Chapter 3 — Soil Conservation Methods

Soil conservation methods are broadly classified into three categories: Agronomic (biological), Mechanical (engineering), and Special measures. Because multiple Past Year Questions (PYQs) directly ask for these methods, you must always be prepared to discuss all three categories in detail.

3.1 Classification of Soil Conservation Measures

• Agronomic (Biological) Measures: These involve manipulating the crop, the soil surface, and the vegetative cover to reduce erosion. They are generally low-cost, based entirely on crop husbandry principles, and are best suited for areas facing mild to moderate erosion.
• Mechanical (Engineering) Measures: These involve constructing physical earth structures (such as bunds, terraces, and check dams) that actively modify the flow path, velocity, and storage of surface runoff. They carry a higher capital cost and are absolutely necessary for areas facing severe erosion or steep slopes.
• Special Measures: These are unique interventions designed for specific, extreme problem environments. Examples include establishing shelterbelts for severe wind erosion, stabilizing migrating sand dunes, and managing severe salinity.

3.2 Agronomic Conservation Measures

A. Contour Farming

• Principle: Farmers till, plant, and manage crops horizontally along the contour lines of a slope (lines of equal elevation) rather than plowing vertically up and down the hill. By doing this, every single furrow acts as a miniature barrier to surface runoff.
• Effect: It significantly increases the time available for water infiltration, slows down runoff velocity, and physically traps eroded soil within the furrows. On gentle slopes (2 to 7% gradient), contour farming alone can reduce soil loss by 50%.
• Limitations and Applicability: It is only effective on mild slopes (less than 8%). On steeper slopes, heavy runoff may breach the furrows at weak points, causing concentrated gully erosion. It is highly ideal for the semi-arid cotton and sorghum fields of the Deccan Plateau and the gently undulating alluvial slopes of the Indo-Gangetic Plains.

B. Strip Cropping

• Principle: This practice involves planting alternate strips of close-growing, dense crops (like grasses or small grains) alongside wider-spaced row crops (like maize, sorghum, or cotton).
• Types: * Contour Strip Cropping: Strips are planted along the contour lines to act as a primary defense against water erosion.
• Field/Wind Strip Cropping: Strips are planted perpendicular to the prevailing wind direction to act as a primary defense against wind erosion in arid areas.
• Effect: The dense-growing strips act as biological filters, trapping sediment and slowing water. Compared to a monoculture field of clean-tilled row crops, strip cropping reduces total soil loss by 60 to 70%.

C. Cover Crops and Green Manure

• Cover Crops: These are fast-growing crops (like cowpea, Dolichos, sunhemp, and clovers) planted specifically to cover and protect the bare soil during fallow periods or between the primary cash crops. Their canopy intercepts falling raindrops, their roots physically bind the soil, and their residue increases infiltration.
• Green Manure: This involves taking a lush cover crop and intentionally plowing it back into the soil while it is still green. This massively adds organic matter, improves soil structure, increases aggregate stability, and drastically reduces future erodibility.
• Indian Context: Farmers commonly use Sesbania (dhaincha) in the Indo-Gangetic Plains before transplanting Kharif rice. Cowpea and moth bean are used in the drylands of Rajasthan, while berseem is heavily used during Rabi fallows in Punjab and Haryana.

D. Mulching (PYQ 2025 Q6c)

• Definition: The application of organic or inorganic material directly to the soil surface to physically shield it from raindrop impact, heavily reduce evaporation, and suppress weed growth.
• Types of Mulches:
• Organic Mulches: Utilizing crop residue (straw, sugarcane trash), dried leaves, or compost. As they decompose, they add vital organic matter, making them the most sustainable option.
• Inorganic Mulches: Utilizing plastic polyethylene films. While highly effective at conserving moisture and suppressing weeds, they present a severe, persistent plastic pollution risk.
• Live Mulches: Maintaining low-growing cover crops alongside the primary crop to provide continuous biological cover.
• Gravel/Rock Mulch: A traditional practice in arid Rajasthan where stones cover the surface to prevent evaporation, though it provides no decomposition benefit.
• Effect: A heavy straw mulch (4 to 6 tonnes per hectare) absorbs the kinetic energy of raindrops, prevents surface sealing, maintains an infiltration rate 2 to 5 times higher than bare soil, and cuts overall soil loss by an incredible 70 to 90%. Furthermore, it reduces evaporation by 30 to 50%, improving dryland crop yields by 20 to 30%.

E. Crop Rotation

• Principle: Systematically rotating crops that possess different rooting depths, planting densities, and residue characteristics. This physically breaks pest cycles, improves soil structure, and reduces erosion.
• Best Practices: The best SWC rotations always include a deep-rooted legume. The deep roots increase soil macroporosity (aiding water infiltration), the legume residue adds organic matter, and the biological nitrogen fixation fuels vigorous crop growth that provides a denser protective canopy.

