What does soil pH actually do to my plants?

The single most common feeding mistake in a garden is treating a pH problem as a nutrient problem. A rhododendron with bright yellow leaves on chalky soil is not short of iron — the soil around it contains plenty. It is unable to dissolve that iron into a form its roots can take up, and adding more iron to the soil changes nothing at all.

Step by step

1. Understand what the number is.pH measures how acidic or alkaline the soil water is, on a logarithmic scale from 0 to 14 with 7 as neutral. Logarithmic matters: pH 5 is ten times more acidic than pH 6, and a hundred times more than pH 7. Small numbers describe large changes.
2. Understand the mechanism: solubility, not quantity.Nutrients are only useful to a plant when dissolved in soil water. pH governs which compounds stay dissolved and which precipitate out as solids the roots cannot touch. The nutrient is still there; it is simply out of reach.
3. Learn what goes wrong at the alkaline end.Above roughly pH 7.0, iron, manganese, zinc, copper and boron become progressively insoluble. Iron chlorosis — yellow leaves with sharply green veins, on the newest growth first — is the signature, and it is what kills acid-lovers on chalk.
4. Learn what goes wrong at the acid end.Below roughly pH 5.5, phosphorus locks up with iron and aluminium, calcium and magnesium leach away, and free aluminium starts to damage root tips directly. Growth stunts, roots stay short and stubby, and nothing responds to feeding.
5. Note that phosphorus is squeezed from both ends.It is locked by aluminium and iron in acid soil and by calcium in alkaline soil. It is only genuinely free in a narrow window around 6.5, which is a large part of why that figure keeps appearing.
6. Note the effect on soil life.Most bacteria, including the nitrogen-fixing bacteria in pea and bean roots and the ones that convert organic nitrogen into a usable form, work best near neutral. Below pH 5.5 the whole nitrogen cycle slows down. Fungi tolerate acidity better, which is part of why woodland soils are fungal and acidic.
7. Match your plants to the range you have.Blueberries, rhododendrons and heathers want 4.5 to 5.5. Most vegetables and border plants want 6.0 to 7.0. Brassicas prefer 6.5 to 7.5, partly because higher pH suppresses club root. Lawn grasses are happy at 5.5 to 6.5.
8. Stop reaching for fertiliser when the pattern says pH.Yellowing between green veins on new growth, on several unrelated plants, in a garden with chalky stones, is a pH diagnosis. Feeding harder makes no difference and eventually causes salt damage.

Tips

• Chelated iron sprays relieve iron chlorosis for a few weeks by supplying iron in a form that stays soluble at high pH. They are a symptom treatment and they need repeating; they are not a soil correction.
• Ericaceous fertiliser works partly by being acidifying, not just by containing iron. That is why using it in a pot of ordinary compost gives poorer results than using it in ericaceous compost.
• Rainwater is slightly acidic; hard tap water is alkaline and carries dissolved calcium. Watering an ericaceous pot with hard tap water raises its pH steadily over a season.
• Peat-free composts are not automatically acidic. Check the pH stated on the bag if you are growing anything fussy.

Questions people ask