Particulate Organic Matter Composition and Pythium Damping‐Off of Cucumber

Abstract

Organic matter–mediated root rot suppression is unpredictable in field soils. This study was conducted to determine whether particulate organic matter (POM) composition and content were related to Pythium damping‐off (DO) incidence in a sand amended with sawdust‐bedded dairy manure compost (15% compost: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 determined with a Cucumis sativus L. (cucumber)/Pythium ultimum DO bioassay. Particulate organic matter composition was determined spectroscopically. The compost‐amended sand supported suppression of DO for a period of ∼1 yr. Suppression was sustained by the degradation of the less decomposed coarse and mid‐sized POM fractions. After these fractions stabilized in mass, suppression was lost. Plant constituents were highly degraded during composting before amendment to sand. Compost‐derived POM composition changed little as suppression was supported for 1 yr. In contrast, aromatic and aliphatic 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‐decomposed soil physical fractions may be the only fractions compositionally capable of supporting suppression of DO in field soils.