The Complete Guide to Potting Soil, Compost and Growing Media

Why the Growing Medium Matters More Than Almost Anything Else

A plant's roots are its foundation. Everything above ground — leaves, stems, flowers, fruit — depends on what is happening below the surface. If the roots are sitting in a medium that holds too much water, they suffocate and rot. Too little water, and they dry out between waterings. If the medium is too dense, roots cannot expand. Too loose, and the plant has no physical support. If the pH is wrong, nutrients present in the medium become chemically unavailable — the plant starves surrounded by food it cannot access.

The growing medium is not just "dirt in a bag." It is a carefully balanced system that must simultaneously provide physical support, water retention, drainage, aeration, and access to nutrients. Getting this right is the single most impactful thing you can do for any container-grown plant, from a windowsill herb pot to a raised bed vegetable garden.

The good news is that choosing a growing medium does not require a degree in soil science. Understanding a handful of key ingredients and what each one contributes gives you the knowledge to select — or mix — the right medium for any purpose.

A Note on Terminology: Compost, Potting Soil, and Growing Media

Terminology in this category varies by region and can be genuinely confusing.

Garden compost is the material you make at home from decomposed kitchen scraps and garden waste. It is a soil improver, added to garden beds to enrich existing soil. It is not the same as the bagged products sold for growing plants in containers.

Potting compost / potting soil is a commercially produced medium designed for growing plants in pots, containers, hanging baskets, and seed trays. Despite the name, it often contains no actual soil (compost) and no actual soil (earth) — it is typically a blend of peat or peat-free materials, perlite, fertiliser, and lime.

Growing media is the broad, technically accurate term that covers everything a plant's roots grow in — from traditional soil to soilless mixes, bark chips, coconut coir, perlite, and hydroponic substrates.

In this guide, we use "growing medium" or "compost" interchangeably for bagged products, and "garden compost" when referring to the material you make yourself.

The Peat Question: Why the Industry Is Changing

For decades, peat moss was the dominant ingredient in commercial potting composts worldwide. It is excellent for growing plants — lightweight, consistent, sterile, moisture-retentive, and available in vast quantities. The problem is where it comes from.

Peat forms over thousands of years in waterlogged bogs — ecosystems that are among the most important carbon stores on the planet. Peat bogs hold roughly twice as much carbon as all the world's forests combined. When peat is extracted for horticulture, this stored carbon is released into the atmosphere as carbon dioxide. The bogs themselves — unique habitats supporting specialised wildlife — are destroyed far faster than they can regenerate.

The horticultural industry is actively transitioning away from peat. In some countries (including the UK, where a ban on peat-based growing media for amateur gardeners is planned), the shift is being driven by legislation. Globally, it is being driven by environmental awareness and improving alternatives.

What this means for you as a buyer: Peat-free composts are now widely available and, in many cases, perform as well as or close to traditional peat-based products. The quality of peat-free mixes has improved dramatically in recent seasons, though some variation remains between brands and batches. We recommend choosing peat-free where possible — not because every peat-free product is perfect, but because the environmental case is compelling and the alternatives are now genuinely practical.

Understanding the Ingredients

Every bagged compost is a blend of ingredients, each contributing something specific. Understanding what each one does allows you to read a product label intelligently and choose — or mix — the right medium for your purpose.

Peat Moss (Sphagnum Peat)

The traditional base ingredient. Excellent water retention (holds up to twenty times its weight in water), lightweight, sterile, and consistent. Naturally acidic (pH 3.5–4.5), which means lime must be added to raise the pH for most plants. Decomposes very slowly, maintaining its structure over time.

The limitation beyond environmental concerns: Peat is difficult to re-wet once it dries out completely. A pot of dried-out peat-based compost resists water, causing it to run down the sides of the pot rather than being absorbed. This hydrophobic quality is one of the most common frustrations for gardeners using peat-based composts.

Coconut Coir

The most widely used peat alternative. Made from the fibrous husk of coconuts — a byproduct of the coconut industry. Available as compressed blocks or bricks (which expand when soaked), loose fibre, or chips.

