Agriculture & Soil Improvement

In acid soils, yield loss is usually caused not by the pH figure itself but by the aluminium that becomes soluble at that pH. Below pH 5.5, Al³⁺ moves into the soil solution and destroys the root tip meristem; roots cannot go deep, and the crop is exposed to drought and nutrient deficiency at the same time. A liming decision therefore rests not on the question “what is the pH” but on “how much exchangeable acidity sits in the root zone”.

The material used in agricultural liming is not a caustic chemical but a ground carbonate. Ground calcium carbonate reacts with hydrogen ions in the soil solution: CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂. The rate of that reaction is set not by purity but by the particle surface actually in contact with the soil solution. Two batches carrying the same certificate will behave differently in the field if their sieve curves differ.

This page works through how the lime requirement is calculated: what a buffer pH test actually measures, how the gap between neutralising value (NV) and effective neutralising value (ENV) changes cost per hectare, where the calcium–magnesium balance becomes critical, and why several months should sit between application and sowing. What lime does in a field is fundamentally different from what it does in soil improvement and stabilisation: there the aim is to bind the clay mineral permanently, here it is to hold root-zone chemistry inside a defined band.

A pH read in a 1:2.5 water suspension shows only active acidity — the H⁺ currently free in the soil solution. That is often less than a thousandth of the total acidity present. The rest sits as exchangeable acidity (Al³⁺ and H⁺) held on clay and organic matter surfaces, and every hydrogen ion neutralised in solution is replaced from that reserve. Of two plots both reading pH 5.2, one may reach target with 1.5 t/ha of lime while the other is still short at 6 t/ha.

The difference is buffering capacity, set together by cation exchange capacity (CEC), clay content and organic matter. A sandy soil sits around 5 cmol(+)/kg CEC; a heavy clay rich in organic matter goes above 30. This is why lime requirement is calculated from a buffer pH test (SMP, Adams–Evans or Mehlich buffer) or an exchangeable acidity determination, not from pH. The analysis to request from the laboratory is not “pH” but “buffer pH + CEC + exchangeable cations”.

The second misleading point is sampling depth. Lime barely moves in soil; spread on the surface without incorporation, its effect advances only a few centimetres a year. In no-till fields acidity accumulates in the top 0–5 cm while 10–20 cm still looks normal. A single 0–20 cm composite hides that stratification; only separate depth samples show whether the problem is at the surface or through the profile.

The third is the opposite error: over-liming. Above pH 7 the availability of zinc, manganese, iron and boron falls, and phosphorus is fixed again — this time as calcium phosphate — which is far harder and slower to reverse than adding lime. Acid soils are a regional problem, concentrated in high-rainfall tea, hazelnut and pasture land and in leached granite–schist derived soils. With fast-reacting products such as hydrated lime that ceiling is crossed considerably sooner.

The field value of a liming material is the product of two numbers. The first is neutralising value (NV): its acid-neutralising power relative to pure calcium carbonate. Pure calcite gives 100%, dolomite CaMg(CO₃)₂ 108–109%, Ca(OH)₂ 136% and CaO 179%. Commercial ground aglime typically falls between 85 and 100%; the shortfall is quartz, clay and moisture. NV is measured to EN 12945 by dissolving in excess acid and back-titrating.

The second number is usually skipped, yet it drives the field result: the fineness factor. Particles above 2 mm effectively do not react in the first season; 0.85–2 mm counts about 20–40%, 0.25–0.85 mm about 50–60%, and below 0.25 mm is taken as 100% effective. Effective neutralising value (ENV) is NV multiplied by the weighted average of those fractions. A material with 95% NV but half its mass above 2 mm falls below 50% real effectiveness: the tonne price looks cheap while the true cost per hectare doubles.

The default product is ground natural calcium carbonate. It is not caustic, it is safe to handle, it forgives a dosing error and it does not disturb soil biology. Where dust and spreading uniformity are a problem, granulated calcium carbonate is preferred; the granule disintegrates in soil back to the same fineness, so it does not lower ENV — it only improves handling and working width.

Magnesium balance decides the product directly. When Mg base saturation drops below 10%, dolomitic lime does two jobs at once: it corrects acidity and closes the Mg gap. If Mg is already adequate, dolomite distorts the Ca:Mg ratio; below roughly 4:1 on a cmol(+) basis, heavy soils start to show surface crusting and reduced permeability. Where correction has to be fast — before orchard planting, in greenhouse soil preparation — quicklime or hydrated lime works in weeks rather than months; in exchange it is caustic, it temporarily suppresses microbial activity and it does not forgive a dosing error.

Sugar factory carbonation lime is also a legitimate source: because the lime used in juice purification is re-precipitated as CaCO₃ in the food and sugar industry, the material is already very fine and reacts fast. In return it carries 25–35% moisture, contains organic residue and has a low bulk density. The calculation must be done on a dry-matter basis and the haulage distance judged on that same moisture — otherwise a material that looks cheap ends up expensive per hectare.