22/04/2022
Land degradation assessment 2020.
4.2 Soil organic matter depletion
Soil organic matter (SOM) itself is composed of living biomass like microorganisms, dead tissue or partly decomposed materials, and stable, fully decomposed humus. SOM has long been suggested as the single most important indicator of soil productivity (Berner, 2006; Hijbeek, 2017). It contributes to soil productivity by influencing the physical, chemical and biological properties of soil. It serves as a reservoir of nutrients for crops (chiefly, N, P, S), enhances aggregate stability, increases nutrient exchange (CEC, mainly Ca, Mg & K), improves water holding capacity, water infiltration, soil aeration, buffering capacity (ability of a soil to resist pH change), reduces compaction, helps to reduce runoff, and provides food for the living organisms. The percentage of SOM that occurs naturally in soil varies greatly, from 1 percent to more than 90 percent in muck soils. Most of our productive agricultural soils have between 3 to 6% organic matter (CUCE, 2008; BARC, 2012; NRCS, undated).
Degraded land with respect to soil organic matter depletion in 2020 is around 11.62 million ha which amounts to 78.7% of the country area. Among these, moderate to very severely degraded lands account for about 4.4 million ha which is equivalent to 29.8% of the geographical area of the country.
Overall degradation of soil organic matter content in the whole country during 2000-2020 may be said to be stable with exception of some bright spot areas (0.2%) where it has got a slight increase. In physiographical context, the degradation map reveals, Gopalganj-Khulna Bils and Sylhet Basin are less affected.
The depletion of organic matter should not be generalized across the country. The popular belief is that ‘as time advances, organic matter content in soil declines’. This is particularly true under highland and medium highland conditions, exception is in the areas where high-value vegetables/crops have been produced fertilizing with appropriate doses of chemical fertilizers in addition to organic manures like compost, mustard oil-cake, ashes, etc, and other improve farming system practices, e.g. leaving crop residue, crop rotation and crop diversification introduced by DAE, BARI, other Gov. agencies, soil organic matter content has shown a slight increase (comparing between 2000 and 2020 soil data and Field Survey 2019-20). Examples are Thakurgaon district and Shibpur Upazila of Narshingdi district. The increase in soil organic matter in particular areas of Old Himalayan Piedmont Plain (AEZ 1) was first shown by SRDI in 2010 at its Annual Workshop in BARC (Hossain et al. 2010). Later, BARC also reports that the long-term fertilizer trials indicate that in the rice-rice (anaerobic-anaerobic) cropping system, the organic matter has slightly increased (BRRI & BARI reports, cited in FRG 2012). We have also found similar increasing trend of soil organic matter content in Mymensingh division since 2000 to 2020. This is also supported by the research carried out by Shawkhatuzzaman (2018). In the wheat-rice (aerobic-anaerobic) system the soil organic matter has rather decreased (FRG-2012).
