Cotton monoculture on regur soil near Akola
A farm of 6 hectares near Akola, Maharashtra, has been under continuous cotton for six years. The soil is black (regur), 45–60 cm deep, derived from Deccan basalt, with pH 8.3 and organic carbon of 0.35%. Rainfall is 780 mm, almost all between June and October, while potential evapotranspiration exceeds 1,800 mm. Irrigation comes from a borewell 60 m deep, and the water table has fallen by 1.6 m every year. Lint yield has fallen from 550 kg/ha to 380 kg/ha. After every heavy shower, 40% of the field stands waterlogged for two to three days and then sets into a hard crust. A gully 0.4 m deep has begun at the field edge where the slope steepens towards a stream. The farmer's plan is to drill a deeper borewell, apply more urea, and continue with cotton because last year's market price was high.
1. Explain how the same properties of regur that make it a good cotton soil also produce the waterlogging, crusting and gullying on this farm.
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Regur is a montmorillonite-rich clay derived from basalt. Its defining behaviour is swelling when wet and shrinking and cracking when dry.
Why it suits cotton: the high clay content gives a very high water-holding capacity, so moisture from the 780 mm monsoon is stored and released through the long dry post-monsoon period when the bolls are filling. Its high cation exchange capacity and reserves of lime, magnesia, potash and iron give natural fertility, and its slightly alkaline pH of 8.3 suits cotton.
Why the same properties cause the symptoms:
- Waterlogging: the swelling clay closes its pores when saturated, so saturated hydraulic conductivity is very low. With only 780 mm of rain but falling in intense showers, infiltration cannot keep pace and 40% of the field ponds for two to three days.
- Crusting: on drying, the dispersed clay at the surface cements into a hard crust, which further lowers infiltration and impedes seedling emergence, so the next shower runs off even faster.
- Gullying: the same low infiltration converts rainfall into overland flow, which concentrates where the slope steepens at the field edge, cutting a 0.4 m gully.
- Moisture stress despite a 'moisture-retentive' soil: with potential evapotranspiration near 1,800 mm against 780 mm rainfall, storage is exhausted before the crop matures.
The soil is thus excellent for holding water but poor at accepting and draining it.
2. Diagnose what has gone wrong in six years of continuous cotton and propose a three-year rotation, explaining how each crop addresses a specific symptom.
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Diagnosis:
- Loss of organic carbon - 0.35% is below the 0.5% level at which structure begins to fail; continuous cotton with residue removal and deep ploughing has oxidised the organic matter.
- Nutrient mining - cotton is a heavy feeder; continuous cropping without a legume has drawn down nitrogen and depleted the biological activity that recycles it.
- Structural decline - repeated tillage of a swelling clay destroys aggregates, worsening both crusting and waterlogging.
- Marginal alkalinity - pH 8.3 is not saline, but the combination of alkaline soil with low organic matter raises the risk of sodicity and further dispersion.
- Pest and weed build-up - host continuity favours cotton-specific pests.
Proposed rotation: cotton in year 1, soybean or black gram in year 2, jowar or chickpea in year 3.
- The legume fixes nitrogen, breaks the pest cycle and, if residue is incorporated, raises organic carbon.
- Jowar/chickpea has deeper or more fibrous roots that exploit different soil layers and adds residue for mulch, which protects the surface from crusting and sheds the erosive first showers.
- Reducing tillage and adding 5 t/ha of farmyard manure rebuilds aggregates, and broad-bed-and-furrow layout drains the ponded 40% of the field.
- At the field edge, contour bunds and a gully check stop the 0.4 m gully from extending.
3. Evaluate the farmer's plan of a deeper borewell, more urea and continued cotton.
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The plan treats symptoms with inputs that make the underlying problem worse.
Deeper borewell: the water table is already falling 1.6 m a year, which is itself evidence of over-extraction. A deeper borewell buys time for one farmer but accelerates the decline for the whole aquifer, and pumping costs and electricity rise faster than yield. It does not fix the low infiltration, so it adds no water to storage.
More urea: nitrogen is not the main limiting factor at 0.35% organic carbon - structure, water relations and micronutrient supply are. Extra urea without organic matter worsens the carbon-to-nitrogen imbalance and acidification over time, and leaches or volatilises rather than raising yield.
Continued cotton at high prices: the price signal is precisely what caused the six-year monoculture. Yield has fallen 31% (550 to 380 kg/ha), so the farmer is running to stand still: revenue per hectare may be flat while costs rise.
What the plan misses: the diagnosis is physical and biological (structure, organic matter, drainage, gully) and only secondarily about water supply or nutrients. The economically rational response is to reduce the area under a high-demand crop, adopt micro-irrigation on cotton to cut evaporation, harvest and recharge through farm ponds and check dams so that the 780 mm is retained rather than wasted as runoff, and rebuild the soil with the rotation described. Treating a water-balance problem as a borehole problem simply deepens the debt.