My Soil Is Alkaline — Why Are My Plants Struggling?

Alkaline soil — pH above 7.5 — is common on chalk and limestone-based ground and in areas where building rubble or concrete has raised pH artificially. Despite often looking fine on the surface, high-pH soil can leave plants starved of iron, manganese, and other micronutrients even when those elements are physically present in the ground. Understanding the problem helps you make the right changes rather than adding fertiliser that simply cannot be absorbed.

Why Alkaline Soil Causes Problems

At pH values above 7.5, iron and manganese become increasingly insoluble. Roots cannot absorb them even if the soil contains plenty. The result is interveinal chlorosis — young leaves turn yellow while the veins stay green — a classic sign of iron deficiency. Boron and zinc also become less available at high pH, leading to poor fruit set and slow, distorted growth. Additionally, some beneficial soil bacteria and mycorrhizal fungi are less active in highly alkaline conditions, reducing natural nutrient cycling.

What Causes Soil Alkalinity?

The underlying geology is the most common cause. Chalk, limestone, and some sandstones release calcium carbonate as they weather, continually pushing pH upward. Tap water in chalk areas is hard and alkaline; watering with it over many years slowly raises pH in containers and raised beds. Builders' rubble — concrete, brick, mortar — raises pH dramatically and is a frequent culprit in urban gardens. Over-liming in previous seasons is another common cause and takes years to correct.

Crops That Tolerate Alkaline Conditions

Some vegetables actually prefer near-neutral to slightly alkaline soil. Brassicas — cabbage, cauliflower, broccoli, kale, Brussels sprouts — perform well up to pH 7.5 and benefit from the calcium abundance. Asparagus, leeks, and onions are tolerant of higher pH. Sweetcorn and beans manage reasonably well in alkaline conditions. If lowering pH is impractical on your plot, focusing on these crops while using raised beds with imported compost for acid-loving plants is a sound strategy.

How to Lower Soil pH

Elemental sulphur is the most effective soil acidifier for the long term. Soil bacteria convert it slowly into sulphuric acid over weeks, gradually reducing pH. Apply at 100 to 200 g per square metre for a light correction; stronger corrections may need repeat applications across multiple seasons. Composted pine bark, ericaceous compost, and acidic mulches like wood chip help maintain lower pH once achieved. Avoid adding lime or wood ash. Water with rainwater where possible if tap water is very hard. In containers, switch entirely to ericaceous compost and acidic liquid feeds for acid-loving crops.

Managing Rather Than Fully Correcting

On chalk-based ground, fully correcting pH across an open garden bed is rarely practical — the underlying rock keeps releasing calcium. A more realistic approach is to raise the soil's organic matter content, which buffers pH and improves micronutrient cycling, and to use foliar iron chelate sprays for chlorotic plants as a short-term fix. Raised beds filled with imported acidic loam give you full control for specific crops like potatoes or strawberries without fighting the underlying geology.