How is material moved and then finally lost from the soil?

There is

1) movement of fine material particles to new geographical locations and

2) the movement of material within the soil.

Soils can develop on material that has slipped down hill slopes, sediment material deposited by rivers, and fine silty wind-blown material. The other occurs vertically and horizontally within the soil.

The movement of mineral and organic material down the profile is determined by how easily the material moves in water and the rate of the water movement. Mobile compounds move downwards, so that some parts of the soil profile become enriched. This movement at varying speeds produces a number of distinct layers; referred to as ‘soil horizons’.

Under acid conditions, the upper O Horizon is sometimes visible as a dark-coloured band which can be further divided into 3 separate layers referred to as Litter, Fermentation and Humic.

The other extreme is under neutral-to-alkaline soil conditions, where a combination of readily mineralized residues and high numbers of soil organisms rapidly break down the organic matter and mix it with the underlying mineral layers. Under these conditions there is often no distinctive black-coloured O Horizon and no Litter, Fermentation or Humic subhorizons.

Movement can be in either solution or suspension, and is thus ‘leaching’ or ‘eluviation’. ‘leaching’ involves the movement of soluble ions such as Ca2+ (Calcium), Mg2+ (Magnesium), Na+ (Sodium), K+ (Potassium), NO3- (Nitrate), NO2- (Nitrite) and complex ions based on SiO4+ (Silicates). In the case of iron (Fe), its solubility depends on whether it is an oxidised (Fe3+) or reduced (Fe2+) form.

As leaching continues, the more soluble compounds are completely washed out of the soil profile. Eluviation is the movement of insoluble particles such as clay minerals down the profile (lessivage). Lessivage is detected by a relative increase in clay content of the B Horizon when compared to either the A or E Horizon.

How can we explain soil formation?

Soil formation has 5 soil-forming factors: climate, parent material, topography, organisms (plants and animals) and time. These 5 factors determine the nature of the initial inputs, how they are transformed, and how quickly they are moved and lost from the soil.