Soil pH controls how much of the fertilizer you apply the crop can actually use. Most nutrients are easiest for plants to take up when the soil pH is between about 6 and 7. On strongly acid soils, phosphorus gets locked up and aluminium becomes toxic. On alkaline soils, phosphorus, iron, zinc and boron become harder to reach. In both cases, part of your fertilizer spend is wasted.
What soil pH means
pH measures how acid or alkaline the soil is, on a scale from 0 to 14. Seven is neutral; below 7 is acid and above 7 is alkaline. The scale is logarithmic: a soil at pH 5 is ten times more acid than one at pH 6. Most agricultural soils range from about 4.5 to 8.5.
How pH affects nutrients
| pH range | What happens | Nutrients affected |
|---|---|---|
| Below 5.5 (strongly acid) | Aluminium and manganese become soluble and toxic to roots; phosphorus binds to iron and aluminium | Phosphorus, calcium, magnesium and molybdenum short; legumes nodulate poorly |
| 5.5 to 6 | Acceptable for many tropical crops; some loss of P availability | Phosphorus, molybdenum |
| 6 to 7 | The best range for most field crops | Most nutrients readily available |
| 7 to 7.5 | Mostly fine; micronutrients start to fall | Iron, manganese, zinc |
| Above 7.5 (alkaline and calcareous) | Phosphorus binds to calcium; micronutrients precipitate | Phosphorus, iron, zinc, manganese, boron; urea volatilisation higher |
These effects explain many of the problems described in our nutrient deficiency symptoms guide: iron and zinc shortages on alkaline soils, and phosphorus and magnesium shortages on acid soils, often happen even when the soil holds plenty of the nutrient.
Preferred pH for common crops
| Crop | Preferred pH (approximate) |
|---|---|
| Wheat, maize, soybean, alfalfa | 6 to 7.5 |
| Rice (flooded) | Flooding moves pH towards neutral |
| Potatoes | 5 to 6.5 (lower pH reduces common scab) |
| Cassava, oil palm, coffee | Tolerate acid soils, often 4.5 to 6.5 |
| Tea | 4.5 to 5.5 |
How to raise soil pH (liming)
Acid soils are corrected with liming materials that neutralise acidity:
- Ground limestone (calcium carbonate): the most common and usually the cheapest per tonne.
- Dolomitic lime: adds magnesium as well as calcium; useful where magnesium is short.
- Calcified seaweed (lithothamnium): porous and relatively fast-acting, with calcium, magnesium and trace elements. See lithothamnium calcified seaweed.
- Quicklime and hydrated lime: fast but caustic and harder to handle.
The amount needed depends on the starting pH, the target pH, soil texture and organic matter. Clay and high-organic soils need more lime than sandy soils to shift by the same amount. A soil lab can give a lime requirement. Lime works slowly, so apply it well before sowing, ideally months ahead, and work it into the soil.
How to lower soil pH
Lowering pH is slower and more costly than raising it. Options include elemental sulphur, which soil bacteria turn into sulphuric acid, and acid-forming fertilizers such as ammonium sulphate. On calcareous soils full of free lime, it is rarely practical to lower pH across a whole field; growers instead use banded phosphorus, chelated micronutrients and acidifying fertilizers near the roots.
How fertilizers change soil pH
Fertilizers themselves move pH over time:
- Ammonium-based nitrogen fertilizers (ammonium sulphate, urea, MAP, DAP over the long term) acidify the soil as ammonium is converted to nitrate. Ammonium sulphate is the most acidifying.
- Nitrate fertilizers such as calcium nitrate have little effect or a slight alkalising effect.
- CAN contains limestone, which offsets most of its acidity.
- Potash (MOP and SOP) has little effect on pH.
- Organic fertilizers and manures often buffer pH and add organic matter.
That is why long-term heavy use of nitrogen fertilizer gradually acidifies soils, and why regular liming is part of good management on many farms. See nitrogen fertilizer for the main nitrogen sources.
Testing soil pH
Test every two to four years, and before changing a rotation or planting a perennial crop. Take many cores across a field, mix them, and send the sample to a laboratory. Field pH kits are useful for quick checks but less precise.
Buying fertilizer and amendments in bulk
Agromer has traded agricultural commodities internationally since 1985 from Saint-Avertin, France. We supply urea, NPK, DAP, potash, organic fertilizers and lithothamnium to importers, distributors and large growers. Send your product, quantity, packaging and destination and we confirm availability and terms.
Frequently asked questions
Does soil pH affect fertilizer?
Yes. pH controls how available nutrients are. On very acid or alkaline soils, a large part of the fertilizer can become unavailable to the crop.
What is the best soil pH for most crops?
About 6 to 7 for most field crops, though some crops prefer more acid soils.
How do I raise soil pH?
Apply a liming material such as ground limestone, dolomitic lime or calcified seaweed, based on a soil test, well before sowing.
How do I lower soil pH?
With elemental sulphur or acidifying fertilizers such as ammonium sulphate. It is slow, and impractical on soils with a lot of free lime.
Which fertilizers make soil acidic?
Ammonium-based nitrogen fertilizers, especially ammonium sulphate, and urea over time.
How long does lime take to work?
Several months to a year or more, depending on the material, particle size and how well it is mixed into the soil.