Agriculture Reference
In-Depth Information
each other and so the distinction between permanent and pH-dependent charge
is blurred.
Organic Matter
Soil organic matter is a weak acid and becomes negatively charged by losing pro-
tons. The main functional groups are carboxylates and, to a lesser extent, phenols,
which are weaker acids. Their acid-base behaviour is complicated because of
their heterogeneity and because of the effects of neighbouring functional groups
on soil surfaces. With increasing dissociation, the build up of negative charge
on the surface tends to inhibit further dissociation. Thus a plot of the extent of
dissociation of organic functional groups versus pH tends to be steeper than the
equivalent plot for simple monoprotic acids, and it approaches a straight line over
the usual pH range in soils. This leads to the following rough empirical relation
for the negative charge on soil organic matter as a function of pH (McBride,
1994):
SOM charge
(
mmol
c
g
−
1
organic C
)
=−
0
.
6
+
0.5 pH
(
3
.
41
)
As a rule of thumb, at near neutral pH, each g of organic C per kg of soil
increases the surface negative charge by about 3 cmol
c
kg
−
1
soil.
A further complication is that soil organic matter becomes more soluble at
higher pH as dissociation increases the surface negative charge. Also, organic
matter may form coordination complexes with some metals involving cova-
lent bonds.
3.6 THE SOLID SURFACES IN SUBMERGED SOILS
Many submerged soils are developed in recent in alluvium and are often young
or only weakly weathered (Section 1.3). The overall composition of the clay
fraction is therefore often close to that of the parent sediment. Hence the following
generalizations can be made for rice soils in the humid tropical lowlands (Kyuma,
1978; Binkman, 1985)
•
Soils derived from marine alluvial sediments tend to be dominated by mont-
morillonitic 2:1 clays whereas those from riverine sediments have vermiculitic
2:1 clays with mixtures of 1:1 clays and metal oxides, the sediment being
developed under more strongly weathered conditions.
•
Soils developed in positions higher in the landscape tend to be dominated by
more-weathered material.
•
Soils derived from basic volcanic ejecta, metamorphic rocks and granitic rocks
have corresponding mineralogies.
Various changes in mineralogy are induced by seasonal flooding. The first
factor in this is the change in base status of the soil due to the flow of water
