Soil Science

Waterlogging & Iron Mobilisation

In aerobic soils, most iron occurs as Fe in highly insoluble minerals like the red mineral hematite (Fe2O3) and the yellow-brown mineral goethite (FeOOH).

However, under waterlogged conditions, Fe minerals become more soluble and mobile in the soil.

Soils that are waterlogged semi-permanently exhibit dull grey or bluish-grey colors.

These soil morphological features are termed:

The correct answer is: GLEYING. Gleying is typically found in soil that is subject to frequent long periods of waterlogging, for example, in the lower parts of soil profiles in marshland or adjacent to streams.

In the following screens, you will explore the processes that take place when rainfall causes a texture-contrast (duplex) soil to be intermittently waterlogged.

The first event is the rapid infiltration of rainwater into the sandy A horizon and its accumulation in the soil profile.

Click on the zone in the soil profile that is most likely to become waterlogged.

Water percolates through the sandy A horizon until its progress is impeded by the relatively impermeable clay B horizon. Water therefore accumulates in the lower part of the A horizon. 

Microorganisms in the waterlogged zone continue to respire and therefore rapidly deplete the available ? ?

Water contains a small amount of dissolved O₂. Respiration converts this to CO₂. Once used up, oxygen cannot be replenished by diffusion from the atmosphere as the soil pores are filled with water.

Respiration in waterlogged soils does not cease when O₂ becomes unavailable. Many microorganisms are capable of anaerobic respiration, in which a range of substitutes for O₂ are used. One of these is Fe³⁺. In anaerobic respiration, Fe³⁺ is reduced from the +3 oxidation state to the oxidation state.

Respiration in waterlogged soils does not. cease when O, becomes unavailable. Many microorganisms are capable of anaerobic respiration, in which a range of substitutes for 02 are used. One of these is Fes+ In anaerobic respiration, Fe2+ is reduced from the +3 oxidation state to the ? oxidation state.

Fe is reduced from the +3 oxidation state (Fe³⁺) to the +2 oxidation state (Fe²⁺). The important consequence of this for soil formation is that Fe²⁺ minerals are much more soluble than soil Fe³⁺ minerals. Therefore, Fe minerals tend to dissolve under anaerobic conditions.
 

The Fe²⁺ dissolves in water and moves with it.

As the soil dries, O₂ once more moves through the soil. Fe oxides and hydroxides precipitate because Fe is oxidized back to the ? oxidation state.

Fe²⁺ oxidizes to Fe³⁺, which precipitates. In this way, some areas of the soil become depleted of Fe, while others become enriched in Fe.

After many reduction and oxidation cycles, the intermittently waterlogged horizon from which Fe is removed becomes pale or even bleached.

This horizon is given the designation ?

In Australia, this pale or bleached horizon is called an A2 horizon, and is equivalent to the E horizon in the USA.
  

Zones of Fe accumulation are indicated by patches of red, orange, or yellow-brown in the soil, known as ?

Mottles are patches of different colors in soil. If sufficient Fe accumulates, the mottle will harden, and is then known as a concretion of ironstone or laterite.

 

Mesa with a cap of laterite

Waterlogging and Fe mobilization were widespread in Australia during the warm and wet Tertiary period when much of the continent was covered by rainforest.

The dissected remains of these soils, including the pallid zones of Fe depletion and lateritic zones of Fe accumulation, can be seen today in arid central Australia.

 

 

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