Soil pH measures how acidic or alkaline the soil is, on a scale where 7 is neutral. Most crops grow best between about 5.8 and 7.0. Outside that band, the crop struggles to take up nutrients that are physically present in the soil.
The phosphorus lock-up
In acid soils, phosphorus binds with iron and aluminium into forms roots cannot use. A farmer adding more DAP to acid ground is buying phosphorus the crop cannot reach. Aluminium also becomes soluble and toxic to roots in strongly acid soils, stunting root growth so the crop reaches less water and fewer nutrients.
Why Kenyan soils go acid
- High rainfall leaches calcium and magnesium out of the topsoil
- Ammonium-based fertilisers acidify the soil every season
- Removing crop residues takes away the bases the crop took up
When liming pays
Lime raises pH and supplies calcium. It is bulky and not cheap, and it pays only where pH is actually low. A pH map shows which blocks need it and how much — and, just as usefully, which do not.
| pH | Typical meaning for most crops |
|---|---|
| Below 5.0 | Strongly acid — aluminium toxicity likely; lime first |
| 5.0–5.5 | Acid — phosphorus availability reduced; lime usually pays |
| 5.5–7.0 | Suitable for most crops |
| Above 7.5 | Alkaline — iron, zinc and manganese can become short |
Lime reacts over weeks to months. Apply it well before planting and work it in.