Particulate organic matter composition and Pythium damping-off of cucumber

Organic matter-mediated root rot suppression is unpredictable in field soil s. This study was conducted to determine whether particlulate organic matte r (POM) composition and content were related to Pythium damping-off (DO) in cidence in a sand amended with sawdust-bedded dairy manure compost (15% com post:85% sand, v/v) incubated in pots for 506 d. Suppressive and conducive POM composition was then related to literature values for agricultural and forest soil POM fractions, The suppressive potential of the substrate was d etermined with a Cucumis sativus L, (cucumber)/Pythium ultimum DO bioassay. Particulate organic matter composition was determined spectroscopically, T he compost-amended sand supported suppression of DO for a period of similar to1 yr, Suppression was sustained by the degradation of the less decompose d coarse and mid-sized POM fractions. After these fractions stabilized in m ass, suppression was lost. Plant constituents were highly degraded during c omposting before amendment to sand. Compost-derived POM composition changed little as suppression was supported for 1 yr, In contrast, aromatic and al iphatic contents and alkyl- and O-alkyl C declined as suppression was lost. Suppressive POM was similar in composition to forest soil organic horizons and soil unprotected light fraction (ULF), suggesting that the least-decom posed soil physical fractions may be the only fractions compositionally cap able of supporting suppression of DO in field soils.