A study was made in An Son village, Binh Duong province, Vietnam, where the use of biodigested slurry as fertilizer was investigated. The village is situated close to Ho Chi Minh City, i.e. close to a market for selling and buying. Nutrient flows were calculated for a theoretical mixed farm typical for the village, with pigs, rice (Oryza sativa), sugarcane (Saccarum officinarum) and pak choi (Brassica chinensis) production. The livestock sub-system consists of exotic breeds of pigs fed with feed concentrate, consisting of maize meal, soy bean meal cassava meal and fish meal. The biodigester sub-system consists of a plastic tube type digester of approximately 6 m3. The digester is loaded with 12 kg manure per day and produce about 900 L gas per day. An interview study showed that the farmers with biodigesters did not use the slurry as fertilizer. A theoretical nutrient flow was made in order to show the potential of integrating the crop sub-system with the livestock and biodigester sub-system. The crop sub-system is made up of 0.1 ha pak choi, 0.2 ha rice, and 0.2 ha sugarcane. The theoretical livestock sub-system consists of 6 fattening pigs year around.

The MFA shows a yearly flow where 130 % of the phosphorus, and 75 % of the nitrogen need (based on the removal with pigs and crop harvest) on the farm is covered by the input of feed concentrate. The calculations on crop removal are based on average figures for harvest, and nitrogen and phosphorus content for the crops concerned. The MFA shows that there might be a risk of phosphorus accumulation on the farm even in a system where all three sub-systems are integrated and no mineral fertilizer is added. Nitrogen, however, has to be supplemented to meet crop demand in this system. Nitrogen is also exposed to the risk of losses through leaching and/or volatilization. 

Key words: Vietnam, nitrogen, phosphorus, feed concentrate, pig, excreta, biodigested slurry, rice, Oryza sativa, pak choi, Brassica chinensis, sugarcane, Saccarum officinarum, plastic tube biodigester.

I am a Ph.D. student in Soil Science with a special interest in nutrient balances and recirculation of organic wastes. My Ph.D. project is called: Recirculation of biodigested slurry to the soil - farm sustainability on two soils in southern Vietnam. The project is funded by SAREC/SIDA.
 

Dr. An is a graduate from the University of Havana (Cuba), he finished his MSc. in Biology at the Swedish University of Agricultural Sciences (Uppsala, Sweden) and his Ph.D. in Agriculture at the University of Agriculture and Forestry, Vietnam. He works as a lecturer at UAF. His area of interest includes : Farming System, Rural Development, Biogas Production, Ruminal Nutrition. He is involved in a number of projects on : 
- Better Use of Local Feeds Sources
- Intensifying Livestock and Fuel Production by Using Local Resources
- Developing Integrated Farming System in Vietnam
- Community based Upland Resource Management
- Forage for Small Holders Project
- Biogas Development Project in Vietnam
- Technology Development for Remote and Mountainous Villages