Know your compost: compost types and improvers explained

Val Bourne / 12 March 2019

Buying potting compost isn’t as simple as it once was. Gardening expert Val Bourne looks at what’s on the market, and what you can add to improve drainage and insulation.



The days when you could opt for peat-based or loam-based compost are gone. Peat-based compost grew things well, although once it dried out it was hard to rehydrate. Loam-based compost was more expensive and heavier but held water and nutrients far better.

These days small high-quality compost makers have been bought out by large companies driven by profit and I’ve used composts that were very poor when it came to growing plants. Companies tend to change the recipes to keep the prices lower. Three bags for £10 may sound attractive, but cost isn’t everything! Go for a good brand.


Loam-based John Innes 1, 2 and 3

These loam-based composts originated in the 1930s at the John Innes Horticultural Institute then based at Merton in Surrey. Two scientists, William Lawrence and John Newell, were asked to come up with a good quality compost following problems growing Primula sinensis seedlings for a long-running genetic study. Their recipe was made available in 1938, although John Innes compost was never made commercially by the institute. The only difference between the seed sowing compost, no 1 (for pricked out seedlings), no 2 (for potting on) and no 3 (for mature plants and gross feeders like tomatoes) is the amount of fertiliser added to the mix. And they only contain enough last for four to six weeks, after that you have to feed your plants. However, loam is now in short supply and many gardeners feel that John Innes is not as reliable as it used to be.

Peat-based composts

Peat is gradually being eradicated from many composts because extracting peat from peat bogs damages the flora and fauna. The Government is trying to phase out the use of peat for amateurs by 2020 and commercial growers by 2030, although this isn’t likely to happen by these dates.

The turning point came in 2012 when Eric Pickles, then Secretary of State for Communities and Local Government, upheld Salford City Council’s refusal to give planning permission for more peat extraction at Chat Moss, near Manchester. The reasons he gave were all connected to climate change, water purification, water storage and flood prevention. It’s a contentious issue and many gardeners try to avoid peaty composts, although there is some peat in most composts – including John Innes.

Peat-free alternatives

The compost companies, faced with an approaching ban on peat, are rising to the challenge and producing alternatives.

Wood based

Melcourt has been producing bark-based, peat-free replacements since the early Eighties. Many commercial nurseries use the company’s composts as a growing medium. It’s been successful for me, although containers of spring-flowering bulbs haven’t done as well. The bags are sizeable because wood-based composts are light. SylvaGrow multi-purpose compost with added John Innes is peat-free and contains no green waste. It scored highly in a Which? Gardening trial in 2017. melcourt.co.uk, 01666 502711.

Wool based

Dalefoot has pioneered a compost made from potash-rich bracken blended with Herdwick sheep's wool for improved water retention and slow-release nitrogen. These composts are dark in colour so I found it difficult to tell when my plants needed watering. Dalefoot also sells an autumn mulch named Lakeland Gold. dalefootcomposts.co.uk, 01931 713281

Coir based

Coir-based compost has improved greatly and Fertile Fibre, made from milled coconut husks with added vermiculite, is peat-free and endorsed by the Soil Association. It isn’t sold in garden centres, but there is a home delivery service – although you’d need to be a group. fertilefibre.com, 01432 853111

Soil improvers

Adding a soil improver either means spreading organic matter on to the surface, to feed and suppress weeds, or digging it in to create an airy mixture.

Leaf mould

This normally gets put over the soil and it’s a precious commodity because most of us can’t make enough for the wholes garden. It needs to spread in autumn, round precious plants, and then it’s pulled down by earthworms. It’s good garden sense because leaves can’t be added to the compost heap the lignin in the leaves is hard to break down. The amount of lignin varies. Beech leaves take longer to rot than many others because they’re high in lignin. Turning your leaves three or four times a year speeds up the process and should produce friable leaf mould within a year.

Find out how to make leaf mould

Green waste

Many gardeners are using green waste, bought cheaply from their local authority, as a mulch or soil improver. However it is likely to contain chemical residues, because garden chemicals take a long time to break down, so it isn’t an option for organic gardeners like me. If this doesn’t concern you, it’s an effective weed-smothering mulch. However gardeners who use it tell me it doesn’t seem to contain lots of nutrients. They still add food such as pelleted, nitrogen-rich chicken manure or a slow release balanced fertiliser like Vitax Q4.

Find out more about using soil improvers in the garden

Adding drainage

Coarse grit

This improves drainage and is especially useful for small potted plants and, as a 50% mixture with compost, cuttings. You can also mulch seed trays to keep moisture in, although it does make it easy to overwater when there’s a gritty layer, and some gardeners think it causes bacterial problems. I often surround tender plants with coarse grit, to aid drainage.

Coarse horticultural sand

This can be mixed with compost or used on its own as a growing medium for cuttings. The sand must be coarse, not fine, because coarse sand holds the moisture. Fine sand can dry out in hours. It’s useful for preparing small seed trays: fill the tray with damp coarse sand and then plunge cuttings in, as and when. Cuttings root easily between May and the end of August. After that some bottom heat is needed.

Adding warmth

Airy compost is a better growing medium than compacted compost, because it aids strong root development. Certain materials add air and give a better air-to-water ratio. The two most commonly used substances are vermiculite and perlite, which are mined and then heated so they expand.

Both aerate the soil when mixed with compost and the ratio differs from 50/50 to 20/80 according to preference. They also insulate plants against the cold, raising the compost’s temperature and keeping it stable; and retain water, lessening the need for regular watering. They are often added to seed compost, to speed up germination, and they help cuttings to root more easily.

Perlite

Perlite is an amorphous volcanic glass that expands into round shapes almost like tiny Styrofoam balls. They have lots of cavities, caused by the expansion process, which hold water as well as creating airy spaces. Perlite is more porous than vermiculite so it’s often used with cacti and succulents, because water drains away from the roots more efficiently.

Vermiculite

This is a light, spongy material made from mica and the pieces are irregularly shaped so they create lots of air pockets. Vermiculite holds more water than perlite, so it’s useful for a whole range of moisture-loving and shade-plants including trilliums and other woodlanders. It’s also used for potted snowdrops and other small bulbs. It lessens the need to water, but also allows excess water to drain away. It’s an excellent insulator and you can buy small, medium and large grades.

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The material is for general information only and does not constitute investment, tax, legal, medical or other form of advice. You should not rely on this information to make (or refrain from making) any decisions. Always obtain independent, professional advice for your own particular situation.