Tolerance of Rice (Oryza sativa L.) and Echinochloa Weeds to Growth Suppression by Rice Straw Added to Paddy Soil in Relation to Iron Toxicity

The effects of addition of rice straw to submerged soil on the emergence and growth of rice (Oryza saliva L.) and two paddy weeds (Echinochloa oryzicola Vasing. and Echinochloa crus-galli (L.) Beauv. van crus-galli) were investigated. Rice straw suppressed both the emergence and growth of transplanted plants depending on the amount of rice straw added (0%, 0.3%, 0.6% and 0.9% (w/w)) in the order of E. crus-galli > E. oryzicola > rice. The severe suppression of emergence and growth of E. crus-galli in the presence of 0.9% rice straw in hydroponic culture was thought to be due to high Fe content of the shoots. Since the difference in tolerance for the toxicity of rice straw is an important factor, the addition of organic materials into soil may help to suppress Echinochloa weeds selectively.