Comparative study on the effect of organic fertilizers on plant biomass and on organic carbon content of sandy and loamy soils

Two important requirements for organic fertilizers are to provide nutrients to plants and to improve the organic carbon stock of the soil. Different fertilizers do not fulfill these aspects to the same extent. Four organic fertilizers: digested sewage sludge (DS), DS compost (COM), cattle manure (CM), and pig slurry (PS) were compared in a greenhouse experiment on sandy and loamy soils, using sorghum as a test plant. The materials were applied based on their nitrogen content: 0, 91.5, 183 and 366 kg N ha–1 on sandy and 0, 75.5, 151 and 302 kg N ha–1 on loamy soil, respectively. In the soil total organic carbon (OC), labile organic carbon (POXC) and organic matter quality (Q) by ratio of absorbance determined in NaF and NaOH soil extracts were measured. The effect of fertilizers on average sorghum yield increase followed the same order on the two soils: COM (43%), CM (53%), DS (58%) and PS (87%). Despite the expectations, POXC did not prove to be an appropriate indicator for soil fertility in soils amended with the organic fertilizers. On loamy soil COM and CM while on sandy soil PS reduced the Q. Regarding the yield-increasing effect, the OC content and quality improvement on sandy soil COM and CM while on loamy soil PS proved to be the best organic fertilizer in the short term. Soil properties can significantly modify the effectiveness of organic fertilizers, primarily in terms of their effect on the organic matter quality.