Compost-Bins.co.uk

Summary

Compost leachate is the brown-yellowish liquid that drains out of a compost bin when the contents are wetter than the bin can hold. It is not the same thing as ‘Compost tea’, ie the brown-yellowish liquid made by soaking compost in water and then using the brown-yellowish water.

Neither is a consistent, predictable fertiliser. In a well‑running bin, leachate is usually something to avoid collecting in the first place.

What compost leachate actually is

Leachate is not “compost juice” in the way most people imagine. It is simply water that has moved through partly decomposed material and picked up whatever is easy to dissolve or carry with it.

That means leachate can contain a mixture of:

• dissolved salts and nutrients (sometimes useful, sometimes too strong)
• organic acids and other by‑products from oxygen‑limited patches
• very fine particles washed out of the composting mass

Because composting happens unevenly inside a bin, the liquid draining out reflects whatever zones the water passed through. Two bins fed with the same kitchen scraps can produce very different leachate on different days.

What people mean by “compost tea” (and why the term causes confusion)

The phrase “compost tea” is used loosely for several very different liquids. Lumping them together is where most confusion starts.

In practice, people are usually talking about one of three things:

• Plant-based soaks – leafy plants such as comfrey soaked in water until they break down. This produces an oxygen‑limited liquid similar to a digestate. It is often called compost tea, but it is not made from compost at all.
• Compost water extracts – compost mixed with water, then strained so the liquid can be poured onto soil. This dissolves some fine particles and soluble nutrients, while leaving larger fragments behind.
• Accidental low‑oxygen brews – compost left sitting in water for too long, especially without movement, where the liquid turns sour or smells unpleasant.

All three are liquids. All three are sometimes called compost tea. Biologically and practically, they behave very differently.

The shared name creates a false sense of consistency. It sounds like a defined product, when in reality it describes a range of liquids whose behaviour depends on what was soaked, how wet it became, and whether oxygen was present.

How compost leachate is claimed to work, versus the practical reality

The common claims

You will often see claims like:

• “It’s a free liquid fertiliser.”
• “It’s full of beneficial microbes.”
• “It feeds plants instantly and boosts growth.”

These claims feel attractive because they turn a messy by‑product into a benefit.

The practical reality in a compost bin

A compost bin is a living decomposition mass with wet and dry pockets, airy zones and oxygen‑limited micro‑zones. Liquid forms when moisture builds up faster than it can be held and evaporated.

So, leachate usually appears when conditions have drifted in the wrong direction:

• too much wet input (food scraps, fruit, coffee grounds) relative to structure
• compaction and loss of air pathways
• rainfall entering an open or poorly covered bin

In other words: leachate is often a sign of excess moisture and reduced airflow, not a sign that the system is producing a premium plant feed.

When it can be useful

Leachate can sometimes act like a rough, fast‑acting feed because it may contain soluble nitrogen and potassium. However, its strength is unknown and can swing from weak to harsh.

It tends to be most usable when it is:

• produced in small amounts
• not foul‑smelling
• coming from a bin that is not waterlogged and not sour

Even then, it behaves more like an unpredictable liquid feed than a stable fertiliser.

When it should not be used as a fertiliser

Leachate is a poor choice when it shows signs of oxygen‑limited breakdown or concentrated load:

• strong sour, rotten, or “drain” odours
• thick, sticky liquid with lots of fine solids
• large volumes after heavy rain or prolonged wet loading

In these situations, the liquid is more likely to carry organic acids and reduced compounds that plants do not enjoy, especially in pots or on young seedlings.

Why leachate shows up in some bins and not others

Leachate is mostly about water behaviour.

Bins that shed rain, hold structure, and allow air exchange tend to produce little or no leachate. Bins that trap water, compact easily, or are fed heavily with wet materials tend to produce more.

A useful mental model is this:

• Good composting is mostly aerobic biology.
• Aerobic biology needs air pathways.
• Too much water blocks those pathways.
• Leachate is what happens when the system has more water than it can carry.

So the long‑term “solution” is rarely about finding the perfect way to use leachate. It is about keeping the bin in a moisture range where leachate barely appears.

