N2 fixation of groundnut in the eastern region of South Vietnam.

To determine the need for and potential benefits of rhizobial inoculation of groundnut (Arachis hypogaea) in the southeastern region of Vietnam, surveys of 40 farmers’ fields and 6 inoculation experiments were conducted during winter-spring (1999–2000), summer (2000) and winter-spring (2000–2001) in the Tay Ninh province and Ho Chi Minh City area. All 40 farmer crops in the survey were nodulated, although none had been inoculated. As well, all crops were fertilised with N, at an average rate of 50 kg/ha (range of 10–120 kg/ha). The percentage of crop N derived from N2 fixation (%Ndfa) varied in the range 24–72%, with an overall average of 46%. Cop N fixed ranged 49–205 kg/ha (average 106 kg/ha). Factors enhancing crop N fixed were plant population, soil organic fertility and fertiliser P applied. Plant population was also highly correlated with pod yield. The six inoculation experiments indicated both inoculation and fertiliser N application to be unnecessary because they did not enhance nodulation (in the case of inoculation) or pod yield (inoculation and fertiliser N). Both surveys and inoculation experiments were conducted in established groundnut areas. In new groundnut land, we would expect a response to inoculation and inoculation would be required. In the absence of inoculation, the farmer would have to supply the crop with substantial amounts of fertiliser N or suffer a yield decline because of N deficiency. Surveys of farmers indicated an almost total lack of knowledge of inoculants but a willingness to use them if they were shown to provide a yield benefit. They also saw that information about inoculants would need to be provided to them and to the extension officers. THE EASTERN region of the South of Vietnam (ERS) comprises 2.3 million ha. The region is located in the tropical monsoon climatic zone, with high solar radiation (>130 Kcal/cm/year), high mean temperatures (26–27°C), and high precipitation (1800–2000 mm/year). The ERS is dominated by 744,650 ha (32% of total) of grey soils (Acrisols), which are characterised as acidic, eroded, light texture and low fertility (Lieu 1992). Paddy rice is the main crop of the region with 542,800 ha yielding an average of 3.1 tonnes/ha. Groundnut (Arachis hypogaea) is grown on 50,500 ha with an average yield of 1.6 tonnes/ha and soybean (Glycine max) grown on 13,700 ha with an average yield of 0.8 tonnes/ha (General Statistical Office 2000). Paddy is either grown as a monoculture or rotated with legumes (groundnut, soybean, mungbean (Vigna radiata)), vegetables and other annual crops. In the ERS, farmers commonly apply N-fertiliser to groundnut, at rates of about 50 kg N/ha, a practice that represents an unnecessary expense because of the capacity of groundnut to fix its own N. We report a 2-season survey of N2 fixation of groundnut in farmers’ fields and multi-locational inoculation experiments in ERS. These surveys and experiments were part of a coordinated project, funded by the Australian Centre for International Agricultural Research (ACIAR) in Vietnam. The objectives of the survey were to determine current levels of N2 fixation in farmers’ fields and the cultural practices that either enhance (e.g. inoculation, effective weed and insect management) or suppress N2 fixation (e.g. fertiliser N, poor agronomy). The inoculation experiments built on the outcomes of the survey to determine the need for and assess the biological and economic benefits of inoculation. 1 Oil Plant Institute of Vietnam, 171–175 Ham Nghi St., District 1, Ho Chi Minh City, Vietnam 2 NSW Agriculture, TCCI, Tamworth, NSW 2340, Australia