Compost – or compost? That is the question!

Compost. Now that’s a great gardening term. And yet, although the word itself is simple, it can cause a fair deal of confusion. Geoff Hodge unravels the mystery

Use a quality compost for container plants such as Richard Jackson's Multi Purpose Compost Image: Martin Mulchinock
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For a start, the word compost has two meanings. There’s the stuff you make yourself, either in a compost bin or a heap from waste plant materials from your garden and kitchen. This is properly referred to as home-made or garden compost.

You can make home-made garden compost in a dedicated bin
Image: Geoff Hodge

And then there’s the material you put in your pots, containers and hanging baskets; yes, it’s called compost – not “soil” or “muck”! These are either called potting or multi-purpose composts, and include Container and Basket Compost, which generally contains extra water-holding materials to reduce drying out, and Ericaceous Compost, needed by lime-hating/acid-loving plants like rhododendrons. Seed sowing or seed and cuttings compost is a similar material used for seed sowing or taking cuttings. If you’re posh (or in the gardening trade), composts are referred to as growing media.

The science

Yes, believe it or not, scientists really do formulate what goes into our composts. They don’t just sweep up anything they can find on the floor and just “chuck it together”! Research and growing experiments are carried out to produce a perfect combination of materials to ensure they do what they’re supposed to – grow plants to perfection.

To do this, they need to have a good stable structure (so they don’t compact down to virtual concrete in the pot) that roots will easily grow through, and hold air (yes, roots need air to breath) and good quantities of water and nutrients – without easily becoming too wet and waterlogged or dry out too quickly.

The amount of nutrients also varies from compost to compost. Nearly all will have an amount of nutrients that will feed your plants for a minimum of five to six weeks. Others will contain controlled-release nutrients that can give your plants just about all the nutrients they need for up to six months. Most compost bags now give an indication of how long they will feed for. After that time, you’ll need to add supplementary nutrients.

Seed and sowing composts have low nutrient levels – and a finer structure needed by seeds – as high nutrient levels can prevent germination and burn delicate young roots.

Staggering choice

When you pop along to your local garden centre, you’re probably confronted by a huge number of stacks of growing media, so which one(s) do you need?

For the vast majority of gardening tasks, a multi-purpose will be perfect. But, as the old saying goes – a Jack-of-all-trades is often a master of none. So, you need to buy a good quality one, and one that delivers the results you want.

Use a potting tray to keep your compost in one place
Image: Stewart Garden

While a multi-purpose is certainly fine for growing most plants in containers and growing on and potting on plants, as well as sowing most large seeds, they are usually not great for sowing smaller seeds – so you need a seed and sowing compost too.

You’ll also need an ericaceous compost for any lime-hating/acid-loving plants.

Personally, once I’ve started to use a particular brand of compost and have got used to, and like, how it “works”, I’ll always try to stick to it.

Introducing John Innes

John Innes Composts are another area for possible confusion. John Innes isn’t a brand, and neither is it named directly after or by a person. They are a formula of ingredients, traditionally a mixture of loam, peat and sand plus nutrients.

They were named after the John Innes Centre in Norwich, which came up with the formula in the 1930s. It needed a standard growing media that would give consistently good and reliable results in its plant growing experiments.

There are four types – 1, 2, and 3 and Seed. The Seed Compost is a finer mix of ingredients to ensure good seed germination and seedling growth. The other three are used for potting on and potting up plants. The only difference between the three is the level of nutrients they contain: 1 contains one unit of nutrients, 2 two units and 3 three units. Simple!

John Innes Composts are often the best choice for long-term plants that will stay in the same container for many years – such as trees, shrubs and fruit. As they’re heavier than multi-purpose composts, they’re a good choice for tall plants, especially in windy gardens, as they’re less likely to be blown over! This heaviness is often too heavy for shorter plants, and I prefer to use them mixed 50:50 with multi-purpose compost.

Refresh, recycle, re-use

Questions we regularly receive at Richard Jackson’s Garden relate to whether you have to buy fresh compost each year or can you use old bags of compost still sitting in the shed, do you need to use new compost in containers or can you re-use compost that has already been used to grow plants in?

Obviously, it’s a huge waste of resources to simply throw away unused or old, used compost. And we should try and do everything we can to recycle or re-use such materials.

Because composts contain nutrients, if they sit around in your shed for a year or more, the nutrients break down in the compost, creating byproducts that can turn the compost “sour” and damage delicate roots. So, I would never recommend using old compost – apart from spreading it on the soil as a mulch, where these breakdown byproducts are unlikely to cause any problems.

So always buy your compost fresh each spring, or whenever you’re going to need it, and use it up within the year.

Buying old compost can similarly be a problem. So when you’re at the garden centre, check on the compost before you buy it. Bags that weigh a ton because they have been sitting around in the wet, and those with pale or bleached out writing or covered in green algae should be avoided like the plague! Or buy Richard Jackson’s Premium Multi Purpose Compost  – only fresh batches are mailed out to you!

Use a quality compost for container plants such as Richard Jackson’s Multi Purpose Compost
Image: Martin Mulchinock

When I empty out my containers, I generally tend to use the old compost on my borders, either digging it in to improve soil structure or applying it as a mulch. But if compost is infested with vine weevil grubs, this is the only time I would suggest consigning it to the council’s green recycling bin.

Sometimes, providing the plants were only in it for a few months and there were no pest or disease problems, I’ll thoroughly “fluff” it up again – working through it with my fingers – to break up large lumps and get more air into it, and then add it to the bottom one-third of the container, topping up the remaining two-thirds with fresh compost.

 

Geoff Hodge

About Geoff Hodge

Geoff Hodge is a freelance garden writer, writing for various national
gardening magazines and websites – as well as lots more besides! He answers the questions submitted by Richard Jackson's Gardening Club members. Previously, he was the Web Editor for the Royal Horticultural Society, Gardening Editor of Garden News magazine and Technical Editor of Garden Answers magazine. He has written eight gardening books and broadcasts on BBC Radio Cambridgeshire and is a regular guest on Ken Crowther’s gardening programme on BBC Essex.
www.gardenforumhorticulture.co.uk
@Hodgerow
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