F. Tie Ridging / Basin Listing

• Principle: Farmers create standard ridges but insert small, transverse earthen barriers (cross-ties) at regular intervals. This divides the furrow into a series of small, enclosed basins or compartments.
• Effect: Each basin captures and retains rainfall precisely where it falls, forcing it to infiltrate rather than run off. This practice reduces runoff by 40 to 80% and increases soil moisture by 20 to 30% in dryland areas.

3.3 Mechanical (Engineering) Conservation Measures

A. Contour Bunding (PYQ 2022 Q5c)

• Definition: The construction of earthen embankments exactly along the contour lines of a field to intercept surface runoff and retain water temporarily. It is the most widely adopted mechanical SWC measure across India.
• Contour vs. Graded Bunds:
• Contour Bunds: Built on a strict horizontal line (0% grade). Water is ponded behind the bund to maximize in-situ water conservation. They are highly suitable for low to moderate rainfall areas (< 750 mm) but carry a high risk of breaching during heavy storms.
• Graded Bunds: Built with a very slight cross-slope gradient (0.2 to 0.5%). This allows excess water to safely drain off the field, heavily reducing the risk of a catastrophic breach. They are safer and strictly required in higher rainfall areas, though they offer slightly less in-situ water conservation.
• Benefits: A properly constructed bunding system reduces runoff by 40 to 60% and slashes soil erosion by 60 to 80%. It is a one-time capital investment that easily lasts 10 to 20 years with minor maintenance.

B. Terracing

• Definition: The extreme engineering practice of converting a steep, un-farmable slope into a series of level or nearly level platforms (terraces) separated by steep vertical risers. It effectively divides one long, dangerous slope into a series of short, safe steps.
• Types:
• Bench Terracing: Features a completely flat, level surface. It provides maximum erosion control and is absolutely required on steep slopes (> 15%), making it the standard practice in the Himalayan hills and the Western Ghats.
• Broad-Base Terracing: Features a wide terrace (3 to 5 meters) with a gentle forward slope, allowing modern farm machinery to operate on gradual hillsides.
• Benefits and Costs: Terracing essentially eliminates the slope factor from the erosion equation, reducing soil loss by 90 to 99%. However, it requires massive earth-moving machinery (bulldozers) and is incredibly capital-intensive (costing ₹25,000 to ₹1,00,000 per hectare).

C. Check Dams

• Definition: Small, sturdy barriers built directly across natural drainage channels or gullies to violently slow water velocity, trap eroded sediment, and force water to recharge the local groundwater aquifer.
• Types:
• Loose Rock (LRI) Check Dams: Built using dry stone masonry. They are highly porous, allowing water to seep through slowly, making them excellent for groundwater recharge and relatively cheap to construct.
• Gabion Structures: Wire mesh baskets tightly packed with rocks. They are flexible, adapting to ground movement and settling without cracking.
• Cement/Concrete Check Dams: Completely impermeable and expensive structures used on larger streams to create permanent water storage reservoirs.
• Functions: They dissipate the kinetic energy of flowing water to halt gully erosion, trap sediment to literally rebuild the soil floor of the gully, and significantly raise the local water table, making downstream agricultural wells viable.

D. Farm Ponds

• Definition: Small, excavated ponds (0.1 to 1.0 hectares in area and 1 to 4 meters deep) designed to capture and store surface runoff from the surrounding farm catchment.
• Benefits: They store critical water for life-saving supplemental irrigation during dry spells. They also provide secondary income streams through fish culture and duck farming, supply domestic livestock water, and actively recharge the shallow aquifer immediately surrounding the pond.

E. Wind Erosion Control Measures

• Shelterbelts: Massive rows of tall trees planted perfectly perpendicular to the prevailing wind direction. They break the wind's velocity, protecting the leeward fields over a distance equal to 10 to 15 times the height of the trees. Common species include Eucalyptus, Casuarina, and Prosopis juliflora in arid zones.
• Windbreaks: Shorter vegetative barriers made of shrubs or tall grasses planted at field boundaries. They are easier to establish but provide a more temporary, localized effect.
• Stubble Retention: Leaving crop stubble standing upright in the field (protruding 20 to 30 cm above the surface) over the dry fallow period. This rough surface absorbs wind energy and is the most economical wind erosion control available to a farmer.

📝 Exam Focus / Past Year Question (PYQ) Hooks

• PYQ 2019 Q7(a) 20M: Elaborate account of agronomic measures of watershed management. → To secure 20 marks, rely entirely on Section 3.2. Outline 7 to 8 specific agronomic measures (Contour Farming, Strip Cropping, Cover Crops, Mulching, Tie Ridging, etc.). For each measure, explicitly describe the mechanism of action and tie it directly to its watershed benefit (e.g., reducing runoff, increasing infiltration).

• PYQ 2022 Q5(c) 10M: Contour bunding and Graded bunding. → Utilize Section 3.3A. Clearly define both terms, explicitly stating their gradients (0% vs. 0.2-0.5%). Contrast how they handle water (ponding vs. safe drainage) and identify the specific rainfall environments (low vs. high) where each is suitable.