Weed control by application of fresh organic materials to paddy soil in organic rice production

Monochoria vaginalis and Echinochloa weeds are typical weeds in organic rice production. Applying fresh organic materials to the soil is a widely accepted weed control technique. Although the application of rice (Oryza sativa L.) bran on soil surface is useful for the removal of M. vaginalis, its efficacy fluctuates according to year and field. With the incorporation of this material into soil, toxic substances (e.g. organic acids and ferrous ions (Fe2+)) increase. These harmful substances suppress the germination of this plant. In contrast, paddy soil is typically equipped with the ability to reduce oxygen (O2). This trait enhances the number of M. vaginalis, because the hypoxic condition is preferable for its germination. In other words, paddy soil with rice bran has two conflicting powers: promotion of germination by soil and the suppression by rice bran. The presence of conflicting factors may destabilize the efficacy of rice bran. Rice straw is incorporated into the topsoil after harvesting to maintain soil fertility. Seedlings of rice and Echinochloa weeds exhibit distinct tolerances to growth suppression by rice straw: rice is the most tolerant, followed by Echinochloa oryzicola, and E. crus-galli is the most susceptible. Once these weeds establish seedlings under field conditions; however, the application of organic materials does not completely remove them. Consequently, promising methods such as deep-water management are combined. It is speculated that one of the factors causing the difference among rice, E. oryzicola, and E. crus-galli is the variance in tolerance to iron toxicity. The rice leaf can concentrate a greater amount of Fe than the leaves of Echinochloa weeds. The remarkable resilience of rice to harmful Fe is associated with this tolerance-based mechanism of the leaf. Conversely, the root of E. oryzicola can prevent Fe from being transported to the leaf. This avoidance-based power outweighs that of E. crus-galli.