Soil and Compost: The Part That Decides Everything Else

Most plant failures blamed on the plant are soil problems. It is also the cheapest thing to get right, and the one gardeners spend least time on.

Know what you have

Take a handful of moist soil and squeeze it.

• Falls apart immediately, feels gritty — sandy. Drains fast, dries out, loses nutrients quickly.
• Holds together, feels sticky and smooth — clay. Holds water and nutrients, drains slowly, hard when dry.
• Holds shape but crumbles when poked — loam. What everyone wants.

The fix for both extremes is the same: organic matter. It opens up clay and helps sand hold water. This is unusual and useful — one action improves both problems.

What the bags mean

• Multi-purpose compost — a growing medium for containers, not a soil improver. It is designed to hold water and nutrients in a pot.
• Soil improver / soil conditioner — bulky organic matter for digging into beds. Low in nutrients, high in structure.
• Topsoil — actual soil, for filling and levelling. Quality varies enormously.
• Ericaceous compost — acidic, for plants that need it. Using it generally is a mistake.
• Seed compost — fine textured and low in nutrients, which is what seedlings want.

Filling a border with multi-purpose compost is a common and expensive error: it breaks down, sinks, and is not what plants want long term.

Peat

Peat-free composts have improved considerably and are increasingly the default, with retail peat sales being phased out in some markets. They behave differently — generally drying out faster at the surface while remaining wet below, so judge watering by weight and by feeling deeper rather than by looking.

Making your own

Compost needs a mix of green material (grass, kitchen scraps, soft prunings) and brown material (cardboard, dry leaves, woody stems), plus air and moisture. Roughly equal volumes by eye is close enough. All-green heaps go slimy and smell; all-brown heaps sit there for years.

Turning it speeds things up considerably. Not turning it still works, just slowly.

Mulch

A layer of organic matter on the surface suppresses weeds, holds moisture, moderates soil temperature and feeds the soil as it breaks down. Apply it to moist soil, keep it clear of plant stems, and top it up annually. It is the single highest-return job in a garden.

Feeding

Plants in the ground with good soil rarely need much feeding. Plants in containers do, because the compost is exhausted within weeks and nothing replenishes it. This is the most common reason container plants stall in midsummer.

Before you order

Check whether you need a growing medium or a soil improver, since the two are not interchangeable. Check the bag volume rather than the price, as sizes vary widely. Check whether bulk delivery is cheaper for larger quantities, which it usually is. And check the delivery point, since bagged compost is heavy and kerbside delivery means carrying it.