Agriculture Reference
In-Depth Information
certain soils and thus limits crop production. The amount and form of S present within some soils,
especially in humid regions, is related to the quantity and type of OM since much of the S is in
organic forms (David et al., 1982). In general, a linear relationship has been demonstrated between
the content of SOM and the content of total S in surface soils (Freney, 1986). Total soil S decreased
in soils where no organic material was added (Kirchmann et al., 1996).
4.6.2 a
vaIlaBIlItY
of
m
ICronutrIents
OM plays a key role in the soil micronutrient cycle. Knowledge of the nature of the organic ligands
that form complexes with metal ions and of the properties of the complexes thus formed will lead
to a better understanding of the factors that affect trace element availability to plants (Stevenson,
1991). Organic chemicals with two or more functional groups that can bind with metals to form a
ring structure are known as chelating agents (Soil Science Society of America, 2008). OM fractions
such as FAs can form chelate structures with some metals. These chelates can bind micronutrients
such as copper, iron, zinc, and manganese and improve their availability to plants.
Stevenson (1991) summarized the formation of metal-organic complexes that have the following
effects on the soil micronutrient cycle: (i) micronutrient cations that would ordinarily precipitate
at the pH values found in most soils are maintained in solution through complexation with soluble
OM. Many biochemicals synthesized by microorganisms form water-soluble complexes with trace
elements. Complexes of the trace element with FA are also water soluble, (ii) under certain con-
ditions, metal ion concentrations may be reduced to a nontoxic level through complexation with
soil OM. This is particularly true when the metal-organic complex has low solubility, such as in
the case of complexes with HA and other high-molecular-weight components of OM, (iii) various
complexing agents mediate the transport of trace elements to plant roots and, in some cases, to
other ecosystems, such as lakes and streams, (iv) organic substances can enhance the availability of
insoluble phosphates through complexation of Fe and Al in acid soils, and Ca in calcareous soils,
and (v) chelation plays a major role in the weathering of rocks and minerals. Lichens, for example,
enhance the disintegration of rock surfaces to which they are attached through the production of
chelating agents.
Loeppert and Hallmark (1985) reported that correlation coefficients for visual evaluation of iron
deficiency in sorghum plants and SOM content were highly significant. In all cases, regression mod-
els indicate that the tendency toward chlorosis decreased with increasing OM content, suggesting
that OM may stabilize soil Fe in a form that is more readily available to the sorghum plant. Also, a
higher OM content would be conducive to a more active microbial population, which may cycle Fe
into the biosphere in a form that is more readily available to growing plants. It is well documented
that SOM can readily complex Fe
3+
(Schnitzer and Khan, 1972; Bloom, 1981) and is a good reduc-
ing agent (Szilagyi, 1971) at pH <4.0. There are only a few studies dealing with the Fe
3+
-OM com-
plex at pH >7.0. Mossbaur spectroscopy (Goodman and Cheshire, 1979) has provided evidence that
Fe
3+
on soil HA is readily hydrolyzed and precipitated as ferric hydroxide as the pH is increased
above pH 4.0. On the other hand, there is evidence that the marine Fe(OH)
x
-HA complex may be
soluble (Picard and Felbeck, 1976). Therefore, even though Fe
3+
may be hydrolyzed at pH >7.0, it
may be stabilized as absorbed Fe(OH)
x
species, which are more readily available to growing plants
(Loeppert and Hallmark, 1985). Cellulosic OM added before flooding increased amorphous Fe con-
centration in the flood soils for lowland rice production (Sah and Mikkelsen, 1986). Iron chlorosis
in grain sorghum, soybeans, and cowpeas occurs on calcareous soils in spots throughout the Great
Plains. One reason for this deficiency is that the OM content and cation exchange capacity (CEC) of
these soils is low compared with that of other soils in the region (Mathers et al., 1980). Manure sup-
plied Fe as an organic complex available to plants. Tan et al. (1971) found that extracts from poul-
try litter complexed Fe and other ions. Miller et al. (1969) demonstrated that poultry manure was
beneficial for correcting Fe deficiency in plants. An application of 11 tons ha
−1
of farmyard manure
produced larger-grain sorghum than a similar application of Fe from FeSO
4
(Mathers et al., 1980).