Strengths: Excellent water retention, good aeration when mixed with coarser materials, close to pH neutral (5.5–6.8), re-wets more easily than peat if it dries out. Renewable resource.

Honest limitations: Contains very few nutrients — plants in pure coir need feeding from the start. Some coir products contain residual salts from the manufacturing process — reputable brands wash and buffer their coir to remove excess salts, but checking the label is worthwhile. Imported from tropical countries (primarily Sri Lanka, India, and the Philippines), so the environmental benefit compared to peat depends partly on the transportation footprint.

Composted Bark

Shredded and partially decomposed bark from the timber industry. Provides structure, drainage, and aeration to growing mixes. Decomposes slowly, maintaining an open texture that prevents compaction over time.

Used in: Potting composts (particularly for long-term container plants), orchid mixes (coarse bark), ericaceous mixes, and as a surface mulch. A growing alternative to peat in commercial peat-free composts.

Note: Fresh, uncomposted bark can temporarily deplete nitrogen as it decomposes (soil microorganisms use nitrogen to break down the carbon in the bark). Composted bark has already undergone this process and does not rob nitrogen from plants.

Perlite

Those small, white, lightweight granules you see in potting mixes. Perlite is volcanic rock that has been heated to extreme temperatures, causing it to expand like popcorn. It is sterile, pH neutral, and inert (it does not decompose or react chemically).

What it does: Improves drainage and aeration. Creates air pockets within the medium that prevent compaction and allow oxygen to reach roots. Holds some surface moisture but drains freely.

Best for: Adding to mixes that need better drainage — particularly for plants that dislike sitting in wet compost (succulents, cacti, Mediterranean herbs, orchids). A standard addition to most quality potting composts.

Vermiculite

A natural mineral (from the mica family) that is heated to expand into lightweight, golden-brown flakes. Unlike perlite, vermiculite holds substantial amounts of water and nutrients within its layered structure.

What it does: Increases water retention and nutrient-holding capacity. Lighter than sand. Provides some aeration but less than perlite.

Best for: Seed-starting mixes (retains the consistent moisture seedlings need), and any situation where you want the medium to hold more water between waterings. Often used alongside perlite — perlite for drainage, vermiculite for moisture retention.

Handle gently: Rough mixing can crush vermiculite, reducing its air-holding ability. Mix by hand or with a gentle turning action.

Garden Compost and Worm Castings

Home-made compost (from a compost bin, tumbler, or worm farm) is not a growing medium on its own — it is a nutrient-rich amendment that is added to mixes or to garden soil.

Garden compost improves soil structure, adds organic matter, and provides a slow-release source of nutrients. Quality varies depending on what went into the compost and how well it was managed. Well-made compost is dark, crumbly, and smells earthy. Poorly made compost may be lumpy, slimy, or smelly.

Worm castings (vermicompost) are exceptionally nutrient-dense — one of the richest organic soil amendments available. A small amount added to a potting mix significantly boosts nutrient availability and supports beneficial soil biology. Too much can be too strong for seedlings; a ratio of approximately 10–20% worm castings to other ingredients is a good starting point.

For more on producing your own, see our Composting Equipment Guide.

Sand and Grit

Sharp sand (also called horticultural sand — not builders' sand, which contains salts and fines that are harmful to plants) and horticultural grit improve drainage and add weight to mixes.

Best for: Cacti and succulent mixes (where very fast drainage is essential). Adding weight to potting mixes for tall or top-heavy plants. Improving drainage in heavy, clay-based garden soil.

Loam (Soil)

Sterilised loam — real, mineral-rich soil — is the traditional base of John Innes composts, which remain highly regarded for long-term container growing. Loam provides weight and stability, holds nutrients well, and creates a growing environment that is closer to natural soil than entirely soilless mixes.

Best for: Long-term container plants (trees, shrubs, permanent plantings) where the extra weight provides stability and the nutrient-holding capacity of loam sustains the plant over months and seasons without frequent feeding.

