Soil Degradation, Erosion, and Conservation

Soil degradation is the decline in soil quality and productivity caused by erosion, loss of nutrients and organic matter, compaction, salinization and pollution, and soil conservation is the set of methods used to protect soil and restore its fertility. In brief: soil erosion by water and wind removes the fertile topsoil much faster than it forms; contour plowing, terracing, cover crops, shelterbelts and no-till farming reduce it. Overgrazing, deforestation and poor irrigation are the main human causes.

Healthy soil is vital for food production, water storage and carbon storage, but it forms very slowly. This guide explains the causes of degradation, the main types of erosion, the methods of conservation, compares them in a table, works through calculations of loss and recovery, outlines the history of soil conservation and corrects common mistakes.

What causes soil degradation?

Natural factors such as steep slopes, heavy rain, drought and strong wind make soil vulnerable, but human activity is what turns erosion into degradation. The main causes are deforestation (removing the roots and canopy that protect the soil), overgrazing (too many animals strip the plant cover), unsuitable farming (plowing up and down slopes, leaving bare ground, growing the same crop repeatedly), poor irrigation, construction and pollution. Climate change adds stress through more intense rainfall and longer droughts.

How do water and wind erode soil?

Water erosion develops in stages. Raindrops hit bare soil and detach particles (splash erosion). Runoff then removes a thin layer evenly (sheet erosion), concentrates into small channels (rills) and finally cuts deep gullies that cannot be crossed by a plow. Steeper and longer slopes and bare ground increase the loss.

Wind erosion is most severe in dry, flat, treeless land with loose soil. Fine particles are lifted into the air, and sand is moved by saltation. The Dust Bowl of the 1930s in the central United States, caused by drought together with plowing of grassland, is the best-known example, and dust storms still affect parts of Asia and Africa today.

What other forms of degradation exist?

• Loss of nutrients and organic matter: crops remove nutrients, and without fertilizer, manure or crop rotation, yields fall.
• Compaction: heavy machinery and trampling squeeze out the pores, reducing air and water movement and root growth.
• Salinization: in dry regions, irrigation water evaporates and leaves salts in the topsoil. Poor drainage makes it worse and can make land useless for crops.
• Acidification and pollution: acid deposition, heavy metals, pesticides and waste can harm soil life and crops.
• Desertification: degradation of land in dry regions to the point where it loses its productive capacity.

How can soil be conserved?

Conservation aims to keep the soil covered, keep water in the field and keep organic matter high.

• Contour plowing: plowing across the slope so furrows hold water instead of channeling it downhill.
• Terracing: building flat steps on steep slopes, as in the rice terraces of Asia and the Andes.
• Cover crops and mulching: keeping the ground covered between main crops so rain does not hit bare soil.
• Crop rotation and agroforestry: varying crops and planting trees among them to improve nutrients and roots.
• Shelterbelts and windbreaks: rows of trees that slow the wind.
• Strip cropping and grass buffers: strips of grass between crop rows or along streams to trap sediment.
• No-till or reduced tillage: leaving crop residue and avoiding plowing, which protects structure and organic matter.
• Controlled grazing and reforestation: rotating pastures, limiting herd sizes and replanting trees on degraded land.
• Better irrigation and drainage: drip irrigation and drainage to avoid salt build-up.

Conservation methods compared

Worked example: soil loss and recovery

1. Mass of soil lost: a field of 10 hectares loses 20 tonnes of soil per hectare each year. The total loss is 10 × 20 = 200 tonnes per year.
2. Depth lost: one hectare is 10,000 m². If the soil has a bulk density of 1.25 t/m³, 20 tonnes is 20 ÷ 1.25 = 16 m³. Spread over 10,000 m², the depth is 16 ÷ 10,000 = 0.0016 m = 1.6 mm per year.
3. Years until the topsoil is gone: with 20 cm (200 mm) of topsoil, 200 ÷ 1.6 = 125 years.
4. Formation versus loss: if soil forms at about 1 t per hectare per year, the net loss is 20 − 1 = 19 t per hectare per year, so the loss is about 20 times faster than formation.
5. Effect of conservation: if mulching and contour plowing cut the loss by 80%, the new loss is 20 × 0.2 = 4 t per hectare per year, which is 4 ÷ 1.25 ÷ 10,000 = 0.00032 m = 0.32 mm per year, and 200 mm would last about 625 years.

How did soil conservation begin?

Terracing and other conservation practices are thousands of years old in many parts of the world. Modern programs began after the Dust Bowl, when the United States set up the Soil Conservation Service in 1935 under Hugh Hammond Bennett, who promoted contour farming and cover crops. In the later 20th century, the Green Revolution increased yields but also brought problems of salinization and nutrient loss in some regions. International agreements, such as the UN Convention to Combat Desertification adopted in 1994, now address land degradation, and no-till farming has spread widely in recent decades.

Which mistakes are common?

Mistake 1: Thinking that soil is renewable on human timescales

Soil forms very slowly. Rates of loss from poor farming are often tens of times faster than natural formation.

Mistake 2: Ignoring slow, invisible erosion

Sheet erosion removes a thin layer evenly and can go unnoticed for years until yields fall.

Mistake 3: Believing that fertilizer fixes degraded soil

Fertilizer adds nutrients, but it does not rebuild structure, organic matter or lost topsoil.

Mistake 4: Assuming that irrigation always improves land

Without good drainage, irrigation in dry regions can cause salinization and make the land less productive.