What Is Soil pH and Why Does It Matter for My Garden?

Soil pH is one of the most powerful levers in gardening, yet most growers never measure it. Plants can sit in nutrient-rich soil and still starve because pH determines whether those nutrients are in a form roots can actually absorb. Understanding pH — and knowing where yours sits — is the fastest way to unlock better growth without spending more on fertiliser.

What pH Actually Measures

pH is a logarithmic scale from 0 to 14 that measures hydrogen ion concentration in your soil solution. A reading of 7.0 is neutral. Below 7.0 is acidic; above 7.0 is alkaline. Because the scale is logarithmic, a soil at pH 5.0 is ten times more acidic than one at pH 6.0, and one hundred times more acidic than pH 7.0. This means small numerical shifts represent large chemical changes — moving from pH 5.5 to 6.5 is a dramatic improvement, not a minor tweak.

The Ideal pH Range for Most Crops

Most vegetables, herbs, and soft fruits grow best between pH 6.0 and 7.0. Within this range, all major and minor nutrients remain soluble and available to roots. Brassicas — cabbage, broccoli, kale — prefer the upper end, around 6.5 to 7.0, partly because that range also suppresses clubroot disease. Potatoes and blueberries are exceptions, thriving in more acidic conditions between 5.0 and 6.0. Knowing your target before you test makes the results immediately actionable.

How pH Locks Up Nutrients

At low pH (below 6.0), phosphorus binds tightly to iron and aluminium in the soil and becomes unavailable. Calcium and magnesium also leach out more rapidly in acidic conditions. At the same time, manganese and aluminium become so soluble they reach toxic levels — often the real cause of the slow, purplish stunting that gardeners blame on nitrogen deficiency. At high pH (above 7.5), iron, manganese, boron, and zinc become insoluble. Plants may develop chlorosis — yellowing between leaf veins — even when the soil contains plenty of these nutrients.

How to Measure Soil pH

Home pH test kits are widely available and cost under £10. Mix a small amount of soil with water, add the indicator solution, and compare the colour to the chart. Digital pH meters give faster and more precise readings but need regular calibration. For the most reliable data, send a composite sample to a lab. Take your sample from 10 to 15 cm depth after removing surface debris, mix cores from across the bed, and test the blend. A single spot sample can be misleading if the bed has uneven amendment history.

Adjusting pH: Quick Overview

If pH is too low, add ground limestone (calcitic or dolomitic) to raise it. Work it in during autumn for best results by spring. If pH is too high, elemental sulphur acidifies the soil over several weeks as soil bacteria convert it to sulphuric acid. Composted pine bark, ericaceous compost, and acidic mulches like wood chip also help over time. Always retest before applying amendments again — over-correction is common and harder to fix than the original problem.