Types of Bagged Growing Media

Walk into a garden centre and you will find an overwhelming number of bagged products. Here is what each type is designed for.

Multipurpose Compost

The all-rounder. Designed to work for a wide range of tasks: potting up plants, filling containers, growing vegetables, planting hanging baskets, and general garden use. Available in peat-based and peat-free formulations.

Typically contains: A base material (peat, coir, composted bark, or a blend), perlite or vermiculite for drainage, a balanced slow-release fertiliser, and lime to adjust pH to a near-neutral range suitable for most plants.

Strengths: Convenient, versatile, and available everywhere. Adequate for the majority of garden tasks. The most popular product by sales volume.

Limitations: A jack of all trades. Not optimised for any specific purpose — it will not perform as well for seed starting as a dedicated seed compost, or for acid-loving plants as an ericaceous mix. Quality varies significantly between brands, and between peat-based and peat-free versions of the same brand.

Seed and Cutting Compost

A fine-textured, low-nutrient mix designed specifically for germinating seeds and rooting cuttings.

Why it is different from multipurpose: Seeds need consistent, even moisture around tiny roots; a fine texture without large lumps provides this. High nutrient levels can inhibit germination and burn delicate seedling roots, so seed compost contains little or no added fertiliser. The fine texture also makes it easy to cover small seeds to the correct depth.

Typically contains: Fine-grade peat or coir, fine perlite or vermiculite, minimal or no fertiliser, and lime. Some seed composts include a trace of nutrients sufficient for early seedling growth before potting on.

Best for: Starting seeds indoors, germinating seeds in trays and modules, rooting softwood cuttings.

Potting-On Compost

A slightly coarser, more nutrient-rich mix designed for transplanting seedlings into individual pots and for growing on young plants.

Why it exists: Once seedlings have developed their first true leaves and are ready to move from the seed tray to individual pots, they need more nutrients and a slightly more open structure that allows developing roots to expand easily. Potting-on compost bridges the gap between seed compost and multipurpose.

In practice: Many gardeners use multipurpose compost for potting on, which works adequately. Dedicated potting-on compost is a refinement rather than a necessity.

Ericaceous (Acid-Loving) Compost

A compost with a deliberately low pH (typically 4.0–5.5) for plants that require acidic growing conditions — rhododendrons, azaleas, camellias, blueberries, heathers, pieris, and some Japanese maples.

These plants cannot absorb essential nutrients (particularly iron) from alkaline or neutral soil. In standard compost with a pH above 6, they develop yellowing leaves (chlorosis) despite adequate nutrients being present. Ericaceous compost provides the acidic environment they need.

Typically contains: Peat or peat-free acidic base (composted bark is naturally slightly acidic), sulphur or other acidifying agents, and fertiliser formulated for acid-loving plants.

Important: If your tap water is hard (alkaline), watering ericaceous plants regularly with tap water will gradually raise the pH of even an ericaceous compost. Collecting rainwater for these plants helps maintain the correct acidity.

Houseplant Compost

Formulated for indoor container growing, with good drainage and aeration to compensate for the typically lower light and airflow conditions indoors where compost dries more slowly.

Typically contains: Peat or coir base, a higher proportion of perlite than multipurpose (for improved drainage), balanced fertiliser, and lime. Some premium houseplant mixes include worm castings, charcoal (for adsorbing toxins), or mycorrhizal fungi (beneficial root-associated fungi that improve nutrient uptake).

Specialist Mixes: Orchid, Cactus and Succulent, Bonsai

Orchid compost: Coarse bark chips, perlite, and charcoal. Orchid roots need air around them — they are epiphytes in nature, growing on tree branches rather than in soil. Standard compost suffocates orchid roots. The chunky, open structure of orchid mix allows air to circulate while retaining enough moisture between waterings.

Cactus and succulent mix: A very free-draining blend — typically multipurpose or coir base mixed with a high proportion of perlite, grit, or coarse sand (50% or more by volume). Cacti and succulents are adapted to dry conditions and will rot rapidly in a moisture-retentive standard compost.

