Ideal pH 6.0–7.0 (veg)
Earthworm test 10+ per 30 cm cube
Key input Organic matter
Core principle Feed the soil, not the plant
1 billion

The approximate number of individual bacteria in a single gram of healthy garden soil, across up to 10,000 different species. That figure does not include the fungi, protozoa, nematodes, mites, and earthworms that share that space. The diversity and abundance of this community is one of the most reliable indicators of soil health.

Soil health has three dimensions: biological (the living community of organisms), physical (the structure, texture, and drainage), and chemical (pH and nutrient availability). These three interact closely. A soil with the wrong pH locks out nutrients regardless of how much feeding you do. Compacted soil suffocates the organisms that cycle nutrients. Organic matter influences all three at once. Understanding how they connect makes it possible to diagnose problems accurately and improve soil in ways that last.

The living soil

Bacteria and fungi

Bacteria and fungi are the primary decomposers in soil. They break organic matter down into forms that plant roots can absorb, and in doing so they drive the nutrient cycles that make feeding possible without any external inputs. Without them, organic matter accumulates without releasing nutrients; the soil becomes an inert medium rather than a living system.

The most significant fungi from a growing perspective are mycorrhizal fungi, which form symbiotic relationships with the roots of more than 80% of land plants. Their thread-like hyphae extend far beyond the reach of plant roots, dramatically increasing the surface area through which a plant can absorb phosphorus, nitrogen, and water. In return, the plant supplies sugars. This relationship is easily disrupted: digging breaks up the existing network, and synthetic phosphorus fertilisers cause plants to reduce their investment in the fungal partnership because the nutrient is already available.

Earthworms

Earthworms are the most visible indicator of soil health in a garden, and one of the most important organisms in it. A healthy garden soil in the UK supports 250–400 earthworms per square metre. Britain has around 27 earthworm species falling into three ecological groups: surface-dwelling epigeic worms, topsoil-burrowing endogeic worms, and deep-tunnelling anecic worms. All three contribute different things.

Earthworms process 8–10 tonnes of soil per hectare each year. Their castings contain five times more nitrogen and seven times more phosphorus than the surrounding soil. Their burrows create 1,000–2,000 channels per square metre in the top 30 cm, providing pathways for air, water, and plant roots. In heavy clay soils these channels are often the primary drainage route; without them, the soil becomes waterlogged within hours of heavy rain.

A 2018 survey of English agricultural fields published in PLOS One found that 42% of sites may be over-worked based on the absence or rarity of epigeic and anecic earthworms, the types most sensitive to disturbance. This is not a problem confined to arable farming: any garden where soil is regularly dug, left bare, or exposed to synthetic chemicals is likely to see the same decline.

Soil structure and compaction

Soil structure refers to how individual particles of sand, silt, and clay are arranged together. In a healthy soil they clump into aggregates, small crumb-like clusters held together by fungal threads, bacterial secretions, and earthworm activity. These aggregates create a network of pores that hold both air and water simultaneously: large pores allow drainage and air circulation; small pores hold water against gravity for roots to draw on. Good structure is what allows a soil to drain freely after rain while still retaining moisture during dry spells.

Compaction destroys this structure. When soil particles are pressed together, the pore network collapses. Water cannot move through the profile, roots cannot penetrate, and the organisms that build structure lose the oxygen they need. The main causes in gardens are walking on beds, especially when the soil is wet, and working wet clay soil. Compaction is visually identifiable: water pools on the surface after rain rather than soaking in, and the surface resists a finger pushed into it. Dug up, compacted soil breaks into hard angular lumps rather than crumbling into soft aggregates.

Never walk on beds. The weight of a person standing on moist soil is enough to destroy the structure in the top 10–15 cm. Use permanent paths between beds, and if you need to reach the middle of a wide bed, lay a board to spread your weight before stepping on it.

Soil pH

pH is a measure of acidity and alkalinity on a scale from 0 to 14, with 7 being neutral. Most vegetable crops grow best in soil with a pH between 6.0 and 7.0, with 6.5 often cited as the target for kitchen gardens because bacterial activity, earthworm populations, and major nutrient availability all peak around that point.

At pH 5.0, only 40% of the soil’s available nitrogen is accessible to plants, 35% of its phosphorus, and 50% of its potassium, even if those nutrients are present in the soil in adequate quantities. At pH 5.5, nitrogen and potassium rise to around 70% availability but phosphorus remains at 45%. Below pH 6.0, calcium and magnesium availability drops noticeably. Above pH 7.0, iron and manganese begin to lock out; at pH 7.5 and above, boron, copper, and zinc availability reduces significantly. A soil that appears to be struggling despite good management is often simply at the wrong pH.

Suffolk soils, particularly in the clay plateau of High Suffolk, tend towards slightly acidic conditions and may benefit from occasional liming to raise pH closer to 6.5. Sandy soils in the Sandlings and Breckland areas can also drift acidic more quickly than heavier soils as nutrients leach out. Test pH before applying lime; overliming is difficult to reverse and pushes the soil past the optimal point.

