Compost

Ericaceous compost is an acidic growing medium with a pH of around 4 to 5, specifically formulated for plants that cannot tolerate lime or alkaline conditions. Plants that need ericaceous compost include blueberries, rhododendrons, azaleas, camellias, acers (Japanese maples), heathers, pieris, skimmia, and hydrangeas grown for blue flowers. These plants have evolved in naturally acidic soils — woodland edges, moorlands, and mountain slopes — and are unable to absorb certain nutrients (particularly iron and manganese) when the pH is too high, which causes yellowing leaves and poor growth. If your garden soil is alkaline or chalky, ericaceous compost in pots and containers is the most reliable way to grow these plants successfully. Standard multipurpose compost is too alkaline for ericaceous plants and will cause them to decline over time.

John Innes is not a brand but a specification for loam-based composts originally developed at the John Innes Horticultural Research Institute in the 1930s. The formulas are in the public domain, so multiple manufacturers produce them. John Innes composts contain sterilised loam (soil), peat or peat substitute, coarse sand or grit, and fertiliser — the loam gives them excellent nutrient buffering and stability compared to soil-free composts. The numbers indicate increasing fertiliser levels. John Innes No 1 contains the lowest level of nutrients and is used for pricking out seedlings and rooting cuttings, where high nutrients would scorch delicate roots. John Innes No 2 contains moderate nutrients and is suitable for potting on young plants and most vegetables. John Innes No 3 contains the highest nutrient levels and is designed for mature plants, shrubs, trees, and long-term container planting. There is also a John Innes Seed Compost for germination, and a John Innes Ericaceous Compost for acid-loving plants. Because they are loam-based, John Innes composts are heavier than multipurpose — an advantage for large pots that need stability.

For the vast majority of garden uses, high-quality peat-free compost performs as well as peat-based compost, and in some respects better. The UK retail ban on peat-based compost for amateur gardeners took effect in 2024, and the major manufacturers have invested heavily in peat-free formulations over the past decade. Modern peat-free composts use ingredients such as composted bark, wood fibre, coir (coconut fibre), green waste, and loam. Coir has excellent water retention; composted bark improves drainage and aeration. The main practical differences to be aware of: peat-free compost can dry out faster than peat-based and may be harder to rehydrate once very dry — check moisture regularly. It can also have a slightly coarser texture, which makes some cheaper peat-free products less suitable for seed sowing. For general potting, container growing, raised beds, and border conditioning, a reputable peat-free multipurpose compost is entirely reliable. Avoid very cheap, unbranded products regardless of peat content — quality varies significantly.

Multipurpose compost is a general-purpose growing medium suitable for most potting, planting, and container tasks. It provides a balanced nutrient level for established plants and is the most practical choice for everyday gardening — filling pots, planting bedding, and topping up raised beds. Specialist composts are formulated for specific plants or stages of growth where multipurpose is unsuitable. Seed compost has a much finer texture and very low nutrients, which is essential for germination — high-nutrient composts can inhibit germination or scorch seedling roots. Ericaceous compost has an acidic pH for acid-loving plants such as blueberries and rhododendrons. John Innes composts are loam-based and better for plants remaining in containers for more than one or two years, as the loam provides stable, long-term nutrition. Tomato and vegetable composts have added potassium to support fruiting crops. As a general rule: use seed compost for sowing, ericaceous for acid-lovers, John Innes for long-term container plants, and multipurpose for everything else.

The volume needed depends on the bed's dimensions and depth. The formula is: length (metres) × width (metres) × depth (metres) × 1,000 = litres required. For reference: a standard 120cm × 60cm bed at 30cm depth needs approximately 216 litres — around four to five 40–50 litre bags. A larger 240cm × 120cm bed at the same depth requires approximately 864 litres — 17 to 22 bags. For new raised beds, a 60/40 mix of compost and topsoil gives better structure and long-term fertility than pure compost, which can shrink and compact. For vegetable beds, adding compost with John Innes loam improves nutrient retention. To top up an existing bed each year, a 5cm surface dressing is sufficient — approximately 50 litres per square metre. For beds over 500 litres, buying a bulk bag (one cubic metre = 1,000 litres) is significantly cheaper per litre than individual bags.

Compost can be added at any time of year, but timing affects how it is best applied. Autumn (October to November) is the most recommended time for top-dressing borders and vegetable beds. Spreading a 5–8cm layer on the surface allows frost and winter rain to break it down, and worms pull it into the soil over winter — by spring, the bed is noticeably improved. Spring (March to April) is the best time for incorporating compost into vegetable beds before sowing, especially if the autumn application was missed. Dig or fork it into the top 20–30cm just before planting. For containers and pots, replace the top few centimetres of old compost with fresh each spring, or repot entirely every two to three years. When planting trees, shrubs, or hedging, mix compost into the planting hole at a ratio of around one part compost to three parts excavated soil to improve establishment.

Used compost can be reused in many situations, though not as a like-for-like replacement for fresh compost. The main issue is nutrient depletion — plants extract nutrients throughout the season, so old compost provides little fertility on its own. The physical structure also degrades over time, reducing drainage and aeration. To reuse pot compost, remove any old roots and top up with 30–50% fresh compost, then add a slow-release granular fertiliser to replenish nutrients. This is perfectly adequate for hardy shrubs, perennials, and bedding plants. However, avoid reusing compost that has grown diseased plants — particularly blight-affected tomatoes or club root-infected brassicas — as pathogens can persist. Avoid reusing seed compost; it breaks down completely after one season. Old compost is excellent dug into borders as a soil conditioner, used as mulch, or added to a compost heap. Compost from grow bags, while nutrient-depleted, is useful as a soil improver dug into beds.

Compost stores best in a sealed, cool, and shaded location. Once opened, fold or clip the bag closed tightly and store off the ground — placing bags directly on bare soil allows moisture to wick in from below and can cause waterlogging. A dry shed, garage, or sheltered area against a wall is ideal. Avoid leaving compost in direct sun for extended periods, as UV degrades the bag and repeated heating and drying kills beneficial micro-organisms and depletes nutrients. Unopened bags keep well for up to 12 months; after this, slow-release fertilisers begin to break down and the structure can degrade. If stored compost has dried out, rehydrate it by dampening thoroughly and leaving for 24 hours before use — very dry compost repels water initially and gives poor results straight from the bag. Use compost from opened bags within one growing season for best results.