IMPROVEMENT OF PADDY SOIL DERIVED FROM GLASSY VOLCANIC ASH (SHIRASU) BY SUCCESSIVE APPLICATIONS OF ORGANIC AND INORGANIC AMENDMENTS

A field experiment has been carried out since 1975, in order to mainta in a high yield of rice cultivated in a paddy soil derived from glassy volcanic tsh (Shirasu) which has potentially low productivity. In thi s experiment, farmyard manure or rice straw was applied as organic ame ndment, and slag and calcium silicate as inorganic amendments. The res ponse of rice yield to the organic treatments was resulted of a large quantity of grains per panicle in the plots supplemented with farmyard manure, while by a high percentage of ripened grains in the plots tre ated with rice straw. The percentage of ripened grains in the organica lly treated plots, however, was lower than the plots treated with chem ical fertilizer. The total carbon and nitrogen contents of the soils i ncreased through successive applications of organic matter, resulting in a high carbon-nitrogen ratio. The amount of available phosphorus me asured by the Truog and Bray No. 2 methods also increased through the application of farmyard manure. Organic components of soils such as ce llulose increased by the organic treatments. The lignin content of the plots treated with farmyard manure also increased. The increase of ri ce yield did not exceed 10% in the organic plots compared with the che mical fertilizer plot, in spite of the increase in nutrient-uptake by rice plants through successive applications of organic amendments. The refore, some suggested that the methods of fertilizer application shou ld be improved to further increase rice yield in paddy soils derived f rom glassy volcanic ash (Shirasu).