Stabilizing effect of organic matter

by T.B.S. Christopher 1996

Organic matter is perhaps the life blood of the soil. One of the reasons why the model C-P-OM proposed by Edwards and Bremner (1967) has gained wide acceptability is this model links the two most important soil constituents: organic matter (OM) and clay (C). Together with clay, organic matter forms the seat of soil reactions (Brady, 1990). Organic matter and clay, either directly or indirectly, influence various chemical, physical and biological reactions (Brady, 1990; Stevenson, 1982). It was once said that just as photosynthesis is important to a plant, the same is organic matter to a soil (Jacks, 1963). Although this statement was considered excessively sensational by some, nevertheless, it does highlight the importance of organic matter.

This is because the effect of organic matter on a soil can be dramatic. Addition of organic matter can improve virtually almost all soil properties: looser and more porous soil, lower bulk density, higher water-holding capacity, greater aggregation, increased aggregate stability, lower erodibility, greater soil fertility, increased CEC, and so on.

However, to sensationalize organic matter at this point is inappropriate. The beneficial effects of organic matter are neither infinite nor indiscriminate. Because soils have limited abilities to accumulate organic matter (Allison, 1973), this means there is a limit whereby further addition of organic matter will not further improve aggregate stability. Organic matter also discriminates between soils. The effect of organic matter is greater in more poorly structured soils than in well structured soils. Adding organic matter to already stable or clayey soils may improve little their aggregate stabilities, as observed by some workers (Fortun et al., 1989; Mbagwu et al., 1993).

The beneficial effects of organic matter are also inconsistent. The effects of organic matter depend on the amount and type of organic matter (Greenland, 1971, 1981; Hamblin and Greenland, 1977). Greenland et al. (1975) discovered that for a range of English and Welsh soils, the "critical" amount of organic matter needed for no slaking and dispersion of aggregates was 4%. In addition, de Haan (1981) commented that all types of organic matter, irrespective to their amounts, would be beneficial to soil structure but only in short term. It is the long term period that ultimately determines whether a type of organic matter is useful. This is the test that decides the capability of an organic matter type to produce sufficient humus needed to sustain its benefits (de Haan, 1981).

On the other hand, Quirk and Panabokke (1962) showed it isn't organic matter per se that increases aggregate stability but the disposition (or distribution) of organic matter. They discovered to increase aggregate stability, the organic matter must settle in certain pore class to strengthen the walls of these pores. Porosity relates directly to aggregate stability because some pores act as conduits or fault lines of aggregate failure (Quirk and Panabokke, 1962). Any aggregate breakdown will occur along these fault lines. Therefore, if these pore walls are strengthened by the strategic placement of organic matter, then the stability of aggregates increases (Quirk and Williams, 1974). Correspondingly, Quirk and Williams (1974) and Carr and Greenland (1972) showed that organic matter placed in pores of 15-50 m m was most effective in increasing aggregate strength.

It is, however, the conceptual model presented by Tisdall and Oades (1982) that is most widely used for understanding the relationship between aggregate stability and organic matter. Tisdall and Oades (1982) proposed three types of cementing agents, operating at different stages, responsible for stability: a) transient agents of microbial- and plant-derived polysaccharides that decompose rapidly, b) temporary agents, including roots, hyphae, especially mycorrhizal, and c) persistent agents of aromatic humic materials associated to amorphous Fe and Al compounds, as well as to polyvalent metal cations.

Transient and temporary binding agents are associated with the stability of macroaggregates. Transient agents like polysaccharides are produced rapidly (Harris et al., 1966), and are responsible for the initial increase in aggregate stability when organic matter is first added to a soil (Bonneau and Levy, 1979; Guckert, 1973). These binding agents, however, are decomposed rapidly by microorganisms. Temporary binding agents can stabilize macroaggregates because roots and fungal hyphae, for example, are large and that they grow in the large pores in the soil (Jackson, 1975; Marshall, 1976). Persistent binding agents, however, are responsible for the stability of microaggregates. These agents are relatively permanent and are unaffected by changes to organic matter content or to management practices. Persistent binding agents probably include complexes of C-P-OM as described by Edwards and Bremner (1967).

Consistent with this conceptual model described by Tisdall and Oades (1982), Waters and Oades (1991) used scanning electron microscopy to confirm that soil aggregates are stabilized in increasingly larger units by different binding agents. Waters and Oades (1991) proposed four main stages or levels in the aggregate hierarchy: <20 m m, 20-90 m m, 90-250 m m, >250 m m. Macroaggregates >250 m m are stabilized by roots and hyphae; microaggregates 90-250 m m have a nucleus of recognizable plant debris encrusted with inorganic components; microaggregates 20-90 m m, however, have fewer distinct organic entities, have more voids presumably left over after biological attack, and aggregates in this level began to show clay morphology; and microaggregates <20 m m are clay microstructures, and have no observable organic entities.

This hierarchy proposed by Waters and Oades (1991) does not oppose, but rather develops the aggregate hierarchy presented by Hadas (1987). The former model, however, is better as it relates not only the aggregation of clay particles to form progressively larger aggregates, but it relates the bonding of clay particles to the organic and inorganic components as well.

Nevertheless, even without understanding the mechanism of stability, researchers have for decades attempted to find a strong and consistent relationship between organic matter and aggregate stability. Some workers (Chaney and Swift, 1984; Christensen, 1986) found a significant correlation between organic matter and aggregate stability; others (Dormaar, 1983; Hamblin and Greenland, 1977; Hamblin and Davies, 1977) observed it is the fractions of organic matter, rather than total organic matter that are important to aggregate stability. There are also differences in opinion regarding which organic matter constituents are responsible for aggregate stability. Acton et al. (1963) and Mehta et al. (1960) noted that polysaccharides correlated positively with aggregate stability, but Chaney and Swift (1984, 1986) ascribed it to humic acids. Fortun et al. (1989), however, found that the combination of humic acid and fulvic acid is most effective in increasing aggregate stability. In Malaysian soils, fulvic acids fare better than humic acids (Soong, 1980; Tajuddin, 1992).

It would be a mistake to believe that these inconsistent correlations contradict or challenge one work against another. Believing so assumes organic matter must only behave in a certain way. Rather these correlations express the diverse nature or behaviour of organic matter as mentioned earlier. The relationship between aggregate stability and organic matter is therefore dependent on the following: a) total organic matter, b) type of organic matter used, c) one or more of the organic matter constituents, e) disposition of the organic matter in the aggregates, and f) the aggregate sizes being studied.

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