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Down to earth: gardening on urban soils

by Hazel Sillver

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credit: Will Merydith

The key to a lush, beautiful urban garden is healthy soil. Hazel Sillver unearths the secrets of the brown stuff.

At the bottom of your garden something is stirring… It’s your soil. Often mistaken for a boring bit of mud, a healthy soil is in fact a vibrant ecosystem, with billions of organisms in each handful. The better the soil, the more insects, worms and friendly bacteria and fungi there’ll be, helping to add the air and nutrients to the earth that your plants need.

Why soil is important

When the earth is healthy, plants grow vigorously, resist disease and look or taste good. Soil is therefore the focus of organic farming and gardening – if it’s well looked after, it will (just about) take care of everything else.

Working with your soil type

It’s important to know the geological make-up of your soil so you can work out how to improve it and choose plants that will thrive in it. Cities tend to have one or two predominant soil types. For instance:

NORTH LONDON / MILTON KEYNES CLAY – clay is a heavy soil that will squash like putty between your fingers. It’s fertile but slow to drain and can become waterlogged. Improve it with autumn digging (when the soil is relatively dry) and the addition of plenty of organic matter such as compost, well-rotted manure or leafmould. Roses love clay.

POOLE SAND – sandy soil will feel coarse and grainy. It’s free-draining and well-aerated but it has low nutrient levels and a tendency to dry out. Improve it by adding a thick mulch of organic matter and perhaps sowing annually with a green manure. Foxtail lilies and cistus are amongst those plants that love sand.

BIRMINGHAM LOAM – loam is an enviable mixture of sand, clay and silt. This is the best soil type and you don’t need to do much to it, other than keep it topped up with organic matter.

MANCHESTER ACIDIC – buy a pH test kit (£5-10 from garden centres) to determine the pH of your soil. If it’s acidic, some plants will refuse to grow, but others such as camellias and magnolias will thrive.

BRIGHTON CHALK / ALKALINE – at the other end of the pH scale, chalky soils are very alkaline. They are also pale and may contain lumps of visible white rock. This type of soil drains well, but it can be low in nutrients. Improve it by digging in organic matter and adding a mulch to conserve moisture. Plants such as echinacea, rudbeckia and lavender love chalk.

A neutral pH is best if you want to grow a diverse range of plants. It is possible to alter pH – for example, you can add lime to reduce the acidity of soil.

Identifying your soil type

If you’re not sure what kind of soil you have in your garden, you can use www.landis.org.uk/soilscapes to check the soil type for your specific area.

Problems facing urban soils

DROUGHT

Cities have their own, warmer, microclimate and walls stop rain reaching the soil so in a small urban garden that’s surrounded by buildings, dry soil can be a big problem. “The solution is to improve the soil’s water retention,” says Christopher Bailes, curator of the Chelsea Physic Garden in London. “This is best done by recycling household and garden waste to make compost.” Organic matter such as compost and leafmould can be added as mulch (soil cover) in autumn or spring and will be worked in by the rain and the resident worms. “Another essential is to preserve as much of the water that falls on your garden as possible,” says Christopher.

POLLUTION

“Some sites have a history of industrial pollution,” says Dr. Francis Rayns, Horticultural Research Manager at Garden Organic. “In such cases it may be best to use raised beds and bring in fresh topsoil.” Some gardeners are also wary of using city plants to make compost and leafmould, since some can absorb traffic fumes, but “the evidence that this causes damage is not clear cut,” says Francis.

BUILDING DEBRIS

In a city garden, it’s common to unearth a lot of rubble dumped by builders over the years. Your options are to spend time digging it out; to bring in top soil and create raised beds on top of it, or to plant alpines, which will enjoy the rocky conditions.

Become a soil geek

Look after the glorious underworld of your soil and your garden will grow into a lush Eden. Go on, come over to the dark side.

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