Spectroscopic and thermal investigations on compost improved by iron salt addition

A quality compost obtained from sewage sludge (one part in weight) and yard trimmings and sawdust (two parts) has been investigated as a potential carrier of iron to plants. At the end of the thermophilic phase, the composting materials were added with crystalline FeSO4.7H2O (97%). Chemical properties, respiratory indices, and seed germination tests proved the compost to be suitable as an iron carrier in agriculture. Fourier transform infrared spectroscopy (FT-IR) and fluorescence spectroscopy provided evidence of the effective linking of the iron ion to the organic molecule functional groups, thus preventing the loss of iron ion by leaching and precipitation phenomena and allowing the metal ion to be available to plants as both mineral and organic species. The thermogram obtained on compost without iron was similar to that previously obtained for composted materials of different origins, whereas samples with added iron ion exhibited in addition an exotherm in the medium-temperature region. FT-IR spectra carried out on samples heated at different temperatures indicated a loss of iron ion linked to carboxyl groups.