Soil pH & the major nutrients

You can pour fertiliser on a struggling plant and watch nothing happen. The reason is usually invisible: the nutrients are there, but the soil’s pH has locked them away. Understanding pH and the handful of nutrients plants actually need is the difference between feeding the soil blindly and feeding it intelligently.

The pH scale and why it gates everything

pH measures how acidic or alkaline the soil is, on a scale from 0 to 14. Seven is neutral; below that is acidic, above is alkaline. The scale is logarithmic, so each whole number is a tenfold change — pH 5 is ten times more acidic than pH 6.

Most vegetables thrive in slightly acidic to neutral soil, around pH 6.0 to 7.0, because this is where the broadest range of nutrients stays soluble. A few plants break the rule: blueberries and rhododendrons demand acid soil near pH 4.5 to 5.5.

The crucial point is that pH controls availability, not just presence. A nutrient can sit in the soil yet be chemically “locked up” at the wrong pH:

• In strongly acidic soil, phosphorus, calcium, and magnesium become harder to take up, and some metals can turn toxic.
• In strongly alkaline soil, iron and manganese get locked away, producing yellowing between leaf veins.

So before reaching for fertiliser, it is worth correcting pH — doing so can release nutrients already in the ground.

The major nutrients: N, P, K

Three nutrients are needed in the largest amounts, and every fertiliser label lists them as a ratio — for example 10-5-5 — in this fixed order:

• Nitrogen (N) drives leafy, green growth and forms the backbone of chlorophyll and proteins. It is mobile and easily washed out, so a shortage shows as pale, yellowing older leaves and needs regular topping up.
• Phosphorus (P) supports roots, flowering, fruiting, and the plant’s energy transfer. It moves slowly in soil and is most available near neutral pH; deficiency stunts growth and can tinge leaves purplish.
• Potassium (K) regulates water, strengthens cell walls, and improves flowering, fruiting, and hardiness. A shortage often scorches leaf edges, especially on fruiting crops like tomatoes.

A high first number (lots of N) favours leaves; a higher P and K balance favours flowers and fruit.

Secondary nutrients and micronutrients

Below the headline trio sit the secondary nutrients, needed in fair quantity:

• Calcium builds cell walls — its lack causes the sunken brown patches of blossom-end rot on tomatoes and peppers.
• Magnesium sits at the centre of every chlorophyll molecule.
• Sulphur is a component of proteins.

Then come the micronutrients — iron, manganese, zinc, copper, boron, molybdenum, and chlorine. Plants need only traces, but a shortfall still causes real damage. Iron and manganese deficiency is common on alkaline soils and shows as yellowing between the veins of the youngest leaves.

A useful diagnostic habit: deficiencies of mobile nutrients (like nitrogen) appear in old leaves first, while immobile ones (like iron) show in new growth first.

Testing and adjusting

You do not need a laboratory to start. An inexpensive chemical test kit or a digital pH meter gives a quick reading. For reliability, take several samples from across the plot and mix them, since pH varies within a single garden. A paid lab test adds detail on actual nutrient levels.

To change pH:

• To raise it (reduce acidity), add garden lime. Clay soils resist change and need more than sandy ones.
• To lower it (increase acidity), add elemental sulphur or bulky acidic organic matter.

Either way, change comes gradually — adjust over seasons rather than dumping a large dose at once.

The forgiving factor: organic matter

The single best buffer against pH and nutrient problems is plenty of organic matter. Compost and well-rotted manure resist sudden pH swings, hold nutrients in plant-available forms, and feed the soil life that releases them slowly over time. A rich, living soil is far more forgiving of an imperfect pH than a thin, depleted one — which is why, in permaculture, building the soil comes before feeding the plant.