Evaluation of long-term application of organic residues on accumulation of organic matter and improvement of soil chemical properties in a clay terrace soil of Bangladesh

The field experiment on the long-term application of organic residues was started in 1988 at the farm of Bangabandhu Sheikh Mujibur Rahamn Agricultural University (BSMRAU; named as Institute of Postgraduate Studies in Agriculture (IPSA) until 1998) and is continued until now. Five kinds of organic residues (no-application, rice straw, green manure, compost, and cowdung) were applied every June to cover soil surface uniformly as a thin layer and incorporated into soil to the depth of 10cm. Rice (July to October) and wheat (mid-November or December to March) have been cultivated in rotation under the combination of five organ-ic-residue treatments and three levels of inorganic N fertilizer. Improvement of chemical and physical properties of soils during the first 5 years was summarized and already reported along with the yields of rice and wheat (Karim et al, 1995). Soil samples were taken in April 2003 and analyzed for total C and N, and cation exchange capacity to evaluate the accumulation of organic matter and the improvement of soil chemicai properties after 1 5 years since the start of the experiment. The organic matter and total N contents ranged from 9.7 to 15.2g kg**-1 and from 0.57 to 0.93g kg**-1, respectively. Statistical analysis indicated that the organic matter and total N contents of the soils treated with compost (Cp) and cowdung (Cd) were significantly higher than those of the control (Mo: no application). The organic matter and total N contents increased between the first 5 years and the present, but the difference in the contents between the two terms was rather greater for the control than for the organic-residue treatments under the NO treatment (no application of N fertilizer) . This means that the effect of continuous application of organic residues on the accumulation of organic matter is not positively indicated. Therefore, it is reasonable to consider that the increased level of organic matter performed in the early stage of the experiment is maintained by application of organic residues after 1 5 years. The CEC ranged between 12.9 and 16.0 cmol sub(c)kg**-1. It was significantly higher for the Cp and Cd treatments and reflected the accumulation of organic matter.