Liming

Liming refers to the application to soil of amendments containing calcium and/or magnesium compounds, intended to neutralise soil acidity and improve its physico-chemical properties. This practice chiefly aims to reduce the concentration of hydrogen ions (H+) and, in strongly acidic soils, of toxic aluminium (Al3+), thereby raising the pH value. Liming materials originate from limestone deposits formed in seas of earlier geological periods; they may be inorganic precipitates or carbonate shells, whose physical hardness varies considerably. Reserves are vast in the form of calcitic and dolomitic massifs, although some tropical regions are sometimes remote from them^[1].

Nature and choice of materials

Calcium carbonate is the reference material: it generally contains 75 to 95% CaCO3, equivalent to 42 to 53% calcium oxide (CaO), the basis for calculating liming effect. A magnesium carbonate content above 5% is regarded as beneficial. The required particle size depends on hardness: hard limestones must be ground to less than 1 mm, soft materials (chalk) to less than 4 mm^[1]. Dolomite, with the formula CaMg(CO3)2, contains 40 to 45% CaO and 5 to 20% MgO; it is particularly indicated when the soil requires magnesium input^[2]. Marl, a mixture of earthy material with 10–30% calcium carbonate, was rediscovered in Europe during the eighteenth century and widely employed for improving acidic soils, but its extraction and distribution involve high labour costs^[3]. Carbonatation residues from sugar refineries constitute another usable source^[4].

The choice of material depends on soil texture, local availability and cost. Medium to heavy soils are neutralised rapidly with quicklime, but slow-acting carbonates are preferable for maintaining an optimum reaction. In coarse soils (sands), calcium carbonate is favoured to limit the risk of over-liming^[3].

Agronomic effects

Liming improves soil structure, aeration and the availability of phosphorus and molybdenum. It stimulates biological activity, particularly biological nitrogen fixation^[3]. Adequate calcium input prevents various disorders: internal rot of cabbage, brown spots and bitter pit of apples, empty shells of groundnuts. Total groundnut oil yield is thereby favoured, although the oil content itself does not increase^[5]. Cassava, grown in acidic lateritic soils of pH 4.0–4.5, sees its starch content rise and its hydrocyanic acid content fall after liming^[6]. Sugar cane, suited to soils of pH 5.0–8.0, requires liming under strongly acidic conditions to avoid aluminium toxicity^[6].

European regulatory framework

A European Union regulation defines liming materials as Union fertilising products whose function is to correct soil acidity. These materials must contain oxides, hydroxides, carbonates or silicates of calcium or magnesium^[7]. The minimum neutralising value is set at 15 (CaO equivalent) or 9 (HO- equivalent); the minimum reactivity must reach 10% (hydrochloric acid test) or 50% after six months (incubation test); 70% of the material must pass through a 1 mm sieve, except for quicklime, granular materials and chalk^[7]. Contaminants are capped: cadmium 2 mg/kg, hexavalent chromium 2 mg/kg, mercury 1 mg/kg, nickel 90 mg/kg, lead 120 mg/kg, arsenic 40 mg/kg of dry matter, copper 300 mg/kg, zinc 800 mg/kg^[7]. Labelling tolerances specify a margin of ± 3 for neutralising value, ± 10% for particle size, an absolute deviation of 3.0 percentage points for total CaO, and absolute deviations of 1.0, 2.0 or 3.0 points depending on concentration for total MgO^[8].