Acidic, Neutral or Alkaline Soil? Everything You Need for Fertile Ground

Before the first sowing, one measurement changes everything: pH. It decides what your vegetables can absorb — or cannot. This guide covers the scale, per-crop needs, measurement methods and corrections in doses per square metre.

The scale, in practice

• Theory: the scale runs from 1 to 14, 7 being neutral — but in practice garden soils sit between 4.5 and 8.4.
• The zone that matters: most soils fall between 6 and 8, which causes no particular problem. Most vegetables are happy on slightly acidic soil, between 6 and 7.
• Microbial life: soil micro-organism activity is only really good between pH 6 and 8 — outside that, the soil falls asleep.

What a wrong pH disrupts

• Too acidic (< 6): toxic elements like aluminium are released while useful nutrients get locked — phosphorus and potassium become unreachable, manganese can turn toxic and bacterial activity stops.
• Too alkaline (> 8): nutrients stay trapped in the soil. The classic symptom is iron chlorosis: abnormal yellowing of the leaf blade except along the veins, caused by unavailable iron when active lime exceeds 8% at pH 7.8 or more.
• The ideal window: all major nutrients are only freely available within a narrow band, roughly 6.5 to 7.5 — and that is also where aluminium stays locked out.

pH by crop

• The majority: tomato 5.5–7.5 · cabbage 6–7.5 · carrot 5.5–7 · lettuce and onion 6–7 · leek 6–8 · garlic 5.5–7.5.
• The acid-loving exceptions: potatoes and sweet potatoes prefer slightly more acidic soil, 5 to 6; strawberry aims for 5.8–6.5, raspberry 5–6.5 (never happy in alkaline soil) and blueberry demands 4.5–5.5.
• Citrus: lemon and orange trees grow best between 5.5 and 7.

Reading your soil without tools

• The vinegar test: drop 4 or 5 drops of vinegar on a dried soil sample. If tiny bubbles appear, the soil contains lime and the pH is very likely above 7. The reverse test works too: a pinch of baking soda fizzing on moistened soil signals acidity.
• Witness plants: heather, ling, fern, sheep sorrel, broom, gorse, strawberry tree, birch and sweet chestnut announce acidic ground; sainfoin, thyme, stinking hellebore, box, juniper and holm oak announce alkaline ground. Nettle signals nitrogen-rich soil.

Measuring properly: the protocol that matters

• The sample: take soil from at least 10 different spots in the garden, under the litter layer and in the root zone (20 to 45 cm deep), with a clean trowel, then mix well. If you dilute for a test, use distilled water — it is pH neutral.
• The traps: avoid atypical areas (compost corner, path edge) and never sample within two weeks of adding fertiliser. Never conclude on a single measurement.
• Colourimetric kit: reliable for a ballpark figure — the grit settles at the bottom of the tube and the water changes colour — but it only gives a rough idea.
• Electronic probe: push it into the soil and read the pH after a few moments, repeating on several spots to check consistency. Fast, as long as the probe stays clean.
• Laboratory analysis: useful when setting up a new vegetable plot — it also tells you whether the soil is sandy, clayey, silty or loamy, plus mineral levels.

Correcting: doses that work

• Raising an acidic soil: 200 g/m² of slaked lime on sandy soil, up to 400 g/m² on very acidic or peaty soil. To gain one pH point, count 200 g/m² in sand, 410 g/m² in loam and 640 g/m² in clay.
• Wood ash: you need about 30% more ash than lime to gain one point — but it brings a third calcium plus potassium and magnesium.
• Timing: autumn is best, at least three weeks before planting. Lime takes up to a year for full effect but keeps working for seven years. Never mix it with fresh manure: they release ammonia gas.
• Beware overdosing: excess lime causes deficiencies — phosphorus, manganese, zinc and boron first. For large amounts, split into several applications and re-test after a month.
• Acidifying alkaline soil: 1 kg of flowers of sulphur per m² then 1 kg of acid peat, plus 2–5 kg/m² of leaf mould or manure. Sulphur takes about two months to reach full effect. Long term, pine needles, peat and leaf mould (especially oak) maintain acidity.
• The buffer solution: garden compost at 9 kg/m², or manure at 3 kg/m², acidifies by about one point — and a moderate dose of mature compost helps rebalance pH in both acid and alkaline topsoil.
• Knowing when to give up: fighting the soil's nature imposed by the parent rock is often pointless — sometimes it is wiser to grow the plants that like the ground you have.

And for a complete reading?

pH is only one face of fertility. The Gardiscan soil analyser measures eight parameters in one probe insertion — pH, moisture, temperature, conductivity (EC), nitrogen, phosphorus, potassium and fertility index — with history and export on the free app: enough to track the effect of liming week after week, where a disposable kit gives only a snapshot. Crops to watch closely: the lemon tree (pH 5.5–7) and tomato seedlings (5.5–7.5) tolerate a wider range than blueberry, which is impossible without truly acidic soil.