Information regarding physiological and molecular plant responses to the treatment with new biofertilizers is limited. In this study a fast-composting soil amendment (FCA), obtained from solid waste by means of a Fenton reaction, was tested to evaluate the effects on the growth of Lactuca sativa L. var. longifolia seedlings. Growth rate, leaves and roots biomass, chlorophyll concentration and total soluble proteins of seedlings treated with the 2% FCA soil amendment showed significant increase in comparison with control seedlings. Leaf proteomic analysis revealed that the FCA soil amendment induced the up-regulation of proteins belonging to photosynthesis machinery, carbohydrate metabolism and promoted the up-regulation of the water stress response. Root proteomics indicated that FCA strongly induced the organs morphogenesis and developments; root cap development, lateral root formation, post-embryonic root morphogenesis were the main biological processes enriched by the treatment. Overall, our data suggest that addition of FCA formulation to the base soils might ameliorate plant growth by inducing carbohydrates primary metabolism and the differentiation of a robust root system.