Leaching of Soil: Meaning, Process, Eluviation and Illuviation

Leaching Meaning in Soil Science and Geography

What Is Leaching in Geography?

Soil scientists describe leaching as a movement from one zone of the soil to another. What the topsoil loses is either deposited lower in the subsoil or washed out of the soil altogether into groundwater.

Leaching starts with weathering. Soluble salts such as nitrates and sulphates dissolve easily, so in rainy climates they are leached out without leaving any residue, while in dry regions they accumulate. Limestone minerals dissolve in water made weakly acidic by carbon dioxide, which decaying organic matter adds to soil water.

• Leached soil: A soil that has lost its soluble nutrients and bases to percolating water, becoming poorer and often more acidic; laterite is the classic leached soil of India.
• Leaching in agriculture: The loss of water-soluble plant nutrients from the soil because of rain and irrigation.
• Leaching in decomposition: One of the steps by which dead matter breaks down; water-soluble inorganic nutrients go down into the soil horizon and are precipitated as unavailable salts.

Eluviation and Illuviation: Where Leached Material Goes

Water does not only remove material; it also puts it down again lower in the soil. Excess water carries soil components down through the soil, a process called eluviation, and deposits them further down, a process called illuviation. Eluviation happens where precipitation exceeds evaporation.

Soil scientists make one further distinction. Strictly, leaching moves dissolved materials, while eluviation moves fine particles of clay and organic matter as well. The E horizon, the zone of eluviation, is typically light grey, low in organic matter and rich in quartz silt and sand; the zone that collects leached and eluviated material is the zone of illuviation, the B horizon of the soil profile.

Leaching Process in Soil: How It Works and What Controls It

Leaching Process Step by Step

Leaching follows the path of water from the surface down to the water table, in four steps.

1. Water enters: Rain or irrigation water soaks into the soil.
2. Minerals dissolve: Soluble minerals and nutrients dissolve; carbonic acid from carbon dioxide in soil water dissolves calcium and magnesium carbonates.
3. Water percolates: The solution moves down through pores in the soil, carrying the dissolved ions.
4. Material is deposited or lost: Some of it is deposited in lower layers by illuviation; the rest drains out into groundwater, streams and eventually the sea.

The same washing out also concentrates what is left. When rocks weather, some materials are removed by chemical or physical leaching by groundwater, and the concentration of the valuable materials that remain increases. This enrichment is how weathering produces workable ores of iron, manganese, aluminium and copper.

Factors Controlling Leaching: Rainfall, Temperature, Soil Texture and Vegetation

Climate is the main control. In wet equatorial areas with high rainfall, not only calcium, sodium, magnesium and potassium but also a major part of the silica is removed from the soil, a process called desilication. In dry climates evaporation exceeds precipitation, groundwater is drawn up by capillary action and salts are left behind as crusts called hardpans.

• Rainfall: The more water percolates through the soil, the more is leached; leaching is strongest where rainfall exceeds evaporation.
• Temperature: Chemical activity rises in higher temperatures, so tropical soils have deeper profiles; bacteria also destroy humus fast in hot, wet climates.
• Soil texture: Sandy soil holds little water, so water and nutrients pass through quickly; clay soils retain much more water.
• Vegetation: Litter from conifers such as pine, spruce and fir returns fewer base cations to the soil than deciduous litter, so soils under conifers acidify more.
• Slope: On gentle slopes, where erosion is slow, water percolates well; on steep slopes it runs off instead.

Between these two extremes, in tropical areas of moderate rainfall, calcium carbonate nodules called kankar form in the soil.

Soils Formed by Leaching: Laterite, Podzol and Leached Soil Orders

Laterite: The Soil Formed by Intense Leaching

The soil formed by intense leaching is laterite. It develops in areas of high temperature and heavy rainfall, where the rain washes away lime and silica and leaves a soil rich in iron oxide and aluminium compounds, while bacteria that thrive in the heat remove humus fast. The name comes from the Latin later, a brick.

