Following a dry summer, gardeners will be even keener than usual to lavish composts, manures and other bulky organic matter on their garden to feed soil and plants and to boost moisture-holding capacity. Soil fungi and bacteria turn these materials into plant foods and improve soil structure.
Soil structure is how the clay, sand and soil minerals are bound into little lumps called peds, through which plant roots can grow – seeking out anchorage, nutrients and water.
As a very rough rule, adding at least a 5cm layer of compost or similar material every year to your soil as mulch – or dug in once every three years – produces a plant-friendly soil structure.
Even these minimum rates work out to 50 litres per square metre – a bag is generally 50-80 litres at £6-£8 per bag, which becomes expensive.
Garden compost, however, is free – consign flowers and crops to the compost bin which, over a few months, converts wastes into high-quality soil improver.
However, the volume of composted waste shrinks – with the resulting compost only a fraction of what went in. Adding lawn mowings, cardboard and scrunched-up paper helps add bulk.
Turning a compost heap (Photo: Tim Sandall/RHS)
Most gardens generate masses of prunings. Shredded prunings make good mulch and can also be added in moderation to the compost bin.
Many organic growers swear by wood in compost, claiming that it enhances the beneficial fungal content. Shredders can be hired.
Alternatively, the “cut and drop” pruning method can be used, whereby the prunings are snipped into reasonably small pieces during pruning and dropped on the ground to decompose where they fall, mimicking natural thickets, where the fallen timber improves the soil and supports wildlife.
Local tree surgeons can often deliver chipped prunings – by the tipper load, if necessary. These can be good value for shrubs, trees, blueberries, red and white currants and – unlike composted mulches – will last for three years or more applied as surface mulch.
Mulching a rose after pruning (Photo:Tim Sandall/RHS)
Dug wood chips can lead to yellow plants as they temporarily soak up the soil nitrogen. Bark granules and chips are widely available – more attractive but also more costly alternatives.
Leaf mould is usually made by raking fallen leaves (above) to rot in an enclosure for 12 months. Or they can be raked under trees/shrubs to mulch naturally – they disappear by July. Where fallen leaves are plentiful, they can go a long way to meeting garden organic matter needs. Avoid leaves from busy streets, as they can contain pollutants.
Rural gardeners can obtain manure directly from farms and stables. Where manure cannot be bought ready rotted, stack it over winter. Decayed manures have fewer animal pathogens and parasites. Manures contain more plant nutrients than other mulches, although good-quality compost is nearly as rich.
Utility firms such as Veolia supply composted municipal wastes, which can be good value. Check for plastic content, however – plastic waste can break down into tiny micro and nano plastics, which may cause harms that are not yet clear.
Potting compost is a poor soil improver. It can contain materials that are in short supply, such as wood fibre, or environmentally harmful, notably peat. Ideally, keep these composts for potting. However, used potting compost can be recycled as soil improver.
Even the sun will feed your soil. Harvest sunlight by sowing green manures or cover crops such as rye, field beans, ryegrass, wheat or vetches on bare soil from late summer until winter. These add some organic matter by spring, when they are terminated by digging, hoeing or smothering with mulch and cardboard.
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