Aiming to increase the use of natural resources and unexplored opportunities in industrial and agricultural practices, the marine algae biomass, that causes serious environmental problems in the Romanian Black Sea coast, was used in association with another two organic wastes, farmyard manure from cattle-breeding farms and sewage sludge resulted from the waste water treatment, to produce a compost suitable as organic fertilizer for plant cultivation in ecologic farming systems. Four variants of compost, first representing equal parts (33.33%) of those three components, and the other three proportions of 50% of each component, the difference being ensured in a ratio of 25% of each of the other two components, have been tested in a field experiment to assess their effects, both on the agro-chemical properties of the soil and on the sunflower plants development and crops. Until the phase of sunflower calathidia formation, the compost containing 50% farmyard manure influenced the best plant development in terms of height and number of leaves, then, at the end of vegetation period, the best plant development took place under the influence of compost prevalent in marine algae. The mobile forms of N and P were statistically differentiated depending on the dose of compost, the maximum dose generating the lowest content levels in the soil, as a result of higher absorption of these chemical elements in plants. The content of P and Ca in sunflower leaves recorded significant differences.