The effects over the Organic Matter of the soil have been studied comparing the ones produced by trie aplication of Sewage Sludge, instead of the usual organic fertilization and partly of the mineral one, and the normal fertilization in an open-air culture of carnations in the Maresme area. The initial unbalance of the soil, C/N = 5.76, and the great amount of mineral nitrogen apported to the culture during the assay have favored an important phenomenon of mineralization, even stronger in the case of the sewage sludge application. We have only got a qualitative improvement of the Organin Matter af the soil in the assay where animale manure was applied. We attribute these differences to the different composition of manure and sewage sludge, whose lower C/N ratio makes the unbalance of the soil grow up, its Organic Matter also seems more alterable than the one of the manure. In this experience we want to point out the importance of the quality, and the stability of the organic fertilizer and the culture techniques used which have almost the same incidence in the level of Organic Matter in the soil than its application of the product.