Soil pH, soil acidity and liming
What soil pH measures, why soils turn acid, how pH controls nutrient availability and aluminium toxicity, and how to calculate a lime dose.
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Soil pH is a measure of how acid or alkaline the soil solution is. It is often called the master variable of soil chemistry, because it controls which nutrients a root can reach, which elements become toxic, and which microbes are active.
Acid soils cover roughly a third to a half of the world's potentially arable land, especially in the humid tropics, and liming them is one of the oldest and most profitable soil improvements there is. Exams ask about pH at every level, from its definition to lime calculations.
What pH means
pH is the negative logarithm of the hydrogen-ion activity in solution:
Because the scale is logarithmic, each whole unit is a tenfold change. A soil at pH 5 has ten times the hydrogen-ion activity of one at pH 6 and a hundred times that of one at pH 7.
| pH range | Description | Where it is typical |
|---|---|---|
| below 4.5 | extremely acid | acid sulphate soils, peats, some highly weathered tropical soils |
| 4.5 – 5.5 | strongly acid | humid tropical uplands, leached sands, long-fertilised acid soils |
| 5.5 – 6.5 | moderately to slightly acid | most productive humid-region farmland |
| 6.5 – 7.5 | near neutral | many alluvial and grassland soils |
| 7.5 – 8.5 | alkaline | calcareous soils of dry regions |
| above 8.5 | strongly alkaline | sodic soils (high exchangeable sodium) |
How it is measured
pH is measured with a glass electrode in a soil suspension, most often one part soil to 2.5 parts water (1:2.5), sometimes 1:1 or 1:5. Many laboratories also measure it in 0.01 M calcium chloride, which gives a steadier reading less affected by the season's fertiliser salts; it usually reads about half a unit lower than in water. Always note which method was used before comparing numbers.
Why soils become acid
Acidity is a natural result of rain and plant growth, accelerated by farming.
- Leaching. Rainwater carries carbonic acid. Where rainfall exceeds evaporation, basic cations (calcium, magnesium, potassium, sodium) are washed out and replaced on the exchange sites by hydrogen and aluminium.
- Nitrification of ammonium. Converting one ammonium ion to nitrate releases two hydrogen ions. Ammonium-based and urea fertilisers therefore acidify soil steadily unless the nitrate is fully taken up.
- Crop removal. Harvested crops, especially forages and legumes, remove basic cations.
- Organic acids from decomposing residues and root exudates.
- Oxidation of sulphides. Drained coastal marshes and mine spoils containing pyrite form sulphuric acid and can fall below pH 3.5: these are acid sulphate soils.
Soils of dry regions stay alkaline because there is too little water to leach out the bases, and calcium carbonate accumulates.
Active and reserve acidity
The hydrogen ions measured by a pH meter are only a small part of a soil's acidity.
- Active acidity is the H+ in the soil solution. It is tiny.
- Exchangeable (salt-replaceable) acidity is the aluminium and hydrogen held on clay and humus surfaces, replaced into solution as soon as active acidity is neutralised.
- Residual acidity is bound more tightly in organic matter and aluminium hydroxy compounds.
The reserve is often a thousand times larger than the active pool. This is buffering: a soil resists a change in pH. It is why a clay or organic soil needs much more lime than a sand to raise its pH by the same amount, and why a pH test alone cannot give a lime dose; laboratories use a buffer pH test or the soil's cation exchange capacity.
How pH controls nutrients and toxicity
The chart above is the classic way to summarise it:
- Phosphorus is most available around pH 6–7. In acid soils it is fixed by iron and aluminium; in calcareous soils it precipitates as calcium phosphates.
- Iron, manganese, zinc and copper become less soluble as pH rises. Iron chlorosis (yellow leaves with green veins on young growth) is common on calcareous soils.
- Molybdenum is the exception, becoming more available as pH rises. Molybdenum deficiency in legumes and brassicas is a problem of acid soils.
- Calcium and magnesium are scarce in strongly acid soils simply because they have been leached.
- Aluminium becomes soluble as Al3+ below about pH 5.5 and is toxic to roots: root tips stop elongating, become stubby and brown, and the plant cannot explore the soil for water and phosphorus. Manganese toxicity can appear in acid soils too.
- Microbes: bacteria, including nitrifiers and the rhizobia of legumes, work poorly in strongly acid soil; fungi tolerate it better.
Crops differ in what they tolerate. Tea, blueberry, cranberry and pineapple prefer acid soils; potatoes are often grown slightly acid to reduce common scab. Lucerne (alfalfa), barley and many legumes are sensitive to acidity and aluminium.
Correcting acidity: liming
Liming materials are compounds of calcium or magnesium whose anion consumes hydrogen ions. For calcium carbonate:
The calcium replaces acidity on the exchange sites, and the aluminium that was there precipitates as harmless aluminium hydroxide.
Materials are compared by their calcium carbonate equivalent (CCE), also called neutralising value: how much acid they neutralise compared with pure CaCO3 (100 per cent).
| Material | Approximate CCE | Notes |
|---|---|---|
| Calcium carbonate (calcitic limestone) | 100% | the reference; slow but safe |
| Dolomitic limestone, CaMg(CO3)2 | about 109% | also supplies magnesium |
| Calcium hydroxide (slaked lime) | about 135% | fast; caustic to handle |
| Calcium oxide (quicklime) | about 179% | fastest; caustic, reacts with water |
| Gypsum, CaSO4·2H2O | 0% | supplies calcium but does not raise pH |
The CCE values come straight from molecular weights: 56 g of CaO neutralises as much acid as 100 g of CaCO3, so its CCE is 100 ⁄ 56 = 179 per cent.
