Effects of trees on paddy bund on soil fertility and rice growth in Northeast Thailand

A study was conducted in Northeast Thailand on six rice paddy fields on a farm with similar soil (Aquic Quartzipsamments) and with a single tree on the paddy bund. There were 4 tree species: Parinarium anamense, Dipterocarpus obtusifolius, D. intricatus, and Samanea saman. Samples of soil (0-10 cm depth) and rice were collected at 3 positions (1, 5-7, and 9-11 m) away from the tree base in 3 replicated tree-soil trasects in each paddy field. Significantly higher pH, organic matter, and nutrients (N, P, K, Ca, and Mg) were found in paddy fields with S. saman (a legume tree) but grain yield and biomass of rice were significantly lower. Higher soil fertility was found in the positions closer to tree base while grain yields, biomass, and number of tillers were lower, and rice was taller and had more unfilled grain. Shading was deduced to be the key factor responsible for the depressed rice yields and growth at positions closer to tree base especially in the highly shading S. saman.