Recycling agricultural residues to obtain production factors directly in the farm could be crucial to improve sustainability. The aim of this study was to use plant residues to produce both organic amendments (on-farm compost) and plant biostimulants (liquid extracts). They were applied on a 2-year organic fennel crop, both alone (compost on-farm–OC) and combined (compost on-farm plus parsley extract–OCex) and compared with two commercial organic fertilizers (of vegetal origin–ORG-V; and of animal origin–ORG-A) and an unfertilized control, alone (NF) and combined with the extract (NFex). On average, the agronomic performance of compost treatments was higher than the unfertilized control and comparable to ORG-V. The use of the plant extract induced a synergistic effect with the on-farm compost, likely due to the P, K, Ca, Fe, and Mg supplementation by the extract to fennel crop. This is a relevant result, particularly in the perspective of widening the number of fertilizers produced inside the farm in organic farming, thus avoiding both synthetic fertilizers and commercial organic fertilizers. Finally, on-farm compost use reduced GWP emissions, since an average value of −12.44%, compared to the commercial fertilized treatments, was found, thus suggesting environmental sustainability.