Technical proposals necessary for the yield improvement of organic rice cultivation based on field studies in Kou-Shin-Etsu Areas

Abstract

The application and decomposition of organic materials such as crop residuals is important for improvement in soil fertility in organic farming but the decomposition of the organic materials is the key point. We investigated the relationship of rice plant growth and weed incidence with the decomposition of rice straw during the non-planting period in totally 29 organic paddy fields in 2009 and 33 in 2010 in Kou-Shin-Etsu areas in Japan. The dominant paddy weed Monochoria vaginalis was found in high density at the maximum tillering stage in all the organic paddy fields. Weeds dry weight during the maximum tillering period was increased by rice straw remaining rate in puddled soil (organic matter (>4mm) content in the puddled soil divided by the rice straw amount at previous harvest). In addition, in many cases with high weed density and remaining straw in the soil, rice plant growth is suppressed and further this leads to decreases in spike number because of the low planting density and consequently grain yield decreased, which was ultimately due to the poor initial growth. Therefore, in several cases of the investigation paddy fields, the transplanting time was delayed 10 days than the farmers’ practice in order to increase the rice straw decomposition period. Where planting density was 30% higher than that of farmers’ practice （18.2 plants m^－2), weed dry weight decreased to lower than 10g m^－2 and grain was ensured because of less weed incidence. In conclusion, suggested practices for the improvement of organic rice production in the cool areas of Kou-Sin-Etsu areas are to ensure the sufficient straw decomposition period by early plowing and by postponing transplanting, and increasing plant density by 18.2 plants m^－2.