• Nutrients: Laterite is poor in organic matter, nitrogen, phosphate and calcium, while iron oxide and potash are in excess, so it needs manures and fertilisers before it can be farmed.
• Where it is found: Mainly on the higher areas of the Peninsular plateau, in Karnataka, Kerala, Tamil Nadu, Madhya Pradesh and the hilly areas of Odisha and Assam. In Madhya Pradesh the laterite capping the plateau is 30 m thick.
• Crops on laterite: Red laterite soils in Tamil Nadu, Andhra Pradesh and Kerala suit tree crops such as cashew.
• Uses of laterite: Laterite is widely cut into bricks for houses, and bauxite, the ore of aluminium, forms in laterite on the Peninsular plateau and hills.
• First named in India: Francis Buchanan-Hamilton described and named a laterite formation in southern India in 1807.

In laterite areas leaching follows the seasons. Rocks are leached by percolating rain in the wet season, and the solution of leached ions is drawn back to the surface by capillary action in the dry season, so the zone in contact with water is progressively stripped of sodium, potassium, calcium and magnesium.

Red Soil and Black Soil: Why Leaching Does Not Form Them

Red soil is not a product of leaching. It develops on crystalline igneous rocks in areas of low rainfall in the eastern and southern Deccan Plateau. Its colour comes from the wide diffusion of iron in crystalline and metamorphic rocks, and it looks yellow when the iron is hydrated. Laterite, which leaching does form, is also red, which is why red soil and laterite soil are often confused.

Black soil, or regur, is the opposite of a leached soil. It is clayey, deep and impermeable, swells when wet and cracks when dry, and holds moisture for a long time. It stays rich in lime and magnesia, bases that heavy leaching removes, as well as iron and alumina, and it contains potash, though it lacks phosphorus, nitrogen and organic matter.

Podzols and Other Heavily Leached Soils

Cool, humid forests produce a different leached soil, the podzol. Podzols are the typical soils of coniferous or boreal forests. The name, given by Vasily Dokuchaev in 1875, means under-ash in Russian, because peasants ploughing virgin podzol turned up what looked like a layer of ash, which was the leached E horizon. Podzols are mostly sandy, low in moisture and nutrients, with a low pH, phosphate deficiency and aluminium toxicity, so their best agricultural use is grazing.

Three soil orders of the USDA classification are also products of heavy leaching or weathering.

• Ultisols: The ultimate product of continuous weathering in humid climates, with no calcium carbonate anywhere in the soil.
• Alfisols: Soils with a clay-enriched subsoil, a product of illuviation, and relatively high native fertility.
• Oxisols: Soils of tropical rainforest within 25 degrees of the equator, some once classed as laterite soils.

In India, Alfisols and Ultisols together make up about 16 per cent of the soils.

Effects of Leaching: Soil Fertility, Acidity and Water Pollution

Loss of Nutrients and Soil Acidification

The first effect of leaching is on soil fertility. Water carries away the nutrients plants need, and excessive irrigation speeds this up, lowering both the quality of the produce and the yield.

The second effect is acidity. Soil acidification occurs when base cations such as calcium, magnesium, potassium and sodium are leached from the soil, leaving hydrogen and aluminium behind. Acid rain speeds this up, as calcium is leached and aluminium and hydrogen levels rise. Most nitrogen fertilisers do the same, the ammonium-based ones most of all.

Acid soils have a pH below 5.5 for most of the year. They suffer from aluminium toxicity and molybdenum deficiency, yet crops such as rice, cassava (tapioca), mango and cashew grow on them.

• Remedy for acid soils: Liming, the application of calcium- and magnesium-rich materials such as limestone, chalk or burnt lime, neutralises soil acidity and often improves plant growth; soil tests every two to three years show how much lime a field needs.
• Nitrate: As nitrate leaches, it carries calcium and other nutrients out of the soil with it, which lowers the soil's productivity.

Nitrate Leaching, Groundwater Pollution and Blue Baby Syndrome

Nitrate is the nutrient most easily leached. Fertilisers add nitrate and ammonium to the root zone, but soils do not hold the excess nitrate ions, which move down freely with drainage water and are leached into groundwater, streams and oceans. How much leaches depends on the soil type, the water crops use and the nitrate already in the soil.

