Compost Produced with Addition of Sewage Sludge as a Source of Fe and Mn for Plants

Direct application of sewage sludge to soil is controversial due to, among others, its highly variable composition, odour, and risks for health. The obtained composts with the addition of sewage sludge were tested for the contents and availability of manganese and iron. Once composts were applied to the soil, their effect on the content and availability of Mn and Fe in soil and bioaccumulation in the plant were determined. The addition of sewage sludge enriched composts with manganese and iron, but did not increase the content of water-extracted forms of Mn and Fe. The compost with addition of biochar had more organic matter-bound forms of Mn and Fe. Composts amended with sewage sludge had lower effect on the amount of Poa pratensis L. biomass than maize straw compost. The content of Mn and Fe in Poa pratensis L. was in the range permissible for biomass used as fodder. Smaller addition of all composts to the soil significantly increased the content of mobile manganese forms; however, neither the type nor the dose had effect on the content of iron mobile forms. There was no significant differences in the content of organic matter-bound forms of Mn and Fe in soil after the application of composts.