A practical way to think about using it (without overthinking it)

If you choose to use leachate on plants, treat it as a strong, unknown concentrate rather than a gentle tonic.

That leads to four simple, practical rules of thumb:

• Treat it as a concentrate and assume it is stronger than it looks.
• If you dilute it, keep it heavily diluted (many gardeners use at least ten parts water to one part leachate as a safety margin).
• Use it only on established plants, not seedlings or cuttings.
• Prefer soil beds over pots, because pots concentrate mistakes.

If it smells bad, treat it as a warning sign from the bin rather than something to apply to plants.

This keeps the focus where it belongs: on composting conditions first, liquids second.

Rather than trying to replace leachate with another liquid product, it is usually more helpful to recognise when claims about liquids are drifting beyond what compost bins reliably produce.

How to spot marketing drift in leachate and tea claims

A simple test: if a product description makes leachate sound like a guaranteed benefit, it is skipping the messy reality of bins.

Leachate is not a consistent output. It changes with:

• what went into the bin
• how wet the mix became
• where oxygen was limited
• how long material has been decomposing

So any claim that treats it like a standardised fertiliser is overconfident.

Bridge: where to go next

If you want to reduce leachate at the source, the next page is:

• What materials can be composted (and which ones push moisture too far)

If you are deciding what kind of system fits your household, go to:

• Choosing a compost bin (which designs cope best with wet inputs and rainfall)

Summary

Compost accelerators and inoculators are sold as powders or liquids that claim to “speed up” composting. In a normal compost bin, the biggest drivers of speed are still conditions inside the heap: air, moisture, warmth, and structure.

What are they?

Compost accelerators

These are products sold to make compost “break down faster”. They are usually powders or granules, sometimes pellets.

In practice they tend to be one (or a mix) of:

• A food source (finely ground plant material, dried organic meals, seaweed, or similar)
• Enzymes (proteins that help break down food)
• A small dose of microbes (bacteria and fungi cultures)

Compost inoculators

An inoculator is a product sold to “seed” the bin with composting microbes. Some are packaged as powders, some as liquids, and some are mixed into an “accelerator” product.

Other additives

A wider group of products is sold as compost “helpers”, including:

• Odour reducers
• Moisture absorbers
• pH adjusters
• “Activator” liquids

They are marketed as solutions to slow composting, smells, flies, or a bin that “isn’t doing anything”.

How they are claimed to work, versus the practical reality in a bin

The claim: “Add microbes to kick-start composting”

Composting microbes are already present on food waste, garden waste, and in the bin itself. As soon as fresh material is added, microbes that fit those conditions multiply quickly.

So, in most household bins, the limiting factor isn’t “missing microbes”. The limiting factor is that the material isn’t getting the conditions it needs to stay actively aerobic.

What this looks like in practice:

• A bin can contain plenty of microbes and still be slow if the heap is too wet, too compacted, or too cold.
• A bin can smell even with “added bacteria” if air can’t move through the mass.

The claim: “Add enzymes to speed breakdown”

Enzymes are part of how composting works, but they are not something a compost heap runs out of. Microbes produce enzymes in response to food and conditions.

In a bin, the speed of breakdown tracks the basics: how much oxygen is reaching the active material, how wet it is, and how well the structure holds open spaces.

The claim: “Add a small dose of ‘extra food’ to boost activity”

Many accelerators are, in effect, a small amount of easy-to-digest organic food. That can nudge activity in the same way that any easy, fresh material can.

The practical reality is scale:

• A typical dose of a commercial powder is tiny compared with the amount of fresh material you add to a bin.
• If a heap is slow, it’s usually because the existing mass is not in the right condition for fast aerobic breakdown.

The claim: “Fix smells and problems with additives”

Smell is usually a sign that parts of the heap have become air-limited. Additives that promise odour control often work by masking smells, absorbing moisture, or adding a perfume-like scent.

They don’t replace the role of structure and airflow.

