Why Do My Brassicas Struggle in Acid Soil?

Of all the vegetable families, brassicas are the most sensitive to soil pH. While most vegetables grow acceptably across a moderately wide pH range, broccoli and cauliflower perform best in the narrow band of 6.5 to 7.0, and suffer noticeably when grown in soil with a pH below 6.0. The problems compound: acid soils lock up nutrients, reduce beneficial microbial activity, and critically, greatly increase the virulence of clubroot — the most serious brassica disease in the garden.

How acid soil affects brassicas

At a pH below 6.0, the availability of calcium, magnesium and phosphorus drops significantly even when these nutrients are present in the soil. Brassicas are heavy feeders and very sensitive to calcium in particular — the distinctive tip-burn of inner leaves in cauliflower is often a calcium availability issue linked to low pH. Magnesium deficiency at low pH produces yellowing between leaf veins on older leaves. Neither symptom improves with feeding until the underlying pH problem is fixed.

pH and clubroot

Clubroot (Plasmodiophora brassicae) is dramatically more virulent at low pH. The pathogen's resting spores germinate and infect roots most readily when soil pH is below 7.2. Raising soil pH to 7.5 does not kill the spores — which can persist for 20 years — but suppresses the disease so effectively that previously devastated beds often produce good crops. In gardens with a history of clubroot, maintaining pH at 7–7.5 is the primary ongoing management strategy.

Testing soil pH

Inexpensive pH test kits from garden centres are accurate enough for practical use. Test in several spots across the bed as pH can vary. Alternatively, send a soil sample to a laboratory for a more precise reading that also covers major nutrients. Test every two to three years as soil naturally becomes more acid over time.

Correcting acid soil with lime

Garden lime (ground calcium carbonate) is the standard correction. Apply it in autumn and work into the soil surface — it takes several months to fully react. The amount needed depends on how acid the soil is and its texture: a sandy soil at pH 6.0 needs about 100g per square metre; a clay soil at the same pH needs two to three times more. Dolomite lime also adds magnesium, which is useful on soils deficient in it. Retest and relime annually until the target is reached and maintained.