Agriculture Reference
In-Depth Information
and this is characteristic of most soils following flooding (Ponnamperuma, 1972).
The minimum can be less than zero and can be accompanied by evolution of H 2
gas. It is due to fermentation reactions starting as soon as O 2 and NO 3 are used
up but before populations of Mn and Fe reducing bacteria are established. As
discussed in Section 5.3, the low solubility of Mn(III, IV) and Fe(III) oxides may
initially limit the rate of their reduction. Organic acids produced in fermentation
reactions will help dissolve Mn(III, IV) and Fe(III) from oxide particles and
thereby facilitate the establishment of the Mn and Fe reducers. As Mn and Fe
reduction then proceed, the pe will increase to values corresponding to the Mn
and Fe couples involved, and then gradually decline.
Simultaneously H + ions are consumed in Reactions (4.28) - (4.31) and the
pH tends to increase. Initially the pH of aerobic soils may decrease following
submergence because CO 2 formed in aerobic respiration escapes from the soil
only very slowly, and it therefore accumulates. As CO 2 continues to accumulate
during anaerobic respiration and fermentation, large partial pressures develop,
typically in the range 5 to 20 kPa. The accumulation of CO 2 lowers the pH of
alkaline soils and curbs the increase in pH of acid soils. As a result the pHs of
all soils tend to converge following submergence in the range 6.5 - 7.
As the partial pressure of CO 2 increases, the concentration of HCO 3 in the
soil solution increases and therefore the concentrations of balancing cations in
solution increase. Changes in alkalinity and concentrations of cations in solution
following submergence are shown in Figure 4.6. The NH 4 + ,Mn 2 + and especially
Fe 2 + ions formed in soil reduction displace exchangeable cations into solution.
25
Alkalinity
20
Ca 2 +
Mg 2 +
+
+
NH 4 +
Na +
K +
+
+
15
Fe 2 +
Mn 2 +
+
10
5
0
02468 0 2 4 6
Time (weeks after submergence)
Figure 4.6 Changes in alkalinity and concentrations of cations in the soil solution fol-
lowing submergence (Ponnamperuma, 1972)
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