Abstract
We studied the effect of soil sterilization with hot water on the germination and viability of paddy weed seeds. Laboratory experiments showed that seeds of Echinochloa oryzicola, Scirpus juncoides var. ohwianus, and Oryza sativa (weedy rice) lost their ability to germinate after heat treatment at 70°C for over 30min or at 60°C for over 180min in hot water. Soil was sterilized with hot water at about 90°C sprinkled at the rate of 120lm−2 on soil covered with plastic film under the field condition. The treatment increased the soil temperature at 6cm depth to 73°C and maintained it at over 60°C for 150min. One month after the treatment and following the transplantation of rice seedlings, the heat treatment depressed the number of emerged weeds as much as typical herbicide treatment did, and neither treatment harmed the growth of the transplanted rice.However, at harvest time, the total fresh weight of weeds collected from the heat-treated plot was 2 to 3 times as much as that of weeds collected from the herbicide-treated plot.We investigated the survival rate of weed seeds buried in the soil after the harvest. About 50% of S. juncoides seeds survived in the herbicide-treated plot, but the rate had no correlation with the depth of burial. On the other hand, up to 5% of S. juncoides seeds survived at depths of 3, 6 and 9cm, and about 60% survived at 12cm in the heat-treated plot.
