The Story of Organic Matter Decay in Soils Essay -- Chemistry Chemical

The Story of Organic Matter Decay in Soils To most people, humus is a garlic-y chickpea spread for sandwiches or pita bread. However, to soil scientists, its something entirely different. Soil humus is a mixture of dark, colloidal organic compounds relatively resistant to decomposition. These compounds result from the decay of organic litter and accumulate in the O and A horizons of soils. Soil humus helps glue mineral particals into aggregates, giving structure to the soil and affecting soil stability. There are three main classifications of humus: fulvic acid, humic acid and humin. Humin is insoluble but fulvic and humic acids are soluble in dillute NaOH solution. Humic acids precipitate in acidic solution, but fulvic acids remain soluble. Humic molecules are incredibly varied in composition, but generally are characterized by: 1) many active chemical functional groups exposed to the surrounding solution for reaction with other substances in the solution. 2) a very large cross-linked and "folded" molecule with molecular weights in the hundreds of thousands of grams per molecule" (Miller and Donahue, 1990, p. 185). Humus has a large surface area per unit of mass and is highly charged (similar to clay), and individual humus molecules are dynamic and constantly changing form (but may remain humus for several thousand years). Humus includes sugar amines, nucleic acids, phospholipids, vitamins, sulfolipis, polysaccharides and many other unclassified compounds (Miller and Donahue, 1990). Figure 1 shows a hypothetical structure of a humic acid with many of the characteristic functional groups. Fulvic acid and humin have similar structures. The COOH and phenolic OH groups are weakly acidic, which give humus its pH b... ...II, the environmental conditions alone may determine the rate. (Melillo et al., 1989) Aber et al. supported this conclusion, but measured major differences in the nitrogen content during Phase II, indicating that only the carbon was uniform during this stage (Aber et al, 1990). Other researchers had also separated two stages of decomposition: early decay, when "free" cellulose decomposes quickly, and later decay, when cellulose is shielded by lignin and can be decayed only with the coprocessing of the lignin (Aber et al., 1990). In summary, through a series of complex reactions, microbes mediate the conversion of organic material such as leaves and twigs to the dark humus which colors and glues our soils. Without these processes, the soils would be loose, non-cemented dusts and no life would be able to grow on them, and the world would be a very different place.