Acidification happens when acid-forming inputs and crop removal exceed the soil's ability to buffer acid. The change is gradual. A field can lose several tenths of a pH unit over a decade without any obvious change in appearance, and the yield loss is often attributed to other causes until a soil test shows the trend. By then, the correction takes longer and costs more.
Acid soils are common in high-rainfall regions, where basic cations are leached out of the soil over time, and in intensive cropping systems where acid-forming fertilizers are used regularly and crop removal is high. They are less common in dry regions, where basic minerals remain in the soil. Knowing whether a field is trending toward acidity is therefore an important part of planning its management.
Causes
- Ammonium-based nitrogen fertilizers, which release acidity as ammonium is converted to nitrate in the soil.
- Acid-forming fertilizers applied for many seasons without liming, which gradually lower the buffering capacity of the soil.
- Removal of basic cations such as calcium, magnesium and potassium in harvested crops, which leaves the soil more acid.
- Leaching of nitrate and of basic cations in high-rainfall regions, which carries the buffering capacity away with the drainage water.
- Acid rain and other atmospheric inputs in some regions, where deposition adds acidity to the soil.
Ammonium fertilizers are an important cause, because each kilogram of ammonium nitrogen converted to nitrate releases acidity into the soil. The effect is strongest where the fertilizer is applied in large amounts or on soils with low buffering capacity. Nitrate-based or neutral sources avoid this effect, but they are not always the best choice for every system, so the balance has to be judged field by field.
Effects on the crop
In strongly acid soils, aluminium becomes soluble and toxic to roots, which then grow short and shallow. The aluminium interferes with root cell division and with the uptake of calcium and magnesium. The root system is less able to reach water and nutrients, so the crop shows drought stress and nutrient deficiency even when the soil contains enough of them.
Phosphorus and molybdenum become less available in acid soils, because phosphate binds to iron and aluminium oxides, and molybdenum becomes less soluble. Calcium and magnesium deficiency follows the loss of the basic cations. The crop shows poor root growth and yield loss, even when the nutrients are present in the soil, because the plant cannot reach them or use them effectively.
Measuring and correcting
Measure pH in the root zone where the crop roots actually grow, usually the top 15 to 20 centimetres for annual crops. Measure exchangeable aluminium where it is relevant, because it is the direct cause of toxicity. Calculate the lime requirement from a buffer test rather than a fixed rate, since the amount needed depends on the soil's buffering capacity and its texture.
Apply lime well before planting, and incorporate it where possible, because surface liming takes years to move down the profile. Lime applied on the surface of a no-till field may take several seasons to affect the root zone, so the programme should be planned accordingly. Split large lime requirements over two or more seasons on sensitive soils, and recheck the pH before each application.
| Soil pH | Likely problem | Typical response |
|---|---|---|
| Below 5.0 | Aluminium toxicity, poor root growth, low calcium and magnesium | Urgent liming, check aluminium |
| 5.0 to 5.5 | Reduced phosphorus and molybdenum availability, early aluminium risk | Lime programme over the rotation |
| Above 5.5 | Generally adequate for most crops, monitor the trend | Maintain with regular soil tests |
Managing the acid-forming inputs
Review the fertilizer programme for acid-forming inputs. Where ammonium-based products are used, match the rate to the crop's need, and consider nitrate-based or neutral sources where they suit the system. A grower who uses the same ammonium product every season should record its acidifying effect in the soil test trend, so the lime programme can be adjusted before the pH falls too far.
Organic matter helps to buffer acidity. Soils with more organic matter resist changes in pH better than soils with little organic matter, because organic matter carries charged sites that hold basic cations. Improving organic matter therefore supports the effect of lime and slows the rate of acidification, even though it does not replace liming.
Planning a liming programme
A liming programme works best when it is planned over the rotation, rather than as a single large application. Calculate the lime requirement, split the application if needed, and set a schedule for the next soil test. Record the lime type, the rate, the date, the field and the soil test result, so the effect of each application can be seen.
- Use a lime requirement based on a buffer test, not a fixed rate, and check the calculation with the laboratory.
- Choose the lime type that suits the soil and the programme, including the calcium and magnesium content.
- Recheck the pH about one year after application, because the effect develops slowly and is not complete at once.
- Avoid over-liming, which can reduce the availability of iron, zinc and manganese, and lower the pH to the target rather than above it.
Acidity and the root zone
Acidity is usually strongest in the layer where the fertilizer is placed and where the crop removes basic cations. Below that layer, the soil may still be buffered by minerals that have not yet been affected. This creates a profile in which the top layer is acid while the deeper layer is less so, and roots that go deeper can find a more favourable environment.
Subsoil acidity is harder to correct because lime moves slowly downward. Where a subsoil problem is suspected, a deeper soil sample should be taken and tested separately, and the adviser should decide whether deep incorporation or a longer programme is needed. Reducing the acid load at the surface is often the first and most cost-effective step.
Trends in pH over several years reveal the speed of acidification, which helps the grower decide how often to lime. A field that loses a few tenths of a pH unit every few years needs regular attention, while a stable field may need liming only at longer intervals.
Liming and the rest of the programme
Liming works best when it is combined with the other parts of the soil management programme. Organic matter, balanced fertilization and sensible tillage all help maintain the pH once it has been corrected. A grower who lime-corrects a field and then continues to apply large amounts of ammonium fertilizer without monitoring will need to lime again sooner than expected.
Common questions
Can lime cause problems? Over-liming can raise pH above the target and reduce the availability of micronutrients such as iron, zinc and manganese. Apply lime according to the buffer test, and recheck the pH before deciding on further applications.
How do I know if acidity is limiting yield? Look for poor root growth, aluminium toxicity symptoms and nutrient deficiencies in the crop. Confirm with a pH and aluminium test, and check whether the pattern follows the acid zones in the field.
Does organic matter help? Yes. Organic matter buffers pH changes and holds nutrients, which slows the rate of acidification. It supports the effect of lime rather than replacing it, and it improves the soil in other ways as well.
- FAO, Global Soil Partnership materials on soil acidification and sustainable soil management.
- University extension publications on lime requirement, soil acidity and liming practice.
- Soil Health Institute, public materials on soil acidity and its management.
- Kochian, L.V., published research on aluminium toxicity in acid soils (academic literature).
Recommendations depend on local soil, climate and crop conditions. Follow the product label and local regulations, and consult a qualified agronomist for field decisions.