What really decides speed in a compost bin

A compost bin is a living pile. It speeds up when the active material has:

• Airflow (oxygen can reach the microbes)
• Moisture balance (moist enough for biology, not so wet that air spaces collapse)
• Warmth (warmth builds when activity is strong and heat is retained)
• Structure (enough rough, springy material to keep pathways open)

When those conditions hold, composting tends to “look alive”: the heap settles, warms, and changes texture and smell over time.

What alternatives are readily available

If you’re tempted by a commercial accelerator, it usually helps to think about what you’re actually trying to change.

For “I want it to start faster”

In many bins, a simple inoculation is already to hand:

• A handful of garden soil
• A handful of finished compost or older compost from the base of the bin

These bring a diverse mix of microbes and fine particles that help distribute moisture through the heap.

For “It feels sluggish and inactive”

Sluggish composting is often a condition problem rather than a ‘missing ingredient’ problem. A few common, readily available inputs can support active composting because they add easy-to-digest nitrogen and energy.

Examples people often already have access to:

• Fresh grass clippings (when available)
• Manures or pelleted poultry manure
• A small amount of liquid plant feed or diluted fertiliser (used sparingly so the heap doesn’t become wet and airless)

The point isn’t the brand or the product. The point is that a compost heap responds to available food and workable conditions.

For “I’m composting lots of woody or dry material”

Woody, dry, or very fibrous material breaks down more slowly. No powder changes that basic reality.

What helps most is keeping the heap aerobic:

• Add structural material (for example, wood chip or coarse bulking material) so the heap doesn’t compact.
• Keep moisture in the workable zone so microbes can stay active without squeezing out air.

For “I’m getting smells or flies”

These issues usually come from what the bin is experiencing, not from what it is lacking.

A bin tends to stay calmer when:

• Fresh additions are covered with older material.
• Wet, dense food waste is balanced by structure so air can move.
• The lid and access points are managed so pests can’t find easy entry.

Additives marketed as odour-fixers don’t replace these conditions.

When a commercial accelerator can make sense

There are times when people like accelerators because they are convenient:

• They act as a repeatable “nudge” when the bin is being fed small amounts.
• They provide a tidy, measured input when grass or other fresh materials aren’t available.

Even then, what the product is really doing is usually one of these:

• Adding a small food source
• Adding a small dose of nitrogen
• Adding microbes that were already going to arrive with the next bucket of scraps

So the benefit tends to be modest compared with what happens when airflow, moisture, and structure are working well.

A simple way to decide

If a bin is already warm, active, and changing steadily, there is little for an additive to improve.

If a bin is slow, an additive rarely fixes the root cause. In most cases the heap is asking for conditions: more air pathways, a better moisture balance, or more mix of materials.

That’s why many people find the cheapest “accelerators” are the ones already in the garden shed: a handful of soil or old compost, plus readily available nitrogen-rich inputs when needed.

Bridge to next page

If you want to make sense of what helps most, the next step is usually to look at the materials going in.

Next: What materials can be composted in a compost bin?

Composting can appear to stall when one or more essentials become limiting. This is most often due to dryness, excess moisture, or loss of structure. In some cases, compost has not stalled at all but is simply moving slowly, particularly in cooler conditions.

Understanding that composting does not proceed at a constant rate helps avoid unnecessary intervention.

Tip to improve: Check moisture first, then structure, before making changes. In passive systems, allowing more time is often sufficient.

Pests are attracted by accessible food and shelter, not by composting itself. Open access points, exposed food waste, or gaps at ground level increase risk. Well‑designed bins reduce these opportunities, but user practices still matter.

Flies are often linked to fresh food waste near the surface rather than to the composting process as a whole.

Tip to improve: Bury food waste within the compost and keep lids, bases, and joins secure.

Rather than thinking in terms of strict “can” and “can’t” lists, it is often more helpful to think about composting as a diet for decomposer microbes. Different materials break down at very different rates and place different demands on air, moisture, and structure.

Easy or soft materials are rich in sugars, starches, and other readily available compounds. These include vegetable peelings, soft green waste, and fresh grass. They break down quickly but can cause problems if added in large amounts without structure.

Medium materials are dominated by plant fibres such as cellulose. Leaves, stems, and mixed garden trimmings sit in this category. They decompose steadily and form the bulk of most domestic compost heaps.

