Gypsum application on solonetzic soils as a method of land reclamation and increasing yield

How liming improves soil properties and increases yield

High alkalinity and poor structure of solonetz soils block crop development and reduce the efficiency of applied fertilizers. Liming solves this problem by replacing excess sodium in the soil with calcium. Land reclamation does not just improve the hydro-physical properties of solonetz soils, but also guarantees a direct yield increase of field crops.

The presence of exchangeable sodium drastically worsens the structure of solonetz soils, making them impermeable and dense when dry. When gypsum is applied, calcium displaces sodium from the soil exchange complex into the soil solution, where a neutral salt is formed. The chemistry of this process is as follows:

[Soil exchange complex]Na₂ + CaSO₄ ⇄ [Soil exchange complex]Ca + Na₂SO₄

Simultaneously, active soda alkalinity in the soil is neutralized with the precipitation of calcium carbonate:

Na₂CO₃ + CaSO₄ = CaCO₃↓ + Na₂SO₄

The practical effect of reclamation is expressed in a noticeable growth in crop productivity. According to field trials, the yield increase for grain is as follows:

• Grain increase on chernozem soils — 3–6 centners/ha
• Grain increase on chestnut soils — 2–3 centners/ha
• Area of saline soils under rice — 110 thousand ha

On solonetz and saline soils under rice, liming shows maximum efficiency when applied together with mineral fertilizers. Practical data on the joint use of phosphogypsum and various NPK doses are presented below:

Selection of ameliorants and calculation of gypsum application rate

Both raw natural materials and by-products of the chemical industry are used for these operations. The choice of a specific ameliorant depends on its availability and the agrochemical indicators of the soil. Several basic materials are available on the market and near mining sites:

• Raw ground gypsum (CaSO₄·2H₂O) — a white soft powder obtained by grinding natural gypsum. Contains at least 85% active ingredient (for class A). Fineness of grinding: residue on a 0.25 mm sieve is no more than 25%, on a 1 mm sieve no more than 3.5%. Poorly soluble in water, reacts with soil gradually.
• Phosphogypsum (CaSO₄·2H₂O) — a finely dispersed white or gray powder, a by-product of extraction phosphoric acid production. Contains at least 93% active ingredient (for the 1st grade at 5% humidity), as well as 2 to 3% phosphorus (P₂O₅). Available in large volumes at chemical plants.
• Clay-gypsum (CaSO₄·2H₂O mixed with clay) — mined from loose natural deposits in solonetz distribution zones, does not require preliminary grinding. The content of the active ingredient varies depending on the deposit.
• Alternative ameliorants — limestone flour CaCO₃ (effective on low-sodium soils), calcium chloride CaCl₂ (subject to proper leaching), and iron sulfate FeSO₄·7Н₂О.

The decision on the feasibility of reclamation is made based on a detailed chemical analysis of the soil. Practice shows that liming is necessary when certain threshold values of exchangeable elements are exceeded. These critical indicators are listed below:

• Threshold for exchangeable sodium — more than 10% of cation exchange capacity
• Threshold for exchangeable magnesium — more than 30% of cation exchange capacity
• Permissible sodium level — up to 5% of cation exchange capacity

The reclamation process is planned step by step:

1. Conduct an agrochemical analysis of the soil to determine the cation exchange capacity, as well as the sodium and magnesium content.
2. Calculate the base gypsum rate using the formula, taking into account the parameters of the plough layer.
3. Make adjustments for the active ingredient content in the specific batch of the ameliorant.
4. Apply the ameliorant and, if necessary, perform leaching irrigation against the background of total irrigation.

For precise planning of reclamation work, it is necessary to calculate the dose of the pure substance. In practice, a formula is used that takes into account the exchange capacity and the parameters of the treated layer. The calculation for the pure gypsum rate (t/ha) is as follows:

D = 86 × (Na - 0,1 × T) × 10⁵ × h × ρ / 10⁹

Where the following indicators are used:

• 86 — molar mass of CaSO₄·2H₂O equivalents, mg/mmol-eq (required to displace 1 mmol-eq of Na⁺);
• Na — content of exchangeable sodium in the soil, mmol-eq/kg;
• T — cation exchange capacity of the soil, mmol-eq/kg (the 0.1T indicator corresponds to the permissible 10% sodium level);
• h — depth of ploughing (reclaimed layer), cm;
• ρ — soil bulk density, g/cm³;
• 10⁵ — volume of a 1 cm soil layer on an area of 1 ha, dm³;
• 10⁹ — coefficient for converting milligrams into tons.

Let's look at a specific calculation example. For soil with an exchangeable sodium content of 50 mmol-eq/kg and a cation exchange capacity of 180 mmol-eq/kg, at a ploughing depth of 20 cm and a density of 1.2 g/cm³, the calculated application rate of pure gypsum will be 6.6 t/ha. The actual physical mass of the soil amendment is adjusted based on the percentage of the active ingredient in the fertilizer used.

If the soil has an elevated exchangeable magnesium content, an additional dose of the soil amendment is calculated. For this, the same formula is used, but the expression in the brackets is replaced by the difference (Mg - 0.3 × T). A similar adjustment to the rate is carried out in cases of high soda salinity.

The effectiveness of solonetz reclamation depends on the accurate calculation of the gypsum dosage and the method of its incorporation. The application rate is calculated individually for each soil type, taking into account the nature of its salinity. Inefficient distribution of the soil amendment will lead either to unnecessary costs or to a lack of a pronounced reclamation effect.

Pay special attention to the occurrence pattern of solonetz in the field. They rarely appear in a continuous mass and are more often found in patches. If such solonetz patches occupy less than 30 % of the total area of the plot, there is no need for uniform gypsuming — apply the soil amendment only locally to these patches.

Technology of gypsum incorporation into soil horizons

The choice of equipment and depth of tillage depends on how deep the solonetz layer is located. Improper incorporation can leave the gypsum on the surface or place it too deep, preventing contact with the target horizon.

• Shallow solonetz layer (shallow depth): distribute the gypsum over the field after ploughing and incorporate it into the soil with a standard cultivator.
• Solonetz horizon at a depth of 7 to 20 cm: apply the soil amendment in two stages. Distribute the first part before ploughing under a plough with a jointer, and the remaining part after ploughing under cultivation.
• Solonetz horizon deeper than 20 cm: apply the entire calculated gypsum rate at once before ploughing and incorporate it completely with a plough with a jointer.

In some cases, the purchase of gypsum can be reduced by using the resources of the field itself. If layers rich in natural gypsum lie at a depth of 35–45 cm below the solonetz horizon, perform self-gypsuming. To do this, perform trench ploughing to a depth of 35–50 cm to bring the gypsum-bearing layer to the surface and mix it with the solonetz horizon. On chestnut solonetz, CaCO3 from the sub-solonetz horizon can be used for the same purpose, but remember that its effect is inferior to CaSO4.

To combat small, isolated patches of solonetz in the field, land dressing is used. This method consists of mechanically moving and pushing fertile chernozem soil from adjacent areas onto the solonetz spots.

• Proportion of solonetz for local application — less than 30 %
• Depth of natural gypsum occurrence — 35–45 cm
• Depth of trench ploughing — 35–50 cm
• Depth of solonetz horizon for two-stage application — 7–20 cm