Humic substances

by T.B.S. Christopher 1996

Humic substances constitute 70-80% w/w of the organic matter in most mineral soils (Schnitzer, 1989). However, it is the fractions, rather than the whole of humic substances that are of immense interest. This is because each fraction expresses several distinct and important properties. Each fraction has a strong ability to interact with metal ions, oxides, hydroxides, minerals and other organic substances to form water-stable associations (Schnitzer, 1989).

Humic substances are divided into three crude fractions based on their solubilities in aqueous acids and bases: humin, humic acid and fulvic acid. It is important to realize that each of these three fractions, despite having a name denoting singularity, is not made up of a single pure compound. Each fraction is a heterogeneous mixture of organic substances having a wide range of molecular weights and negative charges (Hayes and Swift, 1990).

Because of the heterogeneity of compounds and ambiguities about the characteristics and structure of humic substances, it has led many to question whether humic substances of different soil types are similar to one another. All attempts to isolate single pure compounds and to identify their structures have failed (Hayes and Swift, 1990). This problem is akin to identifying the people in a stadium when all of them shouted out their names at once (Hayes et al., 1989). Although this problem may appear specific to only those whose work is to identify the structure of humic substances, it has nevertheless crossed over to the study of aggregate stability.

Comparing the overall efficiency between humic acid and fulvic acid on aggregate stability would become agonizingly difficult if, for example, fulvic acid in one soil is different from another soil. It is difficult to fully understand how aggregate stability is related to humic substances when the characteristics of humic substances and their compounds are still unclear; or worse still, whether the characteristics humic substances vary from one soil to another.

This problem is expressed in the conflicting opinions about which of the constituent of humic substances is more important (Fortun et al., 1989; Piccolo and Mbagwu, 1990). These conflicting opinions, however, can be grouped into two: opinions derived from the context of temperate soils, and those derived from the context of tropical soils. In temperate soils, humic acids are usually better in improving soil properties than fulvic acids, but in tropical soils including Malaysian soils, fulvic acids usually fare better (Soong, 1980; Tajuddin, 1992; Theng et al., 1989). This dual efficiency inevitably leads to this question: Is humic acid or fluvic acid in tropical soils similar to their respective counterparts in the temperate soils?

Schnitzer (1977), however, found that the humic substances from arctic, temperate, subtropical and tropical soils are structurally alike. Many other researchers also discovered that humic substances from varying climates, soil types and management practices were generally similar (Capriel et al., 1992; Chen et al., 1978; Fründ et al., 1989). However, humic substances from very different environments such as humic substances from soil, soil interstitial waters, streams, groundwaters, sea and ocean waters were widely different from one another (Malcolm and McCarthy, 1991).

If humic substances from different soils are structurally similar, why then this dual efficiency of humic acid and fulvic acid in temperate and tropical soils? Humic acid or fulvic acid may perform better simply because the amount of one is more than another (Soong, 1980). Humic acids may perform better in temperate soils than in tropical soils because the amount of humic acids is generally more in temperate soils. In the same way, fulvic acids may perform better in tropical soils because they occur in greater amounts in tropical soils than in temperate soils (Theng et al., 1989). In Peninsular Malaysia, for example, fulvic acids consist 75-90% of the organic carbon (Zainab, 1977). One reason why there is more fulvic acid in tropical soils is that tropical soils have a higher organic matter turnover rate (Greenland et al., 1992). Following the degradative genesis of humic substances, humic acids would be converted to fulvic acids faster in tropical soils than in temperate soils (Mendonça et al., 1991).

On equal amounts, however, no one is certain which, humic or fulvic acid, is better. Interestingly, Soong (1980) is perhaps the only person who attempted to compare the influence of humic acid and fulvic acid separately and on equal amounts on aggregation. In a 1:1 pure clay-sand mixture, Soong discovered at lower rates of addition (less than about 0.80% w/w), fulvic acid was more superior than humic acid in promoting aggregation. But at higher rates (more than about 0.80% w/w), while the aggregating effect of humic acid surged upward, the effect of fulvic acid, on the other hand, saturated. However, a cautionary note is appropriate here; this so-called critical rate of 0.80% w/w is not an accurate value. Soong (1980) used too few rates (one control and four rates of addition) to accurately plot a curve line graph. These two points undermine the actual relationship and performance between humic acid and fulvic acid on aggregation.

Theory appears to conflict with experimental results. Theory suggests, for two broad reasons, fulvic acids ought to be better than humic acids. For one, fulvic acids are smaller, more highly charged and more polar than humic acids (Hayes, 1991). Repulsion between the more closely arranged negative charges on the strands cause the macromolecules to shape linearly, rather than the spherical shape of humic acids. Large humic acids are ineffective as binding agents because they are too big, are spherical instead of linear, and because of their spherical shape, the functional groups may not be free or be expose enough to attach themselves to clay particles (Allison, 1968). Linear shapes are analogous to a network of ropes where the clay particles string along the lines.

Secondly, fulvic acids are more acidic than humic acids due to their higher concentrations of carboxylic functional group. Together with carboxylic groups, phenolic-OH groups are the major contributors to cation exchange capacities of the organic matter, as well as to the complexation of metals (Hayes and Swift, 1990). Carboxylic groups, however, are more reactive than the phenolic-OH groups because carboxylic groups dissociate easier. Martin and Aldrich (1955) attributed a more positive role of carboxyls to aggregate stability, but Clapp et al. (1962) remarked carboxyls had no effect.

Previous experimental results, however, have shown that humic acids are the better one. Dell'agnola and Ferrari (1971) discovered that the humic substances in stable aggregates have higher molecular weights (> 100, 000) than the humic substances in the unstable aggregates. These higher molecular weights are dominated by humic acids, and that phenolic-OH groups here are more effective than carboxyls to promote aggregate stability. Larger contribution of higher weight humic fractions was also found by Piccolo and Mbagwu (1990). Moreover, Chaney and Swift (1984) found some evidence to indicate that less oxidized and higher molecular weight humic substances were better in aggregation than the highly oxidized and lower molecular weight substances. And there is always Soong's (1980) experiment as mentioned earlier. Although a slower start, humic acid, ultimately, had a longer and stronger effect on aggregation than fulvic acid.

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