Fineness matters as much as chemistry. Lime reacts only at the surface of its particles, so finely ground lime acts within months while coarse grit may take years. Many systems combine CCE and fineness into an effective neutralising value.
Calculating a lime dose
A soil test reports a cation exchange capacity of 12 cmolc/kg and base saturation of 50 per cent. The farmer wants 80 per cent base saturation in the top 20 cm. Bulk density is 1.3 t/m3.
- Bases to add: (80 − 50)% × 12 = 3.6 cmolc/kg of calcium.
- CaCO3 supplies 1 cmolc per 0.5 g (its equivalent weight is 50 g, and a centimole is a hundredth of that). So 3.6 × 0.5 = 1.8 g CaCO3 per kg of soil.
- Mass of soil in one hectare to 20 cm: 10,000 m2 × 0.2 m × 1,300 kg/m3 = 2.6 million kg.
- Lime needed: 1.8 g/kg × 2.6 million kg = 4.68 million g ≈ 4.7 t CaCO3 per hectare.
- With agricultural lime of CCE 90 per cent and a fineness efficiency of 80 per cent, the effective value is 0.9 × 0.8 = 0.72, so apply 4.7 ⁄ 0.72 ≈ 6.5 t/ha, usually split over two seasons and mixed into the soil.
Good liming practice
- Test first; lime to the crop's need, not to neutral. Most crops do well at pH 6–6.5, and many tropical acid soils are limed only enough to neutralise exchangeable aluminium.
- Apply well before sowing and incorporate it; lime moves very slowly down the profile.
- Avoid overliming: pushing pH above 7 in a weathered soil can induce deficiencies of zinc, manganese, iron and boron, and lower phosphorus availability.
- Do not mix lime with ammonium fertilisers or fresh manure at application; ammonia is lost.
- Where subsoil acidity limits roots, gypsum can move calcium down and reduce subsoil aluminium even though it does not change pH.
Lowering pH
Alkaline soils are harder to change, especially when they contain free calcium carbonate, which buffers them strongly. Elemental sulphur, oxidised by soil bacteria to sulphuric acid, is used to acidify soil for acid-loving crops; acid-forming fertilisers help slowly. For sodic soils, the problem is sodium, and the remedy is gypsum and leaching rather than acidification.
Key points
- pH = −log[H+]; one unit is a tenfold change in acidity.
- Soils acidify through leaching, nitrification of ammonium fertilisers, crop removal and sulphide oxidation.
- Reserve (exchangeable and residual) acidity far exceeds active acidity; buffering means clay and organic soils need more lime.
- Most nutrients are most available at pH 6–7; aluminium becomes toxic below about 5.5; molybdenum is the nutrient that becomes less available in acid soil.
- Liming materials are compared by calcium carbonate equivalent; CaO 179%, Ca(OH)2 135%, dolomite about 109%; gypsum does not raise pH.
- A lime dose depends on CEC, the target base saturation, depth, bulk density and the lime's effective value.
What the exams ask
- School-leaving and university entrance papers: the definition of pH, the ranges, acid-loving and acid-sensitive crops, and the effect of pH on nutrient availability.
- Degree and research-fellowship entrances: active versus reserve acidity, buffering, aluminium toxicity, the liming reaction and calcium carbonate equivalents.
- Licensing and professional examinations: lime requirement calculations from CEC or buffer pH, choice of liming material, and the difference between liming acid soils and reclaiming sodic ones.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why can't I lime straight to pH 7?
In many weathered soils, the yield benefit stops once exchangeable aluminium is neutralised, often around pH 5.5–6. Going further costs more lime and can create micronutrient deficiencies. The right target depends on the crop and soil.
Does wood ash work as lime?
Yes. Wood ash contains calcium and potassium carbonates and oxides, with a calcium carbonate equivalent often between 30 and 70 per cent depending on the wood and burning. It is caustic when fresh and its strength varies, so use it with a soil test.
Is a low pH always bad?
No. Some crops, such as tea and blueberry, need it. The harm comes mainly from aluminium and manganese toxicity and from deficient calcium, magnesium, phosphorus and molybdenum, which affect sensitive crops most.
Sources
- Brady, N. C. and Weil, R. R. The Nature and Properties of Soils, 15th edition. Pearson, 2017. Chapter 9, Soil acidity.
- Havlin, J. L., Tisdale, S. L., Nelson, W. L. and Beaton, J. D. Soil Fertility and Fertilizers: An Introduction to Nutrient Management, 8th edition. Pearson, 2014. Chapter 3.
- von Uexküll, H. R. and Mutert, E. "Global extent, development and economic impact of acid soils." Plant and Soil 171, 1–15 (1995).
- Kochian, L. V., Hoekenga, O. A. and Piñeros, M. A. "How do crop plants tolerate acid soils?" Annual Review of Plant Biology 55, 459–493 (2004).
- FAO. FAO Soils Portal: Acid soils. Food and Agriculture Organization of the United Nations.
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