Stage 0
Soil hydromorphism may be grouped in five degrees of increasing intensity.
Stage 0
Stage 0 represents a soil without any hydromorphism.
It deals with horizons with high chroma, without bleached mottles, saturated with water during only a few days, though they may be moist during several months, and in which an intense oxidation predominates.
Stage 1
Stage 1 is defined by a soil with a matrix with high chroma and short periods of saturation. It represents an incipient hydromorphism.
The soil shows high chroma but with development of mainly Mn nodules and coatings.
The hydrological regime of this soil is characterized by a complete saturation with water during a few days only, although the soil may be moist during several months. Under these conditions, the interior of the aggregates remains humid during a long time and especially Mn is reduced and mobilized, and when the soil dries, the solution migrates towards larger pores, which are completely dry and therefore constitute and oxidizing environment where the Mn is oxidized and covers the walls of the aggregates.
Stage 2
Stage 2 is defined by the presence of mottles with low chroma and short periods of saturation. This stage represents already a hydromorphic soils, referred to as a weakly developed pseudogley.
Its characteristic feature is the presence of clods with low chroma: the interior of the aggregates is bleached whereas their surface presents abundant Fe and Mn coatings and hypocoatings, and iron nodules .
These soils are only completely saturated during a few days, but this situation is repeated many times during the year, and the soil remains very humid during many months; as a result the micropores remain saturated during long periods and an important reduction of Fe and Mn takes place inside the aggregates. These elements move during dry periods to the surface of the aggregates where oxidation causes their immobilization. As Mn is more difficult to oxidize, most of it will be leached. The migration of Fe and Mn from the interior of the aggregates leaves there bleached zones.
Stage 3
Stage 3 is also defined by peds with low chroma, but with long periods of water saturation. It represents a strongly hydromorphic soil, although still considered as pseudogley.
Its micromorphological features are very distinct: the matrix has flecks with low chroma (<2), the interior of the aggregates being partially bleached, whereas the surface is totally bleached, forming Fe-depletion hypo-coatings; between both zones one sometimes observes Fe-quasi-coatings and abundant Fe-nodules in the more internal parts of the aggregates.
The figure shows two micrographs representing two evolutive phases corresponding to this stage, with an initial phase in which the surface of the aggregates is not yet completely bleached (a very frequent situation in the Spanish hydromorphic soils) and another stage in which Fe-depletion hypo-coatings are extensively developed. With respect to the hydromorphic conditions one is dealing with a soil already saturated during long periods (several months) and which remains very humid, near to saturation, during practically the whole year (as well in macro- as micropores). Under these conditions the environment is clearly reducing, and the solutions migrate now from the large pores to the interior of the aggregates which are less saturated and constitute therefore a partially oxidizing environment (this means that the solution moves in the opposite direction as described for stages 1 and 2). This distinctive hydrological regime is expressed by a different morphology. By moving towards the interior of the aggregates, the solutions reduce and mobilize all Fe and Mn present, leaving a leached zone (Fe and clay depleted hypo-coatings) and precipitates part of the Fe more towards the interior, forming quasi-coatings. Besides, a large part of the Fe (and naturally of the Mn) are leached from the profile, leaving a bleached matrix.
Stage 4
Stage 4 represents a soil with a maximal development of
hydromorphism:
it is a gley with a totally bleached matrix, a chroma <1 and a total reduction of Mn and Fe due to a practically continuous saturation with water during the whole year.
COMPARATIVE STUDY OF THE FIVE STAGES
Hydromorphic conditions
