Fertilization of ecological cultivating systems for producing greenhouse tomatoes with locally produced mulches was studied from 1993 to 1995. The tomato plants were cultivated in limited growth beds filled with nutrient-poor substrate, consisting of farmyard manure (FYM) compost, straw and soil subsequently mixed with peat and gypsum. Mulches of freshly cut grass or clover or a mixture of the two were placed on the surface of the substrate to assess their value as fertilizer. Fruit yield, fruit quality, plant length and NPK-content of mulch materials and bed water were regularly measured. Comparisons made on four occasions during each growing season showed that the mulches containing clover produced significantly higher yield than the grass mulch and this treatment also had the best utilization of P and K. Nitrogen utilization was highest in beds fertilized with grass mulch, even though the yield was lower. Combined mulch of grass and clover produced intermediate yield and had good utilization of N, P and K. However the clover-grass mulch seemed to have potential for higher yield than was found in the experiment.