Soil pH controls which nutrients a plant's roots can actually take up, regardless of how fertile or well-fed the soil looks on paper. A bed that's been composted and fertilised properly can still produce stunted, yellowing growth if the pH is badly wrong, because at the wrong pH, nutrients that are genuinely present in the soil get chemically locked into forms roots can't absorb. It's one of the quieter causes of a struggling garden, since nothing about the soil looks obviously wrong from the surface.
Soil pH controls which nutrients a plant's roots can actually take up, regardless of how fertile or well-fed the soil looks on paper.
Two articles already on this site show pH doing exactly this kind of work. Blueberries need genuinely acidic soil, roughly pH 4.5 to 5.5, and fail to thrive in the near-neutral soil most New Zealand vegetable beds sit at, not because of watering or feeding but because of pH alone. Hydrangeas show the same mechanism from a different angle: the same plant's flowers turn blue in acidic soil and pink in alkaline soil, because pH controls whether the plant can take up the aluminium that drives that colour change. Neither of those results is about the plant's care routine. Both come down entirely to what the soil's pH is doing underneath it.
Testing is simple and doesn't need to be expensive. A basic soil pH test kit from a garden centre, usually a small vial, a soil sample and a colour-comparison chart, gives a readable result in a few minutes. An electronic pH meter with a probe is a step up, giving a direct numeric reading without needing to interpret a colour chart. Whichever method you use, test more than one spot and more than one depth: take a sample from the surface and another from around 7cm down, and compare a few different spots around a larger garden rather than assuming one reading applies everywhere, since pH can genuinely vary from bed to bed depending on past use, mulch and what's grown there before.
Most vegetables and lawns are happiest somewhere in the 6.0 to 7.5 range, broadly neutral to very slightly acidic, which is also roughly where most unmodified New Zealand garden soil naturally sits. A reading outside that band is the signal to act, and the direction of the problem tells you which way to push it: a low reading (acidic) needs raising, a high reading (alkaline) needs lowering.
Raising pH, for soil that's too acidic, is done with garden lime or dolomite lime, worked into the top 15-20cm of soil well ahead of planting. Dolomite lime has the advantage of also supplying magnesium along the way, useful on soils that tend to run short of it. Wood ash from an untreated-timber fire will also raise pH in smaller amounts, though it acts faster and less predictably than lime, so it's easier to overdo.
Lowering pH, for soil that's too alkaline, is most reliably done with elemental sulphur, the same amendment this site's blueberry guide recommends for exactly this reason. Soil bacteria convert it to sulphuric acid gradually, which is why it takes months rather than days to show its full effect. Digging in peat, well-rotted pine needles or plenty of compost helps the same way over a longer timeframe, through genuinely organic, slower-acting acidification rather than a single chemical hit.
Whichever direction you're correcting, the process is inherently slow and that's worth planning around rather than fighting. A single large application rarely fixes a badly off-target pH in one go, and overcorrecting is a real risk in both directions, so the safer approach is a moderate application, several weeks of settling time, then a retest before adding more. Retesting once a year, or whenever a bed starts underperforming for no obvious reason, catches drift back toward neutral long before it becomes a mystery problem with no visible cause.