India measures the problem every year. The Central Ground Water Board samples groundwater once a year before the monsoon, analyses it in 16 regional chemical laboratories and publishes the results in an Annual Ground Water Quality Report. It counts a sample as contaminated when nitrate exceeds 45 mg per litre.

• Health effects: High nitrate in water causes methemoglobinemia, known as blue baby syndrome, so drinking water limits are set at 45 to 50 mg of nitrate per litre. Bottle-fed infants under six months are most at risk. Once the water mixed into their feed holds more than 50 mg of nitrate per litre, it becomes their main source of nitrate.
• Ecosystems: Nitrate and phosphorus reaching water bodies cause eutrophication, which can kill fish and other marine life.
• Phosphorus: Phosphorus is held more tightly by soil, but decades of heavy fertiliser use fill the soil's holding sites, so legacy phosphorus leaches too.
• Industrial waste: At Daurala near Meerut, untreated industrial wastewater leached down to the water table and polluted the village's water.

Using and Controlling Leaching: Salinity, Liming and Metallurgy

Leaching to Control Soil Salinity

Leaching is not always harmful. On irrigated land the danger is the opposite one, as salts build up when irrigation water evaporates. Farmers then apply a little more irrigation water than the crop needs, the leaching fraction, to wash the added salts below the root zone, and drainage must carry the excess water away.

Nature does the same over long periods. As soil minerals weather and release salts, drainage water flushes them out of the soil wherever precipitation is sufficient, while in droughts the salts rise by capillary action and gather near the surface.

The balance is delicate. Where drainage is disrupted, the salts that should be leached accumulate instead, and the result is the saline and alkaline soils that have to be reclaimed before they can be farmed.

Leaching Process in Chemistry and Metallurgy

In chemistry, leaching means extracting a substance from a solid by dissolving it in a solvent. Metallurgy uses it when an ore is soluble in a suitable solvent, so the metal is dissolved out and the impurities are left behind.

• Aluminium: Bauxite is digested with concentrated sodium hydroxide, which leaches out alumina as sodium aluminate and leaves impurities such as iron oxides behind.
• Gold and silver: The metal is leached with a dilute solution of sodium or potassium cyanide in the presence of air, then recovered by replacement.
• Copper from low-grade ores: Copper is leached out using acid or bacteria, and the solution is treated with scrap iron or hydrogen.
• Heap leaching: Ore is piled on a liner and chemicals are dripped through it to extract precious metals, copper and uranium at lower cost than conventional processing.

The same process can harm the environment. Acid rain corrodes water pipes, leaching heavy metals such as iron, lead and copper into drinking water, and waste heaps can release metals into soil and water in the same way.

Sources

• NCERT: Fundamentals of Physical Geography (Class XI), Geomorphic Processes and soil formation
• NCERT: Contemporary India II (Class X), Resources and Development
• NCERT: Biology (Class XII), Ecosystem, decomposition
• NCERT: India People and Economy (Class XII), Daurala case study
• PIB: Parliament Question, Annual Groundwater Quality Report, 2025
• Central Ground Water Board: Ground Water Quality
• WHO: Nitrate and nitrite in drinking-water, background document
• FAO Soils Portal: Acid soils
• Wikipedia: Leaching (pedology)
• Wikipedia: Leaching (agriculture)
• Wikipedia: Eluviation
• Wikipedia: Illuviation
• Wikipedia: Podzol
• Wikipedia: Laterite
• Wikipedia: Soil acidification
• Wikipedia: Liming (soil)
• Wikipedia: Leaching (chemistry)
• Wikipedia: Heap leaching
• Wikipedia: Ultisol
• Wikipedia: Alfisol
• Wikipedia: Oxisol
• Wikipedia: Soil salinity
• Wikipedia: Leachate
• Wikipedia: Methemoglobinemia
• UPSC: Civil Services Examination question papers
• Central Ground Water Board: Annual Ground Water Quality Report, 2025, State-wise details of nitrate
• PIB: Groundwater Depletion, Over-Exploitation and Quality Monitoring, 2025