Rice straw mulch reduces soil salinity and increases beetroot and maize production in the Mekong Delta of Vietnam

Salinity is becoming a major threat to the growth of continuous rice crops in the low-elevation Vietnamese Mekong Delta. Seawater is increasingly permeating up the river and irrigation water distribution systems in the dry season. There is insufficient knowledge of possible alternatives to rice and management practices during this season to maintain crop production. We assessed the growth of beetroot and maize and tested the value of rice-straw mulch at four rates (0, 3.5, 7.0, and 10.5 t ha⁻¹) at three sites over two years. At the two higher elevation locations (Lieu Tu and Long Phu, 0.80 and 0.96 m above mean sea level (amsl), respectively), dry season crops were successfully grown for 2 years and high levels of production were possible with beetroot and maize (average commercial crop yields up to 42 and 5.4 t ha⁻¹, respectively) while crops could not be established at the low elevation site (Long My, 0.21 m amsl) due to inundation effects. For beetroot and maize, application of mulch increased commercial crop yields by up to 114% and 49%, respectively, decreased the concentrations of Na⁺ in the shoots of plants at harvest by 19% and 37%, respectively, and decreased the salinity (EC_1:5) of the topsoil (0–15 cm) by 13%. Mulch effects were maximized at an application rate of 7.0 t ha⁻¹. Analysis of relationships between soil salinities at different depths and yield and between mulch effects and yield suggested that the rooting depth of beetroot may have been constrained at these sites compared with maize. Very shallow water tables (~20 cm depth) were implicated as the likely cause of this constraint. In overview, our work suggests that exceptional yields of beetroot and maize are possible as alternative crops to rice in this delta environment where salinity and waterlogging are key risks.