Bonsai compost: A specialist mix designed for the shallow pots bonsai trees grow in. Needs to hold sufficient moisture and nutrients in a small volume while draining freely. Typically contains akadama (a Japanese clay granule), pumice, and composted bark in specific ratios depending on the tree species.

Raised Bed and Border Mixes

Large-volume mixes designed to fill raised beds, borders, and garden planters. Typically a blend of topsoil, composted green waste, and composted bark — heavier and less refined than potting compost, but appropriate for the larger volumes and more natural growing conditions of a raised bed.

Not the same as potting compost. A raised bed mix is closer to improved garden soil than to the lightweight, refined mix you would use in a pot. Using potting compost to fill a large raised bed is expensive and unnecessary — the raised bed mix provides the structure and drainage, and the plants' roots access nutrients from the larger volume of medium and from additional feeding.

Soil Improvers and Mulches

Soil improver (soil conditioner) is coarse organic material — composted bark, green waste compost, or farmyard manure — dug into garden soil to improve its structure, water-holding capacity, and fertility. It is not a growing medium for containers; it is an amendment for existing ground.

Mulch is material spread on the soil surface around plants to suppress weeds, retain moisture, and gradually improve the soil as it decomposes. Bark chips, composted bark, wood chips, and well-rotted manure are common mulching materials.

Mixing Your Own: A Practical Guide

Mixing your own growing media gives you control over exactly what your plants grow in. It is particularly worthwhile for houseplants, seed starting, and specialist plants where off-the-shelf products may not be ideal.

A simple all-purpose houseplant mix: Two parts coir or peat-free multipurpose compost, one part perlite, one part vermiculite. Add a small amount of worm castings (approximately 10% by volume) for slow-release nutrition.

A seed-starting mix: Equal parts fine coir (or fine peat-free compost) and vermiculite. No added fertiliser — seeds carry their own energy for germination.

A cactus and succulent mix: One part multipurpose compost or coir, one part perlite, one part horticultural grit. This produces a very fast-draining medium that prevents the waterlogging cacti and succulents cannot tolerate.

A chunky aroid mix (monstera, philodendron, pothos): One part coir or peat-free compost, one part perlite, one part coarse orchid bark. The bark creates large air pockets that these epiphytic plants appreciate around their roots.

Mixing tips: Pre-moisten coir or peat before mixing — dry coir repels water until thoroughly soaked. Wear a dust mask when working with dry perlite or vermiculite — the dust is irritating if inhaled. Mix in a large container or on a tarp. Store unused mix in a sealed container to maintain moisture.

How to Judge Quality

When buying bagged compost, you cannot see inside the bag before purchase. Here are indicators of quality:

Check the ingredients list. A quality product lists its components. Avoid products with no ingredients listed or only vague descriptions.

Look for certification marks. In the UK, the RHS Endorsement or Soil Association certification indicates products that meet specific quality standards. In other countries, equivalent horticultural certifications provide similar assurance.

Feel the bag. A quality compost feels evenly textured through the bag — no large hard lumps or excessively wet spots. If the bag feels waterlogged, the compost may have been stored poorly.

Check the date. Bagged compost has a limited shelf life. Old stock may have depleted fertiliser, compacted structure, or anaerobic conditions (a sour smell when opened). Buy fresh stock where possible.

Smell it when opened. Good compost smells earthy and pleasant. A sour, ammonia, or rotten smell indicates anaerobic decomposition — the compost has been waterlogged or poorly stored. It may recover if spread out and aerated, but it is not ideal for immediate use.

Storage: Keeping Bagged Compost in Good Condition

Store unopened bags in a cool, dry, shaded location. Sunlight degrades the bag and heats the compost, accelerating nutrient depletion and potentially encouraging harmful fungal growth. Damp conditions lead to the bag deteriorating and the compost becoming waterlogged.

Once opened, fold the bag closed or transfer unused compost to a sealed container. Open bags exposed to rain become waterlogged; exposed to dry air, the compost dries out and becomes hydrophobic (particularly peat-based products).

