Salinity and acid sulfate soils of the Vietnam Mekong Delta: Agricultural management and adaptation

Abstract

O ne of the largest concentrations of acid sulfate soils in the world is found in the Vietnam Mekong River Delta, a large low-lying river plain scarcely above sea level, covering 1.6 million ha (4.0 million ac; figure 1) (van Mensvoort 1996; Vietnam Ministry of Agriculture 1978; Huu et al. 2022). Acid sulfate soils have high concentrations of aluminum (Al), sulfates (SO4 2–), and iron (Fe), and when drained produce sulfuric acid (H2SO4) that reduces soil pH below 4 (van Mensvoort 1996; Huu et al. 2022). These metals accumulate in the topsoil during the tropical dry season and are toxic to plant root growth and development and suppress yields making them some of the most difficult soils in which to grow agricultural crops. Yet, the Vietnam Mekong Delta produces 50% of Vietnam’s rice crop; 95% of rice exports; 65% of aquaculture production; 60% of exported fish; and 70% of the country’s fruit production (Loc et al. 2021). One of the keys to acid sulfate soil productivity is water-soil adaptive management that maintains crop-specific balance between reducing and oxidizing conditions in the plant rootzone at critical vegetative, bloom, and fruit development stages (Hanhart et al. 1997). More than 50 years ago vast areas of this delta were covered permanently by wetlands, brackish lagoons, tidal marshes, and mangrove forests. The prevailing winds of the southwest monsoon season brought predictable continuous heavy rains, a consistent 20-fold increase in Mekong River discharge and extensive prolonged flooding inundating lowlands for months (Adamson et al. 2009; Taylor 2014; Ngan et al. 2018). The monsoon is followed by a dry season when the rains stop and farmers adapt their cropping systems by growing flooded rice (Oryza sativa L.) varieties in the wet season and digging ditches and canals to drain the floodwater and convey fresh water from the Mekong (Song Tien) Lois Wright Morton is professor emeritus of rural sociology, College of Agriculture and Life Sciences, Department of Sociology and Criminal Justice, Iowa State University, Ames, Iowa, United States. Nghia Khoi Nguyen is associate professor in soil and environmental microbiology, College of Agriculture, Can Tho University, Can Tho City, Vietnam. M. Scott Demyan is associate professor of soil and environmental mineralogy, School of Environment and Natural Resources, The Ohio State University, Columbus, Ohio, United States. Received March 21, 2023. and Bassac (Song Hau) rivers and their tributaries to their fields for dry season crop irrigation. A changing climate—sea level rise, a stronger and increasingly variable SW monsoon, and more frequent and prolonged drought (Adamson et al. 2009)—in concert with amplified tidal effects and saltwater intrusion reaching 50 to 130 km (31 to 81 mi) upstream into the main rivers since February of 2020 threaten freshwater resources (Loc et al. 2021; World Bank 2022). A growing population, land use decisions, saline soils, loss of mangrove coastal protection, degraded wetlands, water and sediment flow dynamics affected by upstream dams, and mining of river sediments are challenging delta agriculture capacities to continue to provide an abundance of food and nutrition Figure 1 Saline and acid sulfate soils of the Vietnam Mekong Delta map. Saline soils of the Vietnam Mekong Delta peninsula are represented by dark purple (coastal mangrove soils) and two shades of lavender: strongly saline soils and moderate and slightly saline soils (lighter lavender). Acid sulfate soils are represented by shades of blue, with dark blue salt-affected potential acid sulfate soils where saltwater intrusion reaches into freshwater regions; mid-blues strongly acid sulfate soils; and lighter blue moderate and slightly sulfate soils. Alluvial soils (green), grey degraded soils, and grey degraded gleyic soils derived from old alluvium soil (very light green) are found along the mainstem Mekong and Bassac rivers and their distributaries. Scale 1:1,000,000. Image by Vietnam Ministry of Agriculture (1978).