Testing Soil pH and Nutrients

Divide the area into zones that genuinely differ — bed, lawn, under a tree, near a wall. In each zone take at least ten small cores from root depth, roughly 10-20 cm, along a zig-zag path.

Combine each zone's cores into one bag and label it.

Avoid the first few centimetres and avoid within a metre of paths, walls and compost heaps — mortar leaches lime and will hand you an alkaline result that is about the wall.

Spread each composite thinly and air-dry — do not use an oven. Crush lumps, remove stones and roots, and pass through a sieve or a coarse cloth.

Mix thoroughly and take your sub-sample from the mixed whole.

Oven drying changes soil chemistry, particularly for nitrogen and potassium, so a result from oven-dried soil is not comparable with a standard method.

Weigh soil and distilled water at a fixed ratio — 1 part soil to 2.5 parts water by mass is a common standard. Stir, leave 30 minutes with occasional stirring, and read the pH of the settled liquid.

Record the ratio alongside the number.

A pH quoted without its ratio is not a measurement, because dilution shifts the reading — deliberately try 1:1 and 1:5 and watch the number move.

Put a spoonful of dry soil in a dish and add vinegar.

Expect fizzing if free carbonate is present.

Score each zone none / slight / strong.

This is the most useful ten-second test in the whole blueprint. A fizzing soil is buffered alkaline by carbonate, which means acid-loving plants will never thrive there and that adding sulphur to lower the pH is a losing battle against a large chemical reservoir.

Shake a soil sample with water in a tall straight jar and leave it to settle for a day. Sand falls in seconds, silt in minutes, clay over hours.

Measure the depth of each band and compute percentages.

Texture governs how a soil behaves far more than nutrients do — it decides drainage, how much water is available to roots, and how strongly the soil holds onto nutrients in the first place.

Draw the plot and write each zone's pH, fizz result and texture on it. Compare with what actually grows well where.

Expect the map to explain problems you had already noticed.

Match the plant to the soil before trying to match the soil to the plant. Changing a soil's pH meaningfully is slow, expensive and, in carbonate soil, largely futile — and the map tells you which battles are worth fighting.

The availability curve is the whole point. Most plant nutrients are most available in a band roughly between pH 6 and 7.5. Below that, phosphorus binds with iron and aluminium, and aluminium itself becomes soluble enough to be toxic to roots. Above it, iron, manganese and zinc precipitate out of reach — which is what causes the yellow-between-green-veins chlorosis seen on acid-loving shrubs planted in chalky ground. The plant is not short of iron; it is standing in it and cannot get at it.

Buffering is why changing pH is hard. A soil's resistance to pH change depends on its clay and organic matter content and, decisively, on free carbonate. A sandy soil with little organic matter shifts easily; a chalky clay is buffered by a reservoir of calcium carbonate that will neutralise anything you add for years. This is why the vinegar fizz test matters more than the pH number — it tells you not where the soil is but how hard it will fight to stay there.

What a colour test actually measures. Home kits for nitrogen, phosphorus and potassium extract a fraction of what is in the soil with a chemical reagent and compare a colour. Different laboratories use genuinely different extractants — Olsen, Mehlich, Bray — and they give different numbers for the same soil, all correct within their own method. A home NPK result is a rough indication and cannot be compared against a figure from a lab using another extractant. Nitrogen in particular is nearly meaningless to test at home: it moves and transforms so quickly that a reading is out of date within days.

When to use a real laboratory. For anything that matters — a new allotment, a food-growing site on former industrial or urban land, persistent unexplained failure — send a sample. A lab will give you pH, buffer pH, organic matter, cation exchange capacity and a proper nutrient panel for a modest fee. And on any site with an industrial, roadside or demolition history, ask specifically for heavy metals: lead in urban garden soil is a genuine and well-documented risk, especially where children play or root vegetables are grown, and no colour-chart kit will detect it.

Safe practice. Wash hands after handling soil, do not test soil in food containers, and keep samples away from food preparation areas. Do not taste soil to judge texture, an old field method that has no place in a garden of unknown history.