Spatial Variations in Water Availability, Soil Fertility and Grain Yield in Rainfed Lowland Rice: A Case Study from Savannakhet Province, Lao PDR

Rice is the single most important food crop in Laos. Savannakhet province, the largest area of rainfed lowland rice of any single province in the country was selected for the present case study to quantify the spatial distribution of two major limiting factors, water availability and soil fertility, and rice productivity in rainfed lowlands. Field water availability, fertilizer application and other crop management practices, and grain yield information were collected from over 100 farmers to provide basic information relating to rainfed lowland rice productivity and potential bio-physical constraints in this province over two rice cropping seasons. Poor soil fertility is identified as a major yield constraint with yield responding strongly to fertilizer application rate. The results also show that rainfall distribution pattern, soil type and position of rice fields on a sloping land, affect paddy water availability, and this in turn influences sowing time and is also expected to have effect on grain yield. To improve the productivity of rainfed lowland rice, combination of appropriate crop phenology, increased fertilizer use that is matched with water availability, and an understanding of soil water condition for the rice growing environment, is required. In a long term increasing soil fertility is required, and a significant improvement in rice productivity cannot be achieved by improved water availability alone, in a situation where the majority of paddy soils have low levels of fertility.