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In a field experiment conducted at the International University of Greece farm (Sindos, Thessaloniki) during the 2017 growing season, the effects of different levels of sludge or manure on maize crop yield and the improvement of soil components were evaluated. In particular, the effect of 20, 40 and 80 tons of manure or sludge/ha was assessed in combination with or without the use of synthetic fertilizers (80 and 220 kg/ha of nitrogen applied before sowing and 40 days after sowing, respectively, 109 kg phosphorus/ha, and 109 kg potassium/ha) on the maize plant parameters (height, number of leaves, fresh weight, silage and grain yield). Their effect on the concentration of nitrogen, phosphorus and potassium in plant tissues was also evaluated. In addition, the impact of their use on soil characteristics (pH, organic matter, nitrogen, phosphorus and potassium concentration) was evaluated. The results showed that the grain yield was significantly affected by the type and amount of material used and by the synthetic fertilizer. Overall, the yield was higher in the 40-ton/ha sludge or manure combined with or without synthetic fertilizer plots and lower in the control (without synthetic fertilizer) plots. However, fresh weight, plant height and number of leaves per plant were not significantly affected by the use of manure, sludge and inorganic fertilizer. The nitrogen concentration in maize leaves was lower in the plants grown in the experimental plots where 20 tons/ha of sludge and manure with and without inorganic fertilization were applied. However, the concentration of phosphorus in all four levels of sludge was higher where inorganic fertilizers were used, instead, the concentration of potassium which was lower respectively. The percentage of organic matter in the soil was higher in the experimental plots where 40 and 80 tons/ha of sludge without the use of chemical fertilizer had been applied, while the pH was slightly higher where no chemical fertilizer was applied. The concentration of nitrogen, phosphorus and potassium in the soil was higher in experimental plots with manure and chemical fertilizers.