Soil Science

Leaching

Macro view

Leaching is the removal of soil materials dissolved in soil water.

The soil profile shown here is in a high rainfall zone and is highly leached. It is low in soluble salts, acidic (pH = 4) and has high levels of exchangeable aluminium. Why does it have these properties?

Sub-micro view

The effects of leaching can be understood by looking at cation exchange processes.

The animation schernatically shows exchangeable cations adjacent to a negative colloid surface. Further away are the cations and anions of salts dissolved in the soil solution.

There is more material on exchangeable cations in the "Adsorbed Ions" module in the "Nutrient Cycling" section.

Soluble salts

The first (rapid) action of percolating rainwater is to remove soluble salts from the horizon.

If the entire profile is leached (eg in locations where rainfall exceeds evapotranspiration), then the whole soil becomes low in soluble salts.

However, if only the upper horizons are leached, (e.g., in drier areas where evapotranspiration demand prevents percolation below the root zone) then the lower parts of the soil will accumulate salts.

The animation below shows this process.

Exchangeable bases

Rain water is not pure water.

Among other things, it contains a small amount of the weak acid carbonic acid, produced from dissolved atmospheric carbon dioxide.

Over a long period, the hydrogen ions displace the other cations (exchangeable bases) from the exchange sites. The exchangeable bases are then leached from the horizon.

The animation below shows this process.

Exchangeable aluminium

Exchangeable H+ ions on clay surfaces are unstable and rapidly react to release aluminium ions from within the clay crystal lattice.

The liberated Al3+ ions replace most H+ ions on the exchange sites.

Both exchangeable Al3+ and H+ are acidic. Thus, the replacement of exchangeable bases with H+ and Al3+ lowers the pH of the horizon.

 

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