Soil Organic Matter

Soil Organic Matter organic material, mainly of plant origin, from the natural flora, or animal residues added by man, and infiltrated into the surface horizons of the soil. These materials on decomposition give rise to humus. crop residues, weeds, grasses, tree leaves, worms, bacteria, fungi, actinomycetes, animal manures, municipal sewage sludge, logging and wood manufacturing refuse, industrial organic residues, and food processing residues are the main sources of organic matter. Organic matter in addition to containing elements essential for plants (17 elements for proper growth) also contains elements necessary for livestock and man. These are sodium, cobalt and iodine, which are essential for animals. Organic matter also contains traces of nickel, fluorine, gold and other elements.

In Bangladesh, alluvial deposits in depressions remain wet throughout the year, and as a result, organic matter gradually accumulates on the surface, derived from decaying aquatic vegetation or from the residues of crops such as boro rice that may be grown in such sites. This organic matter imparts a darker colour to the topsoil. In older depressions, organic matter may accumulate on the surface in the form of peat or muck. In soils that dry out seasonally, the organic matter content is related to the duration of wet conditions and to the age and use of the soils. The longer the period of seasonal flooding, the shorter the period during which rapid aerobic decomposition of organic matter can take place. In general, Bangladesh basin soils tend to contain more organic matter than associated ridge soils. Young floodplain ridge soils generally have 1% organic matter in the topsoil, so do well-aerated ridge soils on older floodplains. Older floodplain soils, except those on the highest ridges, generally have 1-2% organic matter in the surface layer, and topsoils in deep basin centres generally have 2-5%. Black Terai soils commonly have 3-6% organic matter in their thick topsoils. These older soils are believed to have formed under tall grassland or forest which provided more organic matter than do the present cultivated crops, the residues of which are often removed from the soils. The largest forests in Bangladesh such as the Madhupur Sal forest, Sundarban forest and the forested parts of the Chittagong Hill Tracts are vanishing rapidly, thereby decreasing the accumulation of organic matter in soil.

Available reports indicate that most soils of Bangladesh have a low organic matter content. About 70% of the net cultivable area in high and medium-high lands have a soil organic matter content of less than 2%. This low organic matter content is considered one of the main reasons for the low productivity of many of our soils. The positive roles of residues, including green manure on soils and crops have been well documented. In most instances, crop residues should be left on or returned to the soil. This general guide takes advantage of the benefits of organic matter in soil, its aggregation of soil particles, its improvement of water infiltration and aeration, its reservoir of plant nutrients, and its energy reservoir for microorganisms.

Generally, organic matter content of 3-8% or higher in the soil improves plant growth, but to a farmer, the bottom line of what is a good level of organic matter concentration bears on cost and convenience. The need for proper soil organic matter management should be emphasised in view of the low organic matter content of most soils in Bangladesh. [Md Khalilur Rahman]