Liming as a method of reclamation and improving soil fertility of acidic soils

Why acidic soils lose yield and how to fix it

Acidic soils are a major barrier to achieving high crop yields. Increased acidity slows down the growth of the root system, and plants absorb fewer nutrients from the soil and applied fertilizers. In acidic environments, the structure of the plough layer deteriorates, water permeability decreases, beneficial microflora is suppressed, and toxic mobile forms of aluminum, iron, and manganese are activated. To solve this problem and increase soil fertility, liming is performed—a fundamental chemical land reclamation process involving the application of neutralizing substances.

• Acidic arable land in Russia — more than 43 million hectares
• Annual crop yield loss (in grain equivalent) — about 20 million tons
• Annual loss of bases (in terms of CaCO₃) — 350–450 kg/ha
• Optimal pH level for most crops — 6–7

The historical experience of applying lime fertilizers spans thousands of years. As early as Ancient Rome, Gaul, and the British Isles, marl and chalk were used to improve meadows and fields. However, in the absence of scientific knowledge, lime was for a long time erroneously considered a complete substitute for manure and applied in excessive doses, which led to soil depletion. Systematic studies and the first field experiments in the 19th and 20th centuries allowed for the development of scientifically grounded liming rates, which formed the basis of modern agrochemical technologies.

After liming, the properties of acidic soils change for the better:

• active and exchangeable acidity are completely eliminated, and hydrolytic acidity is significantly reduced;
• a favorable environment is created for root growth and the vital activity of beneficial soil microorganisms;
• soil structure, water permeability, and aeration are improved;
• toxic mobile forms of aluminum, iron, and manganese are bound and converted into an inactive state;
• nitrogen-fixing, ammonifying, and nitrifying bacteria are activated, improving plant nitrogen nutrition;
• soil phosphates are mobilized, converting into forms of calcium phosphates accessible to plants;
• molybdenum mobility is increased, and plant nutrition with calcium is optimized.

How limestone flour works in the soil

The most common lime fertilizer is limestone flour (CaCO₃), obtained by grinding natural limestone. It is a white or yellowish dusty powder containing 75 to 100% active ingredient in terms of calcium carbonate. The effectiveness of the fertilizer directly depends on the fineness of the grind. According to current standards, the residue on a sieve with a mesh size of 0.25 mm should be no more than 15–45%, and on a 1 mm sieve — no more than 6%.

Since CaCO₃ is practically insoluble in water, its interaction with the soil solution occurs under the influence of carbon dioxide and moisture. This process proceeds in several consecutive stages:

1. Calcium carbonate, under the influence of carbon dioxide and water, converts into more soluble calcium bicarbonate:
   CaCO₃ + CO₂ + H₂O = Ca(HCO₃)₂
2. Calcium bicarbonate dissociates and partially undergoes hydrolysis, forming calcium ions and hydroxide ions:
   Ca(HCO₃)₂ + 2H₂O = Ca(OH)₂ + 2H₂O + 2CO₂
Ca(OH)₂ = Ca²⁺ + 2OH⁻
3. Calcium cations displace hydrogen ions from the soil adsorption complex (SAC). The displaced hydrogen is bound into weakly dissociating carbonic acid and water:
   [SAC]2H⁺ + Ca²⁺ + 2HCO₃⁻ ⇄ [SAC]Ca²⁺ + 2H₂CO₃
[SAC]2H⁺ + Ca²⁺ + 2OH⁻ ⇄ [SAC]Ca²⁺ + 2H₂O

In parallel, calcium carbonate directly neutralizes free organic acids, as well as nitric acid, which is formed in the soil during nitrification:

• Neutralization of organic acids: R(COOH)₂ + CaCO₃ → R(COO)₂Ca + H₂O + CO₂
• Neutralization of nitric acid: 2HNO₃ + CaCO₃ → Ca(NO₃)₂ + H₂O + CO₂

As a result of applying the full rate of limestone flour, active and exchangeable acidity are completely eliminated in the soil. At the same time, hydrolytic acidity is significantly reduced and the base saturation degree of the soil is increased. The content of calcium, which is necessary for complete plant nutrition, increases significantly in the soil solution.