Use opened bags within one season. The slow-release fertiliser in most composts has a limited effective period — compost that has sat open in a shed for two seasons has likely depleted its nutrient charge and may harbour fungal spores.

Common Mistakes Buyers Make

Using Garden Soil in Pots

Soil from the garden — however good it is for borders — compacts in containers, drains poorly, and may contain weed seeds, diseases, and pests. Always use a formulated potting compost for containers.

Using Multipurpose for Everything

Multipurpose compost works for most general tasks but is not ideal for seeds (too coarse, too nutrient-rich), acid-loving plants (wrong pH), or cacti and succulents (too moisture-retentive). Matching the medium to the purpose produces noticeably better results.

Ignoring the Peat-Free Transition

Peat-free composts require slightly different watering habits than peat-based products — they tend to dry out faster at the surface while remaining moist deeper in the pot. Adjust your watering technique: water thoroughly but less frequently, and check moisture at depth (push a finger into the compost) rather than judging by the dry-looking surface.

Not Adding Drainage Material

Plants in containers without adequate drainage develop waterlogged root zones. Adding perlite or grit to multipurpose compost — particularly for plants in pots without excellent drainage holes — is a simple improvement that prevents a common cause of plant death.

Using Old Compost Without Refreshing It

Compost in pots loses its structure, nutrients, and aeration over time. Repot plants annually into fresh compost, or top-dress with fresh material if repotting is impractical.

Buying the Cheapest Compost Available

Budget multipurpose compost can contain inconsistent or low-quality ingredients — excessive wood fibre, inadequate nutrients, or poor pH balance. The difference between the cheapest and a mid-range product is modest per bag but significant in plant performance.

Growing Media Comparison Table

Pre-Purchase Checklist

• What are you growing — seeds, houseplants, vegetables, acid-loving plants, cacti, orchids, or filling raised beds?
• Does the plant require a specific pH? Acid-loving plants need ericaceous compost; most others grow in near-neutral mixes.
• Is the compost peat-free? If transitioning from peat-based products, expect slight differences in watering behaviour.
• Does the bag list ingredients? Avoid products with no transparency about their contents.
• Is the compost fresh? Check for a date or lot code, and avoid bags that feel waterlogged or have been stored in direct sunlight.
• Do you need drainage amendments (perlite, grit) to add to the mix? Many plants benefit from improved drainage beyond what comes in the bag.
• For mixing your own: have you sourced the individual components — coir, perlite, vermiculite, bark, worm castings?
• How much do you need? Calculate the volume of your pots or raised beds before buying. Bagged volumes are listed in litres — a standard 10-litre pot uses approximately 8–10 litres of compost.
• Do you have somewhere cool, dry, and shaded to store unused bags?

Why Buying Through Amazon Makes Sense

Amazon's growing media range covers everything from single-litre specialist mixes to bulk bags of multipurpose compost and individual amendment ingredients. Product listings include volume, ingredients, pH range, and intended use for direct comparison.

Customer feedback is particularly valuable for growing media, where the true test is plant performance over weeks and months. Reviews from growers who describe what they planted, how the compost handled watering, whether seeds germinated reliably, and how plants performed across a season provide insights that ingredient lists alone cannot convey.

Individual ingredients — perlite, vermiculite, coir, bark, worm castings, and grit — are well represented on Amazon, making it straightforward to source everything for custom mixes in a single order.

Amazon Marketplace Protections

When you purchase through Amazon, several consumer protections apply. Products sold directly by Amazon are covered by standard consumer rights legislation, including the right to return items within a specified period.

For products sold by third-party sellers on the Amazon Marketplace, the A-to-Z Guarantee provides additional protection. If an item does not arrive, arrives damaged, or is materially different from what was described, Amazon will investigate and, where appropriate, issue a refund.

Amazon Renewed products — professionally inspected and restored items — come with their own guarantee, providing confidence when purchasing pre-owned equipment at reduced prices.

Manufacturer warranties apply in addition to Amazon's protections, covering manufacturing defects and premature component failure under normal domestic use.