Hard or woody materials contain tougher fibres and lignin that only a smaller group of specialist microbes can break down. Twigs, wood chips, and woody prunings decompose slowly and tend to control the overall time horizon of the composting mass.

Most composting problems arise when one group dominates the mix, rather than from the presence of any single material.

Tip to improve: Aim for a mixed ‘diet’. Balance easy, fast‑rotting materials with enough fibrous and woody material to maintain air spaces and keep the composting process stable over time.

Turning and mixing redistribute material, moisture, and oxygen within a compost bin, but their effects are often temporary and limited. Any increase in oxygen availability from turning is usually short‑lived and is quickly consumed in active material. Turning also releases stored heat and can disrupt developing microbial structure.

In static, non‑tumbler bins, turning is often partial and uneven. It does not reliably improve structure and, under high‑moisture conditions, can actually increase compaction rather than relieve it. As a result, turning is not a requirement for composting to occur in passive systems, although it may be used occasionally to address specific problems.

In hot composting systems, regular turning is rarely needed. These systems are designed to manage airflow, moisture, and biological activity internally, and frequent disturbance can work against their intended mode of operation.

Whether to turn therefore depends less on composting necessity and more on how the system is designed and managed.

Tip to improve: Only turn when there is a clear issue, such as strong odours or visible compaction. If you do turn, keep it gentle and limited, and focus on restoring structure with dry, bulky material rather than frequent mixing.

Food waste can be composted at home, but success depends on experience, preparation, and bin capability as much as on the waste itself. Many problems arise not because food waste is unsuitable, but because it is added too quickly or in volumes that overwhelm airflow and structure.

For those new to composting, it is sensible to begin with easy food wastes such as vegetable peelings, salad trimmings, and uncooked plant scraps. These break down readily and help you learn how your bin behaves. As confidence grows, small amounts of cooked food plate scrapings can be added, provided they are well mixed into the compost mass.

With experience, and in bins designed to cope with food waste, even more challenging materials — such as whole chicken or duck roast carcasses — can be composted. However, this sits at the demanding end of domestic composting and requires good structure, moisture control, and patience.

It is worth remembering that even skilled composters occasionally have a bin that drifts off course due to weather, inputs, or timing.

Tip to improve: Build up gradually. When adding food waste, always mix in dry paper or coarse bulking material. If the bin starts to smell or compact, add more dry, structured material to bring conditions back into balance.

Experience with domestic composting systems shows that timeframes depend strongly on system type and input materials.

In ambient composting systems, especially where garden waste such as woody prunings and mixed trimmings dominate, it is realistic to allow 12–18 months for material to reach a generally usable state. Softer materials, such as grass clippings or leafy waste, often break down sooner within the mass, but the overall composting time is governed by the slowest materials present.

In specialist hot composters handling kitchen food waste, material can sometimes look composted within around 30 days under favourable conditions. However, appearance can be misleading. Allowing around 90 days provides a more realistic window for broader stabilisation within the bin.

It is important to remember that composting acts on the whole mass, not just the most active fraction. Final timing is therefore always constrained by what goes in, how often new material is added, and how conditions are maintained.

Tip to improve: Judge progress by overall balance and smell rather than surface appearance. Adjust expectations based on the slowest inputs in the system, not the fastest.

Not all compost smells mean something is wrong. In practice, three broad odour types are commonly experienced, each with different causes and implications.

Mild, cabbage‑like or earthy smells are often associated with periods of high microbial activity. These smells are usually not unpleasant to most people and commonly occur as fresh material is actively decomposed. In enclosed hot composting systems, such odours may be reduced as air passes through carbon filters fitted in the lid. They are typically short‑lived and resolve naturally.

Sharp or ammonia‑type smells are most often linked to large additions of fresh grass clippings or other nitrogen‑rich materials. As these break down, excess nitrogen can be released faster than it can be stabilised, producing a pungent but usually temporary smell.

Strong, unpleasant, or putrid odours are more indicative of oxygen limitation. These develop when compost becomes too wet, compacted, or structurally collapsed, allowing anaerobic conditions to dominate locally.

