Soil fertility or the ability to support plant growth, depends on its acidity, drainage, texture and nutrient availability.
Add organic matter
Adding organic matter improves drainage and water-holding capacity, helping crops resist drought and downpours. Digging in one bucketful per square metre every three years is usually sufficient. Domestic garden compost is ideal, but few people can make sufficient for all their needs. Buying-in straw to make more compost is one option to increase output. Alternatively, buy well-rotted manure from stables or farms.
Manure is hard to get hold of now, as farmers are restricted in how and what animal waste can leave their holdings. Stable manure is treated as trade waste and is similarly restricted in where it can be disposed of. However, manure from domestic horses and ponies can be moved to gardens without restriction. Bagged or bulk deliveries of composted wastes are increasingly available as substitutes for manure.
Adding organic matter replenishes soil nutrient reserves, but some plants, such as sweet peas and roses, may require supplementary fertilisers. These are applied to established plants in late winter/early spring. Fertilisers can also be added before planting or as a 'top dressing' during the growing season.
Many gardeners, particularly ones who garden on stiff clay soils and those who use raised beds, avoid digging by merely laying the compost on the soil as a mulch for it to be incorporated by natural processes, such as worm activity. However, digging can also be a good way of getting rid of weeds and crop debris while mixing in organic matter.
Get digging
Where soil has become compacted, consider digging or at least loosening with a fork so plant roots can grow more freely and drainage is not impeded.
Deep digging can be helpful in increasing the depth of soil available for plant roots to explore for water. Double digging (loosening and enriching the soil below a spade’s depth) can be helpful, particularly where space is short, to get the best out of limited areas.Digging, forking and other cultivations, especially if organic matter such as well-rotted manure or compost is added, promotes good texture with adequate pore formation. Around established plants organic mulches have a similar effect by encouraging soil organisms, earthworms for example, which burrow into the soil carrying organic matter with them. Organic matter binds soil into particles creating pores.
Fertile soils contain numerous air spaces. Large ones allow water to drain and air to reach the roots, while smaller pores hold moisture that can be extracted by plants roots.
Test the pH
Acid soils are ideal for ericaceous plants, but other plants may thrive best where the soil is neutral. The RHS Gardening Advice team recommend an annual test using a simple chemical kit. Lime should be added to maintain pH of 6.5 for most plants, and above 7.0 for most brassicas. Lime takes several years to work in if left on the surface, so is best dug or forked into the soil.
Once the soil structure and pH are ‘tuned up’, consider nutrient supply. It is worth taking a soil test sample at least every four years to check potassium and phosphorus levels. Do this before adding any organic matter. Any serious deficiencies can be remedied by adding suitable fertilisers in winter and mixing these into the soil by digging, or when the soil is tilled and made ready for sowing in spring.
Further information
The RHS provides a Soil Analysis Service for a fee (see Members’ Handbook 2007, p448). Tel: 0845 260 8000, 10am–4pm, Mon–Fri; or visit The Advisory Services webpage.