How to choose right compost for the job

Azaleas must have a compost without chalk.

Azaleas must have a compost without chalk.

Polyanthus: dig them up when they finish blooming.

BRIAN KIDD: on how to save polyanthus and potted roses

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It’s going to be a bit scientific this week – sorry.

This is the time for seed sowing and potting and this week’s feature is due to letters I have received from George, who lives in Cosham, and Sharon, who is quite new to gardening and lives in Whiteley.

Which compost do I use? I must be careful here as I have my favourites but it’s facts we need.

Garden centres don’t sell rubbish compost but the cost of a bag is often a guide.

For all purposes, seed sowing and potting, any good quality Universal loamless compost is suitable.

These composts contain a percentage of peat, coir or cocoa fibre and some recycled material too. They have the correct pH and fertilisers and the compost has been sterilized so that there are no weed seeds, bad bacteria or fungi present. Price is the guide.

Be careful, some bags of compost are so cheap the price may be too good to miss and the compost is mainly recycled material.

It looks and feels good but the material is not fine enough for seed sowing. It is, however, wonderful to use to improve poor soils or as a top dressing on top of clay or to top up raised beds.

I use a lot of it, particularly where carrots are planted in a raised bed.

John Innes composts have been sterilized, there will be no weeds or fungus problems and they are quite specific.

· I seed compost is ideal for seed sowing and propagation of cuttings.

· I number 1 is designed for seedlings which are to be planted into three-inch diameter pots.

· I number 2 is designed for potting from three-inch diameter pots into four or five-inch diameter pots and J I number 3 is for planting the plants which were in the five-inch pots into the largest pots.

Many of you tell me you don’t like John Innes composts because they seem to become hard and poorly drained. I agree but these composts can be considerably improved if 10 per cent extra potting sand is mixed into the original compost.

If you are thinking about shrubs or fruit trees in pots –wonderful idea. Always use

· I number 3 compost because the essential trace elements are in the loam and they are less likely to be washed out of the compost.

Blueberries, azaleas, rhododendrons and heathers must have a compost without chalk.

Choose John Innes Ericaceous compost. Keep little feet underneath the pots and feed once a month from April until September with Maxicrop tomato feed.

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