Transport Processes in Submerged Soils - The Biogeochemistry of Submerged Soils - page 18

2.1 MASS FLOW

Submergence greatly alters a soil's hydraulic properties. Following submergence,
air trapped in the pores inside aggregates becomes compressed. Further compres-
sion develops as volatile products of respiration accumulate in the pores and as
2:1 type clays swell. As a result, large aggregates tend to rupture. Further rup-
ture occurs as a result of the dissolution of organic matter and oxides, which
act as cementing agents within aggregates (Greenland, 1981). Hence in the first
few days following submergence, the permeability of the soil increases as gases
accumulate in the pores. But as the soil begins to disaggregate, the permeability
gradually declines. The decline accelerates as pores become clogged with dis-
persed clay and other debris. Allison (1947) found the decrease in permeability
was less if the soil was sterilized, indicating that the effects of microbes were
important, presumably because of increased disruption of aggregates with the
accumulation of respiratory gases and dissolution of cementing agents.
The extent of disaggregation varies greatly between soils and with the quality
of the water. In pure layer silicate systems with high pH or sodium saturation,
the disintegration of aggregates and dispersion of clays can be near complete.
Whereas in highly structured soils with large contents of organic matter or
hydrous oxides, aggregation may be little affected, although it may be easily
disrupted by subsequent application of force.
In wetland rice cultivation, further disaggregation is caused by the process of
tilling the soil when wet, which is an integral part of the land preparation prior
to transplanting. Wet tillage results in near complete destruction of water-soluble
aggregates and dispersion of fine clay particles. The aim is to reduce losses of
water through percolation, both to conserve water and to control weeds, and to
facilitate transplanting. Some flow through of water should be maintained so that
the soil does not become entirely anoxic. Also, if the structure is completely
destroyed the soil will dry only very slowly following the rice crop, and this
will delay the establishment of a following dryland crop. Table 2.1 shows the
effect of puddling on percolation rates in a range of flooded soils measured

Table 2.1 Effect of puddling on percolation rates in a range of flooded Philippine soils

Soil Mineralogy Clay Percolation rate

( cm day − 1 ) (%)

Unpuddled Puddled

Psamment Siliceous 9 267 0.45

Fluvent Mixed 24 215 0.17

Aquept Montmorillonitic 30 183 0.05

Aqualf Montmorillonitic 40 268 0.05

Ustox Kaolinitic 64 155 0.05

Andept Allophanic 46 214 0.31

Source : Sanchez (1976). Reproduced by permission of Wiley, New York.