A field experiment was conducted to study the effects on yield and soil properties of the continuous application of rice straw compost to an alluvial soil in the Mekong Delta, Vietnam. Fourteen rice crops, two crops per year, were grown by direct seeding of the crop. There were seven treatments: F0C) (no fertilizer and compost), F0C+ (compost with no fertilizer), F20C+, F40C+, F60C+, F80C+ (20, 40, 60 and 80% of the fertilizer application rate with compost, respectively), F100C) (full strength fertilizer application as N:P 2 O 5 :K 2 O at 100:30:30 kg ha )1 in the dry season and 80:30:30 kg ha )1 in the wet season without compost). Compost prepared from the rice straw was applied at 6 Mg ha )1 (fresh weight) at the cultivation of each crop. The F0C+ treatment showed higher yield than the F0C) treatment by 0.68 Mg ha )1 on average in the wet season, but not in the dry season. The rice yield in F0C) declined at a rate of 0.163 Mg ha )1 year )1 in the wet season, but there was no decline in rice yield in F0C+. In treatments with compost, the yield reached its plateau at F40, suggesting that compost could replace part of the fertilizer. Although there were no significant differences in the total C concentrations in the soil among the treatments, even after 12 consecutive crops, soil penetration resistance appeared to be lower with compost than without compost. This long-term field trial showed that the continuous application of rice straw compost has some positive effects on rice yield as well as on soil physical properties.

This study aimed to determine the rice (Oryza sativa L.) yield under double cropping in the Mekong Delta, Vietnam, and identify which climatic factors cause a reduced yield in the wet season (WS) compared with the dry season (DS). A series of field experiments was conducted in the field from 2014 to 2017, along with a supplementary pot experiment, by using the rice variety OM 6976. The yield was 38% lower in WS than in DS, mainly due to a decrease in the spikelet number per panicle and percentage of ripening. This yield reduction observed in WS was most closely related to lower radiation at 51-80 days after transplanting, and was also related to higher temperature. These results suggest that low radiation and high temperature during the reproductive to grain-filling stages were mainly responsible for causing a low yield in WS. In addition, a need to further consider soil reduction was suggested based on more extensive root damage and sulfide levels on roots observed in WS as compared with DS.