Dolomite flour CaCO3·MgCO3. Active ingredient 95-109 %. In appearance, a white or yellowish powder. Produced by grinding dolomite. MgCO3 is practically insoluble in water and slightly soluble in water containing carbonic acid. Therefore, it acts somewhat slower than ground limestone.

Quicklime CaO. Active ingredient up to 178 %. Obtained by calcining limestone and as a waste product at lime plants. Lump lime is slaked with water before application to obtain slaked lime; pulverized lime can be applied directly. A very potent and fast-acting liming material.

Slaked lime Ca(OH)2. Active ingredient up to 135 %. Obtained by slaking quicklime and as a waste product at lime plants and during the production of bleaching powder. Slaking is carried out with water or by covering with moist soil. The effect is strong and fast.

Defecate CaCO3+Ca(OH)2. Active ingredient 60-75 %. Obtained as a waste product of sugar beet production – defecation mud. After air drying, it becomes free-flowing and can be used as a fertilizer. It is available in large quantities in the Krasnodar Krai.

Cement dust CaO+Ca(OH)2. Active ingredient up to 86 %. A fine, dusty powder. Obtained as a waste product at cement plants. A fast- and strong-acting fertilizer.

Soft lime rocks (limestone tuff, bog lime, marl) and a number of other industrial waste products can be used as fertilizers for liming.

Application of lime fertilizers. To carry out liming, it is necessary to determine the need for this agrotechnical practice, calculate fertilizer doses, and establish methods of application.

Need for liming. To determine the necessity of liming, agrochemical soil acidity maps compiled by agrochemical laboratories are used. Exchange acidity (pH of salt extract) and base saturation are taken into account.

Table 154 – Soil need for liming Need for liming pH of salt extract Base saturation, V %

 Strong < 4.5 < 50 Moderate 4.6-5.0 50-70 Weak 5.1-5.5 70-80 Absent >5.5 > 80

For a more precise determination of the need for liming, the content of mobile aluminum and manganese compounds in the soil and the particle-size distribution of the soil are also taken into account.

Calculation of the lime fertilizer application rate. The amount of lime required to establish a slightly acidic soil reaction favorable for most crops (pH of water extract 6.2-6.5, salt extract – 5.6-5.8) is called the full application rate. It is determined by hydrolytic acidity, taking into account the mass of the soil in the ploughing layer of 1 hectare, using the formula:

50  H g  10 5  h   Hg 3)  9

10 200 where 3) – mass of CaCO3, t/ha;

50 – molar mass of CaCO3 equivalents, mg/mmol-eq (required to displace 1 mmol-eq H+);

Hg – hydrolytic acidity of the soil, mmol-eq/kg;

105 – volume of a 1 cm soil layer per hectare, dm3; h – depth of ploughing, cm; ρ – soil density, g/cm3 (numerically equal to density in kg/dm3);

109 – coefficient for converting milligrams into tonnes.

For example, on a soil with a hydrolytic acidity of 60 mmol-eq/kg, at a ploughing depth of 18 cm and a density of 1.1 g/cm3, the mass of CaCO3 is 5.9 t/ha.

The lime fertilizer application rate is then calculated taking into account the content of the active ingredient in the fertilizer used (as in mineral fertilizers). On low-buffer light soils, the lime rate is reduced by 25-30% of the full application rate.

Application of lime fertilizers. The full lime application rate is applied at once or in several stages, depending on the farm's capabilities. The determined lime rate is applied evenly across the field surface using lime spreaders. Fertilizers are applied in the autumn before ploughing or in the spring before secondary tillage.

If phosphate rock is used, it is applied before liming. Small doses of lime, 1/4–1/5 of the full application rate (0.5–1 t/ha), are applied in rows during sowing and in holes during the planting of transplants, or incorporated shallowly for crops particularly sensitive to acidity. When lime is applied at the full rate, repeat liming is carried out after 7–10 years. To refine the timing, an agrochemical soil survey is conducted after 5 years.