Organic amendments are believed to help increase the soil carbon storage and therefore improve soil quality, which may be important in the context of climate change. However, the added value of organic amendments for farmers must be clearly demonstrated in order to convince them of the utility of their use. The aims of this study were: (i) to investigate the impact on maize of compost and vermicompost combined with two levels (negligible and significant) of plant water stress; and (ii) to determine how the organic amendments affected the soil’s physical properties and maize productivity. Water stress levels were imposed by controlling the matric potential of soil columns in which cultivated soil characteristics was mimicked (10 cm topsoil with organic amendments, above a 50 cm subsoil without any inputs containing the majority of the roots). Plant and soil characteristics were monitored daily for 70 days. Our results show that the use of organic amendments is profitable for farmers as: (i) maize performances were increased in both moisture regimes; and (ii) the improvement was particularly striking in terms of yield. No additional benefits were measured when using vermicompost instead of compost. The data suggest that the improvement in plant characteristics did not result from increased water storage in the soils with organic amendments, but rather from better access to the water, resulting in faster root development in the macroporosity of the amended soils.

Organic amendments may improve the quality of acidic tropical agricultural soils with low organic carbon contents under conventional management (mineral fertilization and irrigation) in Southeast Asia. We investigated the effect of biochar, compost and their combination on maize growth and yield, soil physical, biological and chemical properties at harvesting time at four sites in three countries: Thailand, Vietnam and Laos. Treatments consisted of 10 t·ha−1 cow manure compost and 7 t·ha−1 of Bamboo biochar and their combination. Maize biomass production and cop yields were recorded for two seasons. Elemental content, pH and nutrient availability of soils were analyzed after the first growing season. We also characterized macrofauna abundance and water infiltration. Few changes were noted for maize biomass production and maize cop yield. Soil chemical parameters showed contrasting, site-specific results. Compost and biochar amendments increased soil organic carbon, pH, total K and N, P and K availability especially for sandy soils in Thailand. The combination of both amendments could reduce nutrient availability as compared to compost only treatments. Physical and biological parameters showed no treatment response. We conclude that the addition of compost, biochar and their mixture to tropical soils have site-specific short-term effects on chemical soil parameters. Their short-term effect on plants is thus mainly related to nutrient input. The site-dependent results despite similar crops, fertilization and irrigation practices suggest that inherent soil parameters and optimization of organic amendment application to specific pedoclimatic conditions need future attention.

Hand-held hoses and watering cans are widely used by smallholder farmers to irrigate vegetables in Cambodia and Laos. Overwatering is common. Technology change (e.g., low-pressure drip irrigation) has been used to improve irrigation efficiency but can be unaffordable for many smallholder farmers. The purpose of this study was to identify an appropriate method of predicting crop water demand, develop and field-test improved irrigation schedules for smallholder leafy vegetable farming based on that method, and then develop extension tools to communicate the schedules to smallholder farmers. Improved irrigation schedules for leafy vegetables were developed based on a crop water use prediction technique that is well established (the Penman–Monteith method) but beyond the capacity of smallholder farmers to implement without access to simple aids. Compared to conventional practice, the method approximately halved water and labour use and improved irrigation water productivity 2–3 fold in field research and demonstration trials. Simplified extension tools to assist smallholder farmers with practice change were developed. This work showed that significant efficiencies could be gained through improved irrigation scheduling without changing application technology.

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Diversification of food sources and agricultural production systems has potential to enhance domestic supplies and provide export market opportunities for Laos. Major constraints to agricultural productivity are related to soil management and include inefficient irrigation, poor soil structural stability, low pH and nutrient availability. An experiment at the National University of Laos (NUOL) in Vientiane assessed the effect of lime and irrigation scheduling on growth and yield of lettuce. The soil was a sandy clay loam with pH 4.89 (H2O) in the top 15 cm. Lime (CaCO3) was applied at rates of 2 and 4 tonnes per hectare (t/ha). Irrigation scheduling was based on calculated evapotranspiration (ETc) with frequencies of either twice daily, once daily or alternate days. Urea, chicken manure and rice husks were added to soil in all trial plots. The experimental design was split-plot with two treatments (lime and irrigation scheduling) and four replications. The combination of 4 t/ha lime and irrigation every second day had the highest yield (mean > 2 kg/m2). The combined treatment of irrigation once a day and no added lime showed significantly higher leaf number (p = 0.01) and plant height (p < 0.001) compared to the other treatment combinations. However, increased biomass of individual plants did not translate into increased marketable yield per square metre. The application of lime raised the pH of soil but the effect on plant growth and yield was not conclusive. Separation of the two treatments into single factor trials is needed to elucidate the effects of individual treatments in future trials.

Production of leafy vegetables, such as lettuce, in Lao People’s Democratic Republic (PDR) is limited by low nutrient soils. Organic fertilisers or composts made from agricultural residues may provide Lao PDR farmers with economical and environmentally sustainable alternatives to chemical fertilisers. Research is needed to increase awareness and knowledge of organic fertilisers suited to vegetable production in Lao PDR. An experiment at the Horticultural Research Centre (HRC) in Vientiane assessed the effect of four organic fertilisers on growth and yield of lettuce. Two commercially available fertilisers (fermented manure compost and an organic fertiliser) were compared with a mixture of cow manure plus rice husks, and a fourth compost made from vegetable leaves, straw and cow manure at the HRC. The experimental design was a randomised block with four replicates for each fertiliser treatment. Lettuce was grown in raised beds with 10 tonnes per hectare (t/ha) fertiliser applied before seedlings were transplanted. The fermented manure compost treatment had the highest yield (1.95 kg/m2) and was significantly higher than the other three treatments (p < 0.001). Growth rates were also highest for the fermented manure compost at all measured growth intervals (14, 28 and 45 days after transplanting). Rapid nutrient release from fertiliser is important for short-term crops. The higher growth rates and yields found for the fermented manure compost indicate that nutrients were released sooner and were more readily available compared to the other treatments. Mature compost releases nutrients more rapidly than compost that contains partially decomposed rice husks, vegetable leaves and straw.