Can you put too much compost on a garden?

Compost is the closest thing gardening has to a universal answer and it has acquired the reputation of a substance that cannot be overdone. It can. The failures are slow and undramatic — a border that grows lush foliage and few flowers, a bed that sinks every year, aphids everywhere — and they are rarely traced back to the mulch, which is what makes this worth understanding.

Step by step

1. Know what the right amount is.Twenty-five to fifty millimetres a year as a surface mulch on an established bed. On a hungry annual vegetable plot, up to 50mm. On established ornamental borders, shrubs and anything grown for flowers rather than leaves, 25mm is plenty and every other year is often enough once the soil is good.
2. Understand the nutrient build-up.Compost is low in nutrients by concentration and substantial by volume. Applied heavily every year, the phosphorus and potassium accumulate faster than plants remove them, because both are relatively immobile and do not leach the way nitrogen does. Over a decade a heavily composted bed can test very high in both.
3. Understand why excess phosphorus matters.High soil phosphorus suppresses mycorrhizal associations, interferes with the uptake of iron, zinc and manganese, and runs off into watercourses where it causes algal blooms. It cannot easily be removed once it is there — this is the least reversible consequence.
4. Recognise the soft-growth symptom.Excess nitrogen from repeated heavy composting produces lush, dark, sappy growth: lots of leaf, few flowers, thin cell walls that aphids love, and growth that does not harden before winter so it dies back in the first hard frost. It looks like a healthy garden until you notice how little is flowering.
5. Watch for salinity in green-waste compost.Municipal green-waste compost can be relatively high in soluble salts, and repeated heavy application on a free-draining soil with low rainfall builds them up. Symptoms are leaf-tip scorch and poor germination. A single annual mulch is well within safe limits; three loads a year is not.
6. Notice when the bed stops being soil.Add 100mm a year for five years and the top 300mm of the bed is organic matter with almost no mineral fraction. It holds water badly once it dries, sinks continuously, offers roots little anchorage, and is nutritionally unbalanced. Compost improves soil; it is not a substitute for it.
7. Match the material to what the soil actually needs.If the soil needs structure rather than fertility — an established shrub border, a wildflower area, a Mediterranean planting — use leaf mould or composted bark, which add organic matter with almost no nutrients. Save the richer compost for the vegetable plot.
8. Remember that some plantings want poor soil.Wildflower meadows fail on fertile ground because grasses outcompete everything. Lavender, rosemary, cistus and most silver-leaved plants get soft, floppy and short-lived on rich soil. Composting these areas actively makes them worse.
9. Test if you have been composting heavily for years.A laboratory soil test showing phosphorus and potassium indices is the way to find out. It costs perhaps ten times a home pH kit and is worth it once, on a plot with a long history of heavy manuring or composting.

Tips

• Leaf mould is the answer where you want organic matter without fertility. It is almost nutritionally inert, superb for structure, and free.
• Well-rotted manure is richer than garden compost, so the ceiling is lower. An annual 50mm of manure on the same bed year after year is a genuinely heavy feeding programme.
• If a border is growing enormous leafy plants that flop and do not flower, stop mulching it for two or three years. It will recover.
• The advice to 'add compost' is right for the overwhelming majority of British gardens, which have too little organic matter rather than too much. This guide is about the minority who have taken it to an extreme, not a reason for anyone to stop.
• Compost on paths and around permanent planting where nothing is being harvested has nowhere for its nutrients to go. Wood chip is a better material there.

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