Leaching of Soil: Process, Eluviation, Illuviation and Effects

What the Leaching of Soil Means

Leaching and the Movement of Water in the Soil

The leaching of soil is the removal of soluble materials from one zone in the soil to another by water moving down the profile. As rain and irrigation water sink in, they dissolve soluble matter and carry it downward.

Water is an effective solvent in the soil. After the surplus gravitational water drains away, it takes dissolved nutrients with it, so leaching steadily strips the topsoil of its soluble wealth and redeposits part of it lower down.

Why Leaching of Soil Matters

Why it matters is that leaching helps decide how fertile a soil will be. By washing soluble bases and nutrients out of the reach of roots, it can leave a poorer, more acidic topsoil over time.

Leaching is also a great soil-builder. The same downward washing creates the podzols and laterites of the world, and in farmland it carries nitrates into the groundwater, linking the soil to water pollution.

How Water Redistributes Material in the Soil

Eluviation, Illuviation and Leaching Distinguished

Three linked movements describe how water shifts material down the soil. Eluviation is the transport of soil material out of the upper layers by the downward percolation of water, leaving a pale, washed E horizon.

The accumulation of that washed-down material lower in the profile is called illuviation, and the layer that gains it is a zone of illuviation. Leaching goes one step further, carrying the most soluble matter right out of the profile.

What Gets Leached from the Soil

Not everything moves at the same rate. The most readily leached are the soluble bases, calcium, magnesium, potassium and sodium, together with soluble salts and, above all, nitrates, which the soil scarcely holds.

With stronger leaching, fine clay colloids and even iron and aluminium are stripped from the topsoil. The pale eluvial horizon left behind is clay-depleted and rich in resistant quartz sand and silt.

The Factors that Control Leaching

Climate, Rainfall and the Water Balance

The master control is the water balance. Leaching occurs only where precipitation exceeds evaporation, so that surplus water actually drains down through the soil and out of the profile.

Humid climates therefore leach heavily, while in dry climates evaporation pulls water and salts back upward, and little is lost. The amount and seasonality of the rainfall set how strong the leaching will be.

Soil Texture, Vegetation and Relief

Soil properties decide how fast the water drains. A coarse, sandy, permeable soil lets water pass quickly and leaches fast, while a heavy clay holds water and resists leaching.

A dense vegetation cover takes up nutrients and shields the soil, reducing loss, whereas bare ground leaches freely. Gentle slopes with free drainage and warmth speed the solution and movement of material.

Leaching and the Formation of World Soils

Podzolisation and the Podzol

In the cool, humid forests of the north, intense leaching produces podzolisation. Acidic water washes iron and aluminium down out of the topsoil, leaving the pale, ashy-grey layer that gives the podzol its name.

The result is a strongly leached, sandy and rather infertile soil. Podzolisation is the clearest case of leaching dominating soil formation under a cold, wet climate.

Laterisation and the Laterite

In the hot, wet tropics, leaching takes a different course called laterisation. The heavy rain leaches out the silica and the soluble bases, leaving behind a soil rich in iron and aluminium oxides.

This residual concentration of iron gives laterite its rusty-red colour and its hardness on drying. Despite the lush forest, the deeply leached soil is poor in plant nutrients.

Where Leaching is Limited: Calcification

Leaching is not universal. In the semi-arid grasslands, evaporation is high and rainfall is modest, so water does not drain right through the soil and the bases are not washed away.

Instead, lime accumulates in the profile in a process called calcification, building the deep, base-rich and fertile chernozem. The contrast shows that it is the climate, through the water balance, that decides whether a soil leaches.

The Effects of Leaching

Soil Acidification and Loss of Fertility

Because leaching removes the soluble bases that keep a soil sweet, a heavily leached soil tends to turn acidic over time, as hydrogen ions come to dominate the exchange sites.

It also depletes the soil of the very nutrients that crops need, lowering natural fertility. Strongly leached tropical and forest soils are, for this reason, among the least fertile despite their warmth or moisture.

Nitrate Leaching and Water Pollution

The clearest modern effect is nitrate leaching. Because nitrate ions are not held by the soil, they move down freely with the drainage water and are leached into groundwater, streams and the sea.

High nitrate in drinking water can cause methaemoglobinaemia, the blue baby syndrome, while the extra nutrients drive eutrophication, a fall in the oxygen of the water that can kill fish.

Managing Leaching and UPSC Relevance

Why Leaching Matters for Land and Water

What is the significance of leaching is felt on the farm and in the river. It governs which soils are fertile, builds the great leached soil types, and decides how much of a farmer's fertiliser is lost.

The same loss of nutrients that thins a soil reappears downstream as polluted water, so leaching ties soil management directly to water quality.

Managing Leaching and the Exam

Contemporary linkages run from leaching to food security and clean water. Farmers limit it by liming acid soils, adding organic matter, timing fertiliser to crop needs and keeping the ground under cover crops.

For the exam, leaching is tested as a precise soil-geography process with clear causes and effects. The high-yield points are few and worth holding in mind.

• Leaching is the removal of soluble matter from the topsoil by percolating water.
• Eluviation removes material from the upper horizon; illuviation deposits it lower down.
• Leaching needs precipitation to exceed evaporation; it is strongest in humid climates and porous soils.
• Podzolisation builds podzols and laterisation builds laterites; both are leaching-dominated.
• Nitrate leaches readily into groundwater, causing pollution and eutrophication.

Sources and Further Reading

• Wikipedia: Leaching (pedology)
• Wikipedia: Leaching (agriculture)
• Wikipedia: Eluviation
• Wikipedia: Podzol
• Wikipedia: Laterite
• NCERT: Fundamentals of Physical Geography (Class XI)
• Indian Council of Agricultural Research
• Central Ground Water Board
• Food and Agriculture Organization (FAO) Soils Portal
• United States Geological Survey (USGS)