Mineralization of organic amendments in a sandy soil of Central Vietnam

Farming systems in sandy areas of the tropics require special attention to the management of organic matter. According to a recent survey within the Vietnam-Belgium joint project, a great diversity of organic materials is used by farmers in the coastal sandy zone of Central Vietnam. Research is currently being conducted to evaluate the fertilization capacity of various organic amendments. The poster presents the first results on the mineral-N evolution through the incubation of an acid sandy soil from the Thua Thien Hue Province, amended with 4 types of organic matter: buffalo manure (BM), pig manure (PM), chicken manure (CM), and ash + urine (AU). Along with NH4+ and NO3- measurements, pH, electrical conductivity (EC), and redox potential (Eh) were monitored during incubation. Two laboratory experiments were undertaken according to the incubation method recommended by Keeney (1982) under anaerobic conditions at 37ºC; the first experiment was conducted with 30 g organic amendment per kg of soil for 0, 2, 5, 10, and 20 days as incubation times; the second was carried out with 20 and 40 g organic matter per kg of soil during one week. Initially, significant amounts of NO3- are found with PM and CM, whereas AU released only NH4+. Limited amounts of inorganic N were released by BM. During the first week of incubation, there was a rapid, but temporary, drop of Eh, and increase of pH, EC and NH4+ concentration in the soil solution. NO3- disappeared quickly for PM and CM. Only slight variations of these parameters were observed after 10 days, which justified the short incubation period to screen organic matter amendments for their N fertilization capacity, notwithstanding their long term contribution to the soil organic pool. Total inorganic N release was in the order CM > AU > PM ≈ BM. It is worth noting that organic amendments increased soil alkalinity, or soil acid neutralization capacity (release of bicarbonate anions), which can be roughly inferred by pH increase at similar Eh and CO2 pressure values. For a soil amended with 3% organic matter, the pH values after 20 days were approximately 8 for CM and AU, about 6.5 for PM and BM, compared to 5 for the control soil.