Soil pH — acidic, neutral or alkaline soil in the garden

Soil pH decides not how much food a plant has, but whether it can take it up at all. You can fertilise conscientiously and still watch leaves yellow and growth stall — because at the wrong pH the nutrients physically present in the soil become unavailable to the roots. That is why measuring the reaction is the first thing worth doing at a new site, before you buy any fertiliser or plant.

pH measures acidity on a scale from 0 to 14, where 7 is neutral. The scale is logarithmic: soil at pH 5 is ten times more acidic than soil at pH 6, and a hundred times more than soil at pH 7. This is why the seemingly small difference of a single unit means, in practice, entirely different conditions for the roots.

The pH scale — acidic, neutral, alkaline

Reaction	pH range	Typical plants
Strongly acidic	below 5.5	rhododendrons, blueberries, heathers, azaleas
Acidic	5.5–6.5	most garden plants, vegetables, lawn
Neutral	6.5–7.2	brassicas, many perennials and shrubs
Alkaline (limy)	above 7.2	lavender, butterfly bush, lime-loving plants

These boundaries are conventional but practical: most garden species are happiest in the narrow window of pH 6.0–7.0. Deviations in either direction translate into specific feeding problems.

Why pH decides plant health

The key is nutrient availability, which changes with pH. On overly alkaline soil, iron, manganese and phosphorus get locked up — hence the characteristic yellowing of young leaves between the veins, that is, iron chlorosis. Feeding with iron helps little here until the pH is lowered, because the problem is not a shortage of iron in the soil but its immobilisation by high pH.

At the other end, below about pH 5.5, aluminium and manganese become active at concentrations toxic to roots, while calcium and magnesium run short. The goal, then, is not “the lower the better” but hitting the range right for the particular plant.

How to test soil pH

There are three practical methods, differing in accuracy:

• Test strips and indicator liquids — cheapest, giving an approximate result to about 0.5 pH. That is quite enough to tell acidic soil from alkaline.
• Electronic pH meter — reliable, provided it has a genuine electrode and is calibrated. Popular push-in “probe” meters without calibration are often untrustworthy — treat their readings with caution.
• Laboratory analysis — the most accurate, measuring pH in a KCl solution and stating the recommended lime dose or acidification need straight away. It is worth doing when setting up a larger garden.

Whatever the method, take the sample from several spots and from the depth of the root zone (about 10–20 cm), then mix the samples — pH can vary within a single plot, especially near foundations and concrete paths, where lime from the mortar raises the value.

Acid-loving versus lime-tolerant plants

The split into these two groups is fundamental in the garden, because it decides where a given plant is worth planting at all.

Acid-loving (calcifuge) plants need acidic soil and suffer on limy ground. They include the rhododendron, Japanese azalea, highbush blueberry and common heather. You will find the full list of such species in the plants for acidic soil round-up.

Lime-tolerant plants grow well on neutral and alkaline soil — these include English lavender and the butterfly bush. More suggestions are gathered in the plants for limy soil round-up.

A striking special case is the bigleaf hydrangea: with it, pH controls the flower colour. On acidic soil (pH below about 5.5), where aluminium is available, the flowers are blue; on neutral and alkaline soil, pink.

How to acidify soil

Lowering pH is slower and harder than raising it, because you have to overcome the soil’s natural buffering capacity — heavy clay and humus-rich soils resist change, while light sandy soils respond faster. The options:

• Elemental sulfur (garden sulfur) — the most effective for a lasting reduction. It works indirectly: bacteria oxidise it to sulfuric acid over weeks to months, mainly in warm, moist soil. That is why you apply it ahead of time, ideally in autumn before spring planting.
• Acidic sphagnum peat and ericaceous substrates — the fastest way locally: prepare acidic substrate in the planting hole for a rhododendron or blueberry rather than shifting the whole bed’s pH.
• Iron sulfate — acidifies faster than sulfur, but the effect is short-lived and needs repeating; useful as immediate relief for chlorosis.
• Pine bark and needles, acidic compost — as a mulch they hold a lower pH gently and over the long term.

Bear in mind that on soils high in lime (calcareous ground), lowering pH can be a Sisyphean task — then the more honest solution is to grow acid-lovers in containers or raised beds filled with acidic substrate.

How to raise soil pH — liming

Raising a too-low pH is simpler and more predictable. That is what liming is for:

• Carbonate lime (chalk, ground limestone) — mild, safe, the standard choice on light and medium soils.
• Dolomite lime — additionally supplies magnesium, so it makes sense where that is deficient.

Lime preferably in autumn or early spring, and do not combine lime with nitrogen fertilisers or fresh manure at the same time — the reaction releases ammonia and wastes nitrogen; wait a few weeks. Do not overdo the dose: too high a pH itself triggers iron chlorosis, so liming “as a precaution” can do more harm than mild acidity. It is best to set the dose from a laboratory result.

pH and iron chlorosis

The most common practical consequence of the wrong pH is iron chlorosis — yellowing of the leaf blade between veins that stay green, first visible on the youngest leaves. On alkaline soils its cause usually lies not in a lack of iron but in iron being locked up by the high pH (so-called lime-induced chlorosis). This is why acid-loving plants set in limy ground pale despite feeding — the only lasting solution is to lower the soil pH, not another dose of fertiliser.

Before you plant — measure first, then choose

The simplest rule is: measure the pH first, then choose plants to suit it, not the other way round. Fighting a plot’s natural pH is costly and never fully successful, whereas matching species to what you have gives healthy plants without chemistry. Once you know your reading, use the ready-made round-ups for acidic and for limy soil to pick plants your soil truly suits.