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WILLIAM MACNAE
made available to other organisms by the action of sulphur bacteria
which, in the anoxic muds, corrode the sulphur-containing conchiolin
of the shells. I n this way the shells become more fragile and the water
has access to the crystals of calcium carbonate so raising the alkalinity
of the soil.
The quality of the soil in mangrove swamps depends ultimately on
the source of its alluvium. Rivers draining quartzitic areas carry an
alluvium of poor quality. Rivers draining ancient granitic areas also
carry silt of poor quality. Rivers draining areas of recent or moderately
recent volcanic soils produce an alluvium of high quality. The effects
of soil quality are evident, for example, in Java and in Queensland. I n
Java the Braiitas river draws its water from the volcanic mountains of
East Java and produces a highly productive alluvium (Schuster, 1952) ;
in Queensland the Barron and Johnstone rivers come from areas with
abundant Tertiary volcanics, and on their flood plains is produced
much of Queensland’s sugar and their estuaries support high and dense
mangroves. I n contrast the marl and limestone areas of the island of
Madura off north-east Java yield poor alluvium (Schuster, 1952) ; this
is true, too, of many of the rivers of Africa, a continent of old leachedout soils, and also the quartzites of much of coastal Queensland, which
support the scrub forest known as Wallum (Coaldrake, 1961), and
produce poor soils and low mangroves. Nutrients are scarce in such
soils.
But such sources cannot entirely account for the rapid deposition
of soils in certain areas. I n an area such as the Straits of Malacca silt
and alluvium may be derived from two sources. They may have an
origin in the erosive activity of the rivers of Sumatra and of Malaya.
But also some silt and alluvium may be derived from now submarine
sediments which had been laid down on river flood plains during the
Pleistocene and are now being eroded from narrow submarine gullies by
tidal currents. The fertility of this “ fossil ” alluvium will depend on its
history and origins before being deposited.
Alluvium deposited in an estuary or mangrove area is not of itself
productive. Macnae (1957) has commented that on the banks of the
Gaintoos estuary some 60 miles west of Port Elizabeth the intertidal
muds are barren since they have been deposited too recently. Schuster
(1952) has shown that biological processes are necessary before the
alluvium can be utilized by higher plants. These processes involve a
number of organisms, bacteria, blue-green algae, diatoms and green
algae. Important amongst these are nitrifying bacteria and sulphate
reducing bacteria. Schuster (1952) lists several species of algae, which
he argues are important food for prawns and fish (Chanos chanos (Forsk.)
WILLIAM MACNAE
made available to other organisms by the action of sulphur bacteria
which, in the anoxic muds, corrode the sulphur-containing conchiolin
of the shells. I n this way the shells become more fragile and the water
has access to the crystals of calcium carbonate so raising the alkalinity
of the soil.
The quality of the soil in mangrove swamps depends ultimately on
the source of its alluvium. Rivers draining quartzitic areas carry an
alluvium of poor quality. Rivers draining ancient granitic areas also
carry silt of poor quality. Rivers draining areas of recent or moderately
recent volcanic soils produce an alluvium of high quality. The effects
of soil quality are evident, for example, in Java and in Queensland. I n
Java the Braiitas river draws its water from the volcanic mountains of
East Java and produces a highly productive alluvium (Schuster, 1952) ;
in Queensland the Barron and Johnstone rivers come from areas with
abundant Tertiary volcanics, and on their flood plains is produced
much of Queensland’s sugar and their estuaries support high and dense
mangroves. I n contrast the marl and limestone areas of the island of
Madura off north-east Java yield poor alluvium (Schuster, 1952) ; this
is true, too, of many of the rivers of Africa, a continent of old leachedout soils, and also the quartzites of much of coastal Queensland, which
support the scrub forest known as Wallum (Coaldrake, 1961), and
produce poor soils and low mangroves. Nutrients are scarce in such
soils.
But such sources cannot entirely account for the rapid deposition
of soils in certain areas. I n an area such as the Straits of Malacca silt
and alluvium may be derived from two sources. They may have an
origin in the erosive activity of the rivers of Sumatra and of Malaya.
But also some silt and alluvium may be derived from now submarine
sediments which had been laid down on river flood plains during the
Pleistocene and are now being eroded from narrow submarine gullies by
tidal currents. The fertility of this “ fossil ” alluvium will depend on its
history and origins before being deposited.
Alluvium deposited in an estuary or mangrove area is not of itself
productive. Macnae (1957) has commented that on the banks of the
Gaintoos estuary some 60 miles west of Port Elizabeth the intertidal
muds are barren since they have been deposited too recently. Schuster
(1952) has shown that biological processes are necessary before the
alluvium can be utilized by higher plants. These processes involve a
number of organisms, bacteria, blue-green algae, diatoms and green
algae. Important amongst these are nitrifying bacteria and sulphate
reducing bacteria. Schuster (1952) lists several species of algae, which
he argues are important food for prawns and fish (Chanos chanos (Forsk.)