What Soil Or Compost Should I Use?

By Andy McIndoe

A Gardeners Guide To Growing Medium & Potting Compost

As many gardeners head out to plant up pots, containers and hanging baskets with seasonal bedding plants and more permanent subjects I thought I would try and help you to make the best buying decisions.

The success of those new plants depends upon choosing the right growing medium for the job.

Many gardeners are bewildered by the array of potting soils on sale and, as this is one of the great gardening commodity lines that gardeners buy in volume, you will find potting soils in some form on sale not only in garden centres, but also home depots, supermarkets, and even on garage forecourts.

The price might be attractive, but is it the right product for potting and repotting, seed sowing or taking cuttings?

The term ‘compost’ in itself is confusing. In the UK it is used for growing media as well as to composts used to improve soil in the open ground, while in the US compost refers only the latter.

Tree and shrub planting compost is used for soil improvement, not for growing in pots. Even the people selling it are confused by this and often give the wrong advice.

1 Planting up

Can I use garden soil?

You must never use garden soil in pots and containers, no matter what you are growing or how good your soil is.

Micro-organisms, spores, invertebrates, and weed seeds that are harmless in the open ground can cause problems in the confined environment of a pot. Always use a specially formulated growing medium.

What is multi-purpose Compost?

3 Multi-purpose compost

So called multi-purpose potting mixes should be useable for everything from seed sowing to mature plant potting. Because today many contain peat substitutes, such as green waste, paper waste and wood fibre they may not be suitable for seeds and very young plants.

Check the packaging for recommendations. It is best to use a specific seed compost, especially for fine seeds. A seed compost is low in nutrients, so it does not impede germination or delicate new root development.

Are cheap growing media a good buy?

4 Seed sowing

Growing media are often price promoted. The cheaper options often contain cheaper ingredients and less base fertilizer: in other words the time they can supply plants with adequate nutrients is shorter.

Most contain enough feed for 4 – 6 weeks before supplementary feeding is required. Cheaper composts will be more like 4 weeks. If you are using a lot of potting mix a less expensive growing medium mixed with a soil based medium is a good option.

peat based or peat free?

5 Coir growing medium

There is a general move towards reducing the use of peat for horticultural purposes. Alternatives are available, and even most peat based growing media will contain a proportion of peat substitutes.

This subject is too great to go into here, but whatever your views there is no doubt that peat based media are easier to manage for the amateur gardener. I know many manufacturers will disagree – but I remain unconvinced.

Coir based potting mixes, for example, tend to dry on the surface while they remain saturated lower down. Media with lots of wood fibre are poor when it comes to seed germination.

Green waste takes a lot of screening: so don’t be surprised to find debris like bits of polythene in your bag on new potting mix. My advice would be to buy a good quality product from a reputable retailer and stick with the one you know and get on with.

Can I recycle potting mix?

6 planting in pots

If you grow lots of bulbs and seasonal bedding, and you replant each season, you end up using lots of growing media every year. Yes, of course you can use it as soil conditioner and mulch on the garden, but can you re-use any of it in pots?

My answer is yes, if you have only used it once before, especially for spring flowering bulbs.

Empty it out into a wheelbarrow, check for any nasties (vine weevil etc.), if it is clean take out the plant debris and mix in a bag of loam based potting mix and a handful or two of slow release fertilizer. Then mix well and re-use.

What’s the difference with a hanging basket Compost?

7 Fuchsia hanging basket

The growing medium sold as ideal for hanging baskets and containers usually contain a water retentive gel. This helps the medium (nearly always soil-less) to hang on to water.

This is an advantage during summer with containers that contain little growing media or ones that drain easily like hanging baskets. You should never use these products for permanently potted subjects because they hang on to too much water in winter.

Of course you can buy the water retaining gel crystals separately, but always mix them with water and allow them to hydrate before you mix them into the compost.

I want to grow a shrub in a pot?

8 Loam based potting mix

For permanent subjects like shrubs in pots, loam-based potting soil is your best bet. This is made from sterilized soil, often with the addition of peat or peat substitute, so it has fine particles that hold on to water and nutrients for the benefit of your shrubs.

It will also drain freely and have the right amount of air space between the particles. In the UK this is sold as John Innes compost, a well-established formula available with different amounts of base fertilizer according to recommended use.

what Is ericaceous Compost?

9 Azalea 'Arabesk'

Lime hating or ericaceous subjects such as rhododendrons, azaleas, pieris and camellias require a lime free growing medium.

Most ericaceous potting mixtures are peat based and contain a wetting agent to enable the water to “stick” to the growing medium. This breaks down over time, resulting in drying out and difficulties in watering. Soil based, lime free growing media are the best to use for these subjects in pots.

Can I use a bag of potting mix I bought last year?

10 Petunia pot

This is a difficult one to answer. If it looks as if it is in good condition; not soggy or with weeds growing out of the bag it should be fine.

If it is dry and dust like because it has been in the shed for a couple of years, here again I would tip it into a barrow and mix in a fresh bag of loam-based potting mix. This will help the texture and help it to reabsorb water.

Andy McIndoe

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