Optimisation of fertilization practices for a certified “Organic Farming” Ulva spp production ; Optimisation des pratiques de fertilisation d'Ulva spp dans le cadre d'une production labellisée « Agriculture biologique »

Fertilisation is an essential culture parameter of Ulva spp production in land-based ponds, with a major impact on the seaweed’s. Under the "Organic Agriculture" label, the use of fertilisers is regulated, and only organic fertilisers of 100% plant origin and mineral fertilisers are authorised. Currently, France Haliotis only uses organic fertilisers, which are costly. The possibility of switching to a mineral fertilisation (sodium nitrate and calcium phosphate) and a mixed fertilisation (organic fertiliser supplemented by mineral sodium nitrate) was studied. In addition, six strains of Ulva sp were harvested from the wild and grown with the mixed fertilisation formula developed, in order to compare them with the strain grown by the company. Specific growth rates (SGR) with mineral fertilisation were low (5-10%.day-1) compared with mixed fertilisation (> 20 %.day-1). This fertilisation corresponds to an inorganic phosphate concentration of 42.3 μmol.L-1 provided by an organic fertiliser) and an addition of mineral nitrate of 334 μmol.L-1. With this formula, no wild strain exceeded the SGR of the strain already grown, but several obtained similar results. The mixed fertilization is the formula which maintains the SGR obtained with the old fertilization while reducing costs by a factor of three. In addition, a strain harvested from the Kerazan cove has been selected for its satisfactory growth and its morphology, which is better suited to the food seaweed market. ; Pour la production d’Ulva spp dans des bassins à terre, la fertilisation est un paramètre primordial, avec un impact important sur la croissance des algues. Sous le label « Agriculture Biologique », l’utilisation de fertilisants est encadrée, et seuls les fertilisants organiques (d’origine 100% végétale) et minéraux sont autorisés. Actuellement, l’entreprise n’utilise que des fertilisants organiques coûteux. La potentialité d’un basculement vers une fertilisation minérale (nitrate de sodium et phosphate de calcium) et vers une fertilisation ...