Enclosed compost bins can reduce odour risk by supporting airflow and separating excess liquid, but no bin can fully prevent odours if material balance and structure are lost.

Tip to improve:

• For mild cabbage‑like smells, leave the bin undisturbed; these often fade within a day or two.
• For ammonia smells after adding grass, mix in dry, shredded paper or other carbon‑rich material to rebalance inputs.
• For strong, unpleasant odours, gently stir the contents and add coarse bulking material and dry paper to restore air spaces and reduce excess moisture.

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Summary

Composting works because living microbes break down organic material. As they do this, they release warmth, which tells us how active the process is. A good compost bin helps hold the right conditions so this natural process can keep going.

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What happens inside a compost bin

Inside a compost bin, billions of microscopic organisms are feeding on organic material such as food scraps, garden waste, and plant residues. These microbes need three basic things to stay active: air, moisture, and warmth.

As microbes consume organic material, they use oxygen and release energy. Part of that energy becomes heat. This is why compost heaps often feel warm in the middle. The warmth is not added from outside — it is produced by the biology inside the bin.

At first, microbes focus on the easiest materials to digest. Soft plant tissues, sugars, and proteins are broken down quickly. During this stage, activity rises and temperatures increase. As these easy foods are used up, the pace naturally slows and temperatures fall.

Throughout the process, the compost is changing. Fresh scraps gradually lose their original form. What remains becomes darker, crumbly, and more stable. The rise and fall of temperature simply reflects how active the biology is at each stage.

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The role of warmth in composting

Warmth helps microbes work efficiently, but it does not cause composting by itself. Compost heats up because microbes are active, not because heat is forcing the process to happen.

When conditions are right, compost often passes through a warm phase. During this time, microbes that prefer higher temperatures become more common. They continue breaking down tougher materials that were slower to change earlier on.

There is a useful middle range where composting tends to move along smoothly. In this range, microbes are active without being stressed. If compost becomes too cool, activity slows. If it becomes too hot, some microbes struggle and overall progress can stall.

This is why steady warmth matters more than extreme heat. Compost does not need to stay very hot to finish properly. In fact, cooling over time is a normal and healthy part of the process.

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Air and moisture work with temperature

Temperature never acts alone. It is tightly linked to air and moisture.

Microbes need oxygen to do their work. As compost warms up, microbes breathe faster and use oxygen more quickly. If fresh air cannot reach the centre of the pile, oxygen levels drop and the process slows.

Moisture matters just as much. Microbes live in a thin film of water around organic particles. If compost dries out, microbial activity fades and temperatures fall. If it becomes too wet, air spaces fill with water and oxygen cannot move freely.

Healthy compost usually feels damp but not soggy. When air and moisture are balanced, warmth takes care of itself.

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Why structure and bin design matter

How compost materials are held together makes a big difference to how heat behaves.

Very loose piles lose heat quickly. Very dense piles can trap heat but also restrict airflow. The most effective compost structure sits between these extremes, holding warmth while still allowing air to move through.

A compost bin helps by providing shape, containment, and some insulation. This reduces heat loss and protects the compost from drying winds or heavy rain. The result is not higher temperatures for their own sake, but more stable conditions that allow microbes to keep working.

Good design supports the biology rather than trying to control it.

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What composting does — and doesn’t do

Composting turns mixed organic waste into a more stable, soil-like material that is easier to handle and use. Over time, smells fade, volume reduces, and materials become unrecognisable.

Composting does not happen instantly, and it does not stay hot forever. Cooling is not a failure; it signals that the most active stage has passed. Compost also continues to change after it cools, as slower biological processes finish the job.

Warmth can help reduce some unwanted organisms when conditions allow, but it is only one part of a wider process. Time, oxygen, and moisture all play essential roles.

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How this links to choosing a bin

Different compost bins manage air, moisture, and heat in different ways. Understanding the role of temperature helps explain why some bins keep compost active more reliably than others.

On the next page, you can explore what materials can be composted and how they behave inside a bin — or look at how different bin designs support the conditions composting needs to work well.