Soil pollution

Sources of soil pollution

• Agrochemicals: overuse of chemical fertilisers and pesticides (some, like DDT, persist for years).
• Industrial waste: heavy metals such as lead, cadmium, chromium and mercury from tanneries, mines and factories.
• Municipal solid waste and landfills: plastics and leachate (polluted liquid that seeps out of waste dumps).
• E-waste: lead, cadmium and mercury from old electronics.
• Irrigation with polluted water and over-irrigation, which brings salts to the surface (salinisation) or causes waterlogging.
• Mining and oil spills.

Effects: lower fertility, poisoned crops, harm to soil organisms such as earthworms, and pollutants entering groundwater and food chains.

Soil properties

• Texture: the proportion of sand (2–0.05 mm), silt (0.05–0.002 mm) and clay (below 0.002 mm). Loam, a balanced mix, is best for most crops.
• Structure and porosity: how particles clump and how much air and water space there is.
• Water-holding capacity: clay holds most water; sand holds least and drains fastest.
• pH: most crops grow best around 6–7.5. Acidic soils are corrected with lime; alkaline (sodic) soils with gypsum.
• Organic matter (humus): improves structure, nutrients and water-holding.

Simple soil tests

• pH test: shake soil with distilled water and check with pH paper or universal indicator.
• Jar (sedimentation) test for texture: shake soil in water and let it settle: sand settles first, then silt, then clay on top.
• Feel test: sandy soil feels gritty, silt silky, clay sticky.
• Water-holding capacity: saturate dry soil and let it drain. Worked example: 100 g of dry soil weighs 140 g after soaking and draining. Water held = 40 g, so water-holding capacity = 40 ÷ 100 × 100 = 40%.
• Organic matter: adding hydrogen peroxide makes soil with organic matter fizz.
• Percolation test: measure how fast water drains through a soil column.

Key points

• Sand > silt > clay in particle size.
• Clay holds the most water; sand the least.
• Lime for acidic soil; gypsum for alkaline soil.
• In a jar test, sand settles first.

Exam tips

Expect particle-size order, jar-test settling order, and a simple percentage calculation. Link pollutants to sources: chromium → tanneries; leachate → landfills.

Common mistakes

• Saying clay settles first; it is the finest, so it settles last.
• Using gypsum for acidic soil; lime is used.
• Calculating water-holding capacity from wet weight instead of dry weight.