Laser Levelling a Cotton Field with Shallow Topsoil
A 4 ha cotton field has been surveyed on a 20 m grid. Elevation ranges from 0.35 m high at the head to 0.00 m at the tail, with two depressions holding water after rain. The average cut to a plane at 0.15% slope is 0.12 m, and 30% of the area would be cut deeper than the 0.18 m topsoil layer. The soil is clay loam with a shallow hardpan at 0.25 m. Cotton yield is 750 kg/ha, seed cotton sells at PKR 200/kg, and the farmer estimates a 15% yield increase from uniform irrigation. Laser levelling costs PKR 250/m³ of earthwork, and topsoil stripping and replacement costs an extra PKR 80/m³. The farmer has a 10-day window before sowing and wants a dead-level field to reduce waterlogging.
1. Calculate the earthwork volume and the basic levelling cost. Compare a dead-level field with a 0.15% slope for this soil and crop.
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Earthwork volume = area × average cut = 40,000 m² × 0.12 m = 4,800 m³. Basic levelling cost = 4,800 m³ × PKR 250/m³ = PKR 1,200,000.
A dead-level field is not appropriate here. The soil is clay loam with a shallow hardpan, and the field already has depressions that hold water. Dead level would reduce the ability of excess rain or irrigation water to drain, increasing waterlogging and salinity risk. A slope of about 0.15% is enough to move water to a tail drain without causing erosion on clay loam. The 0.15% plane is technically preferable, and the calculated 0.12 m average cut already refers to that plane.
2. Assess the topsoil loss risk and recommend how to protect the topsoil during levelling.
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About 30% of the area would be cut deeper than the 0.18 m topsoil layer. In those cuts, the levelling operation would expose subsoil or hardpan, which has lower fertility, poorer structure, and lower infiltration. That is a serious yield risk for cotton, especially on clay loam.
Protection measures:
- Strip the top 0.18–0.20 m from the areas to be cut and stockpile it separately.
- Strip topsoil from the fill areas as well if fill depth is significant, or at least avoid burying good topsoil under deep fill.
- Level the subsoil to the design plane, then replace the stripped topsoil uniformly over the cut areas.
- Rip the hardpan to about 0.30–0.35 m before replacing topsoil if the hardpan is compacted, but do not mix subsoil into the topsoil.
- Avoid working when the soil is wet; clay loam is easily compacted.
The extra cost is not trivial: if topsoil is stripped over the whole 4 ha at 0.18 m, that is 7,200 m³, and the extra stripping/replacement cost at PKR 80/m³ is PKR 576,000. Even stripping only the cut areas would be substantial.
3. Decide whether to level the whole field now, split it, or postpone. Include the economics and the 10-day window in your recommendation.
Read the worked answer
Yield benefit: 750 kg/ha × 15% = 112.5 kg/ha. Over 4 ha: 450 kg. At PKR 200/kg, the added gross value is 450 × 200 = PKR 90,000 per year.
Basic levelling cost is PKR 1,200,000, and topsoil protection could add up to about PKR 576,000. Even ignoring the extra cost, simple payback is 1,200,000 / 90,000 ≈ 13.3 years. With topsoil protection, payback exceeds 19 years. Whole-field levelling is therefore not economically justified on this cotton yield alone, especially with the risk of exposing subsoil.
Recommendation: do not attempt whole-field dead-level levelling in the 10-day window. Instead, spot-level the two depressions and the worst high spots, maintain the 0.15% slope, and install a shallow tail drain. If the farmer wants full levelling, split the field into two or three sections and level one section per season after topsoil stripping, or hire a contractor with a laser scraper and a defined topsoil-management plan. Postpone full levelling until the economics improve or until a higher-value crop or waterlogging damage justifies it.