6.2.2 Leaching
The leaching of nutrients (i.e., movement with soil water below the root zone or into groundwater) depends on:
  • Water-holding capacity: A soil with high water retention (e.g., high clay or organic matter content) holds more water against gravity, reducing deep drainage and thus leaching potential.
  • Sorption/Buffering: Nutrients strongly adsorbed to soil particles (e.g., phosphate, potassium to clay) are much less prone to leaching than highly mobile anions in solution (e.g., nitrate, chloride).
  • Net percolation: The amount of water draining beyond the root zone. High rainfall or irrigation exceeding evapotranspiration and soil storage leads to high percolation, driving leaching.

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Buffer Power
Buffering capacity (b) indicates the extent of interaction between ions in solution and ions on the solid phase. Mobility (M) is inversely proportional to buffering capacity.,

                              M=1/b

A compound with a buffering capacity (b) of 2 (e.g., calcium) is displaced by 0.5 cm per 1 cm of net percolation (=1/2). A compound with a buffering capacity of 10 (e.g., phosphate) is displaced by 0.1 cm (=1/10).

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Water storing Capacity
The water percolation through soil is inversely proportional to the water-holding capacity of the soil.
  • High water-holding capacity (e.g., in clayey or organic- rich soils) means the soil can retain more water against gravity. This results in lower percolation rates, as more water is stored within the root zone.
  • Low water-holding capacity (e.g., in sandy soils) means the soil retains less water, leading to higher percolation rates, as water drains more quickly beyond the root zone.


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Precipitation, Soil Type
  • Precipitation or Irrigation adds water to the soil.
  • Water first satisfies soil water deficit and evapotranspiration (water used by plants and lost to evaporation).
  • Any excess water beyond what the soil can hold and plants can use becomes net percolation, moving downward through the soil profile.
  • This percolating water carries soluble nutrients (especially nitrate, sulfate, chloride) with it, causing leaching when they move below the root zone.

Not all precipitation causes leaching. Only water exceeding the soil's field capacity and plant demand leads to net percolation.
Heavy, intense rainfall is more likely to cause leaching than the same total amount of water applied gradually, as it exceeds the soil's infiltration and storage capacity more quickly.

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