from soil rather than supplying them, which can make things worse in the short term.
Anything promising to fix everything deserves the most scepticism. Soil problems are specific, and a product claiming to solve pH, nutrition, structure and disease simultaneously is describing compost at a much higher price.
Building soil rather than feeding plants
The organic approach that actually works over years, and the one that reduces spending rather than increasing it.
Add organic matter continuously. Compost, leaf mould, aged manure and mulch that breaks down. Soil rich in organic matter holds water, drains well, resists compaction and supports the microbial life that makes nutrients available.
Mulch everything. It suppresses weeds, moderates soil temperature, conserves water and feeds the soil as it decomposes. Our organic mulch guide covers the options.
Disturb the soil less. Heavy digging breaks up fungal networks and accelerates the loss of organic matter. No-dig approaches build structure faster than repeated cultivation.
Keep something growing. Cover crops over winter hold nutrients that would otherwise leach, add organic matter when turned in, and protect soil structure from rain.
Rotate crops. Different families draw different nutrients and host different pests, and rotation reduces both depletion and disease pressure.
Make your own compost. The single largest saving available in this category, using material you currently pay to dispose of.
What OMRI listing means, and when it matters
Garden inputs sit outside USDA organic certification, which surprises people who assume the seal covers everything organic.
USDA organic certifies food and farms. A bag of fertiliser is not food, so it does not carry the seal in the way a bag of flour does. Certified organic farms still need to know which inputs they may use.
OMRI fills that gap. The Organic Materials Review Institute reviews products against the National Organic Program standards and publishes a list of those permitted for use in certified organic production. An OMRI listed input is one a certified organic farm may legally apply.
It reviews the product, not the marketing. OMRI examines the full formulation including inert ingredients, which is exactly where a nominally natural product can contain something prohibited.
It is voluntary and it costs money, so absence of listing does not prove a product is unsuitable. Small producers frequently skip it. Presence of listing is a genuine verification; absence is merely uninformative.
Where it matters most is anything applied to food crops, and anything where the word organic is doing significant work in the price. Our OMRI listed explainer covers the review process in more detail.
Watch for the usual substitutes. Natural, eco-friendly, earth-safe and plant-based are unregulated terms in garden inputs exactly as they are in cosmetics. Derived from natural sources describes an origin and says nothing about what happened afterwards.
Timing, and why it decides whether anything works
The right amendment applied at the wrong moment does very little, and this is where a good deal of effort is wasted.
Slow-release inputs need lead time. Bone meal, rock phosphate and lime work over months. Applying them the week you plant means the benefit arrives after the season it was meant to serve. Autumn or several weeks before planting is the right window.
Nitrogen is the opposite. It leaches, so applying it far ahead simply loses it to winter rain. Nitrogen goes on during active growth, in smaller amounts more often.
Cold soil suspends everything. Microbial activity, which is what makes organic inputs available, slows dramatically at low temperatures. An organic fertiliser applied to cold spring soil sits inert until things warm up, which is why early-season deficiency symptoms so often resolve on their own.
pH correction is a season-ahead job. Lime and elemental sulphur both shift pH over months, not weeks. Testing in autumn and correcting then means the following spring starts in the right band.
Compost can go on any time and is most useful applied in autumn as a mulch, where it protects soil over winter and is incorporated by worms before spring.
Do not feed dormant plants. Perennials, shrubs and trees take up almost nothing while dormant, and late-season nitrogen in particular pushes soft growth that then gets damaged by frost.
Water it in. Most amendments need moisture for the microbial and chemical processes that release nutrients. A dry spell after application means nothing happens until it rains.
Common questions about soil amendments
What is the difference between an amendment and a fertiliser?
An amendment changes the physical or biological properties of soil, improving structure, drainage and microbial life over seasons. A fertiliser supplies specific nutrients to plants over weeks to months. Compost does both, being an excellent amendment and a weak fertiliser.
Do I need a soil test?
Yes, and it is the cheapest useful thing in this category. Without knowing your pH and existing nutrient levels, every purchase is a guess. County extension services offer inexpensive and accurate tests.
Can I use too much organic fertiliser?
Yes, particularly phosphorus, which binds to soil and accumulates rather than leaching. Excess phosphorus interferes with iron and zinc uptake, suppresses mycorrhizal fungi, and contributes to algal blooms when it runs off.
What is the difference between bone meal and blood meal?
They are near-opposites. Bone meal is high in phosphorus and calcium with little nitrogen, slow-releasing and unable to burn roots. Blood meal is around 12-0-0, the strongest organic nitrogen source, fast-acting and capable of burning plants if over-applied.
Are worm castings worth the price?
For seedlings and potting mixes, yes. As a fertiliser for a whole bed, no. Nutrient content is low, typically around 1-0-0 to 2-1-1, so you are paying a great deal per unit of nutrient. The value is microbial content and humic substances.
Why did my fertiliser not work?
Most often pH. Outside roughly 6.0 to 7.0 many nutrients become chemically locked and unavailable regardless of how much is present. The second most common reason is that the nutrient was never deficient to begin with.
Is compost enough on its own?
For most established gardens, largely yes. Two to three inches annually supplies organic matter, feeds soil biology and delivers modest nutrients. Containers and heavy-feeding crops are the main exceptions.
Should I add sand to clay soil?
No. It is a well-known and expensive mistake that produces something closer to concrete than loam. Organic matter is what improves clay, binding the fine particles into larger crumbs that drain better. The same addition also helps sandy soil hold water.
What does OMRI listed mean?
The Organic Materials Review Institute has reviewed the full formulation, including inert ingredients, against National Organic Program standards and confirmed it may be used on certified organic farms. Garden inputs sit outside USDA organic certification, so OMRI fills that gap.
Why did nothing happen after I applied fertiliser in spring?
Probably cold soil. Organic inputs depend on microbial activity to become available, and that activity slows dramatically at low temperatures. The product sits inert until things warm up, which is also why early-season deficiency symptoms often resolve without intervention.
When should I apply lime or sulphur?
Months ahead of when you need the result, ideally autumn for the following spring. Both shift pH slowly, and treating pH as a season-ahead job rather than a quick fix is the difference between it working and not.
Can I make my own compost instead of buying amendments?
Yes, and it is the largest saving available in this category. You would be composting material you currently pay to have taken away, and finished compost is the single most useful thing you can add to almost any soil.
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