Green manure and compost-enriched in phosphorus can promote the sustainability of cropping systems by increasing soil fertility over the long term. They can also be used to manage crop/weed interactions, a key element in guaranteeing an appropriate level of satisfactory crop yields. We studied how green manuring with hairy vetch (Vicia villosa Roth.) and the application of different types of phosphorous-enriched compost affect weed/maize (Zea mays L.) interactions in an organic stockless Mediterranean agroecosystem for two consecutive dry years. Green manure stimulated the expression of maize traits related to a higher competitive ability against weeds, such as early growth, height and leaf area index, while the effect of compost was less clear. Regarding crop/weed competition, both green manuring and a phosphorus-enriched compost application gave a significant advantage to maize. Neither green manure nor compost increased total weed density and biomass compared to the control. Green manuring significantly affected the weed community composition. The relative density of ruderal and competitive-ruderal species (according to Grime's classification) was higher in plots where the green manure was applied. The use of green manure, together with novel composting techniques, significantly affected crop/weed competitive interactions, favoring maize, but also creating favorable conditions for unwanted weed species such as competitive-ruderals. Increasing nitrogen availability in the early growth stages of maize through green manuring can increase crop competitive ability. However, this may not suffice to preserve the system from future weed problems, should potentially detrimental species be selected. Dedicated strategies for the control of emerging weed species may thus be needed.