Agriculture Reference
In-Depth Information
and hemists are intermediate. Equivalent terms in other classifications are peats,
mucks and mucky-peats, respectively.
By virtue of being unconsolidated and structureless in comparison with mineral
soils, organic soils have much smaller bulk densities, greater porosities and water
contents
(>
80%
)
, and smaller load-bearing capacities. These factors make their
artificial management highly problematic.
Organic wetland soils also tend to be poor chemically. Organic soils tend
to form under nutrient deficient conditions, which limit organic matter decom-
position. This occurs particularly in pluvial wetlands where the only nutrient
inputs are from rainfall and biological fixation from the atmosphere. Although
the organic material may have a high cation exchange capacity by virtue of the
charged functional groups on its surfaces, the exchange capacity tends to be
dominated by H
+
ions and the soil is acid. The acidity is an inevitable conse-
quence of the circumstances in which organic soils form. By contrast, mineral
wetland soils tend to have pHs near neutral as a result of electrochemical changes
accompanying soil reduction.
1.3.2 MINERAL SOILS
The most productive wetlands are on mineral soils, often developed on alluvial
deposits in fluxial wetlands. Nutrients and fertile sediments seasonally flow into
these areas under high rainfall and surface water flow.
Under prolonged submergence, mineral soils develop so-called redoximorphic
features associated with anaerobic soil metabolism (Figure 1.3). As oxygen is
excluded by submergence, soil organisms must use other soil constituents as their
oxidizing agents in deriving energy from organic matter. This typically occurs in
the sequence: nitrate ions to nitrogen, manganic manganese to manganous, ferric
iron to ferrous, and then sulfate ions to sulfide. Subsequently organic matter
is decomposed by methanogenic bacteria to carbon dioxide and methane. This
sequence is predicted by thermodynamics.
The most visible change associated with this process is the reduction of the
red and brown compounds of ferric iron to blue-grey compounds of ferrous iron.
Subsequent translocation of soluble ferrous iron to zones where oxygen enters the
soil-such as at the soil surface or near plant roots-produces reddish-brown mot-
tles of insoluble ferric iron. Likewise there may be movement and re-oxidation
of manganous manganese forming black manganic compounds. These changes
produce the characteristic redoximorphic features of submerged mineral soils.
The soil profile that develops under prolonged submergence is sensitive to the
nature of the water saturation. In very wet areas where the soil is perennially sat-
urated, the profile may be largely reduced throughout with little development of
distinct pedogenic horizons. Such conditions arise in tidal marshes, lake margins,
floodplains or in wet footslope areas. In better-drained areas where the flooding
