Agriculture Reference
In-Depth Information
little is known about rates of pyrite formation under natural conditions indicates
rates of a few kg Sm
−
3
of sediment in 100 years under mangrove vegetation in
stationary or slowly aggrading coastal plains.
Potential acid sulfate soils ripen into actual acid sulfate soils as a result of
drainage and oxidation of the pyrite, forming sulfuric acid. The reaction between
pyrite and oxygen is slow, but oxidation of FeS
2
by Fe(III) in solution is fast
producing Fe(II). This process is catalysed at low pH by the bacterium
Thiobacil-
lus ferrooxidans
which mediates the oxidation of sulfur species and Fe(II), so
regenerating Fe(III) and facilitating further FeS
2
oxidation. The process requires
acid conditions because Fe(III) is insufficiently soluble at pH greater than about 4
and because the growth of
T. ferrooxidans
is inhibited at higher pH. The overall
reaction is
4FeS
2
+
15O
2
+
14H
2
O
→
4Fe
(
OH
)
3
+
8SO
4
2
−
+
16H
+
(
7
.
3
)
Most of the Fe(III) eventually crystallizes as reddish-brown ferric oxide in mot-
tles, coatings and nodules. Under strongly oxidizing severely acid conditions,
pale yellow coatings of the mineral jarosite, KFe
3
(
SO
4
)
2
(
OH
)
6
, may form on
ped faces. At higher pH, jarosite is hydrolysed to goethite. Hence ripe acid
sulfate soils often have a layer of yellow jaorosite mottling adjacent overlying
a still-reduced pyrite layer but overlain by layers from which acidity has been
leached, and hence dominated by reddish-brown goethite. These features are used
to assess the ripening of the soil.
7.2.2
IRON TOXICITY
Iron toxicity is a syndrome of disorders associated with large concentrations of
Fe
2
+
in the soil solution. It is only found in flooded soils. A wide range of con-
centrations produce the symptoms, from 1000 to only 10mg L
−
1
in soils with
poor nutrient status—especially of P or K—or with respiration inhibitors such as
H
2
S. There are large differences in tolerance between rice varieties. The effects
include internal damage of tissues due to excessive uptake of Fe
2
+
;impaired
nutrient uptake, especially of P, K, Ca and Mg; and increased diseases associated
with imbalanced nutrition, such as brown leaf spot (caused by
Helminthospo-
rium oryzae
), sheath blight (caused by
Rhizoctonia solani
) and blast (caused by
Pyricularia oryzae
).
The circumstances of the toxicity are quite well established, though some of
the details of the mechanisms involved are uncertain. Three main groups of Fe
toxic soils are distinguished:
acid sulfate soils, in which extremely large concentrations of Fe
2
+
in the soil
solution arise as a result of the soils' peculiar mineralogy;
•
•
poorly drained sandy soils in valleys receiving interflow water from adjacent
higher land with highly weathered sediments; and