To raise pH: apply garden lime (calcium carbonate) or calcified seaweed in autumn. To lower pH: garden sulphur works slowly over one to two seasons. Always test before and after adjusting. A reliable pH test kit costs very little and removes the guesswork.

Organic matter: the common thread

Organic matter is the decomposed remains of living organisms: plant material, manure, compost, leaf litter. It is the single input that simultaneously improves all three dimensions of soil health. Physically, it promotes aggregate formation and increases water-holding capacity. Biologically, it feeds the entire soil food web from bacteria and fungi up through earthworms and beetles. Chemically, it releases nutrients gradually as it decomposes and buffers against pH swings.

The Broadbalk Wheat Experiment at Rothamsted Research has run continuously since 1843, comparing the effects of organic manure and mineral fertilisers on the same plots. Plots receiving farmyard manure have built soil organic carbon to 2.5 times that of the unfertilised control plots. Notably, yields on the manure plots (averaging 4.8 tonnes per hectare over the last 70 years) have kept pace with those on mineral fertiliser plots (4.9 tonnes per hectare), while the soil biology, structure, and long-term fertility of the organic plots remain significantly higher. This is one of the clearest long-term demonstrations that feeding the soil with organic matter is a viable and sustainable alternative to feeding plants directly with mineral inputs.

In practical terms, a 3–5 cm layer of compost or well-rotted manure applied as a surface mulch each year is the most efficient way to maintain and build organic matter. Soil covered with mulch retains up to 25% more moisture than bare soil, moderates temperature extremes, suppresses weeds, and feeds the organisms below the surface without any digging required.

What damages soil health

Understanding what harms soil is as important as knowing how to improve it. The main threats in a garden setting are:

Keeping beds covered: green manures

A bed left empty between crops will lose nitrogen to rain, dry out at the surface, and provide no inputs to the soil food web. Green manures are fast-growing crops sown specifically to cover bare soil, protect it over winter, and then be cut and incorporated to release nutrients for the following crop.

Legume green manures (winter field beans, crimson clover, trefoil) fix atmospheric nitrogen through bacteria in their root nodules, adding 50–200 kg of nitrogen per hectare depending on the species and growing season. Non-legume types (grazing rye, phacelia, mustard) do not fix nitrogen but hold existing soil nitrogen in their roots and foliage, preventing it from leaching. Grazing rye in particular is effective at this and will overwinter reliably in Suffolk conditions; sow it from August to November. Phacelia and mustard fill shorter gaps, reaching usable bulk within four to six weeks.

Cut green manures down before they set seed and either dig the material in shallowly or lay it on the surface to mulch down. If using a no-dig approach, lay the cut material flat and cover with compost; the green matter will break down beneath it.

Assessing your soil: three simple tests

The earthworm count

Dig a cube of soil 30 cm on each side from a bed that has not been recently disturbed, and place the material on a sheet of cardboard. Count every earthworm you find, including small ones. Fewer than 10 worms indicates poor soil biology; 10–25 is adequate; more than 25 suggests a healthy, active soil community. The presence of all three earthworm types, small surface worms, pinkish-grey mid-depth worms, and large deep-burrowing worms, is a strong indicator of a well-structured, undisturbed soil.

The jar test

Fill a large glass jar about one-third with dried soil from your garden. Top up with water to leave a few centimetres of space, add a few drops of washing-up liquid, seal, and shake vigorously for two minutes. Leave undisturbed for 24 hours. The soil will settle in distinct layers: coarse sand settles first (within a minute), finer silt settles over the first few hours, and clay particles form the top layer after 24 hours. Measure the depth of each layer to estimate your soil composition. A roughly equal balance of all three layers is loam. A thick clay layer at the top explains sticky, slow-draining soil; a thick sand layer at the bottom with little else explains rapid drying and poor nutrient retention.

The pH test

pH test kits and strips are sold in garden centres and online for a few pounds. Follow the kit instructions; most involve mixing a small amount of soil with distilled or collected rainwater and comparing the resulting colour against a chart. Tap water is often alkaline and can give a false reading. Test several spots across the growing area as pH can vary significantly even within a small space. Retest in the second year after any lime application to track the change.

Suffolk soils

Suffolk has two distinct soil challenges. The clay plateau of High Suffolk and the plateau claylands running south from Diss towards Ipswich are dominated by the Beccles Association: heavy, seasonally waterlogged clay soils that compact easily when walked on while wet and bake hard in summer, often cracking visibly. These soils benefit most from steady organic matter addition over several years, permanent bed-and-path systems to prevent compaction, and drainage improvements where pooling is severe.

The lighter soils of the Sandlings to the east and the Breckland area to the west are sandy and free-draining. They are easy to work and warm up quickly in spring but dry out rapidly, leach nutrients faster than heavier soils, and have inherently lower organic matter levels. Green manures during winter and generous compost mulching are particularly valuable here. Both soil types respond positively to the same core practice: regular additions of organic matter, kept on the surface rather than dug in.

Practical steps: a summary

Further reading

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