Seaweed (fresh, dry) or its products (extracts, composts, soil conditioners) have been long used in agriculture to enhance plant growth and productivity. In this study, we evaluated the effects that seaweed composts at different doses and degree of maturation had on the yield of tomatoes (Licopersicon esculentum Mill. cv. platense) grown on a horticultural soil in northeastern Patagonia. We used 10 tomato plants per treatmentplot set in a randomized block design. Treatments were: 1) soil without amendment, control (S); 2) 5 kg m−2 of compost aged 9 months (C9-5); 3) 10 kg m−2 of compost aged 9 months (C9-10); 4) 5 kg m−2 of compost aged 20 months (C20-5); and 5) 10 kg m−2 of compost aged 20 months (C20-10). Total weight and number of tomatoes, and aerial plant biomass (excluding fruits) were significantly higher for the compost treatments than those of the control. Also, compost treated plants bore mature fruits, in average, 9 days earlier and presented higher resistance to diseases than controls. The weight of tomatoes per plant grown in C20 was significantly higher than that of C9, differences that could be attributed to the lower salinity of compost C20 (C20 and C9 electrical conductivities were 1.5 and 15 dS m−1, respectively). The increased yield and resistance to diseases on tomato plants by addition of seaweed compost appear to be related to a complex number of factors not yet fully understood. It seems, however, that a combination of higher nutrient availability (mainly P) due to slight increases in pH of the soil amended, together with increases in readily available K and an improvement in soil physical conditions (increase in pore size and probably amelioration of hydric conditions), may have been responsible for the higher production of seaweed amended plots as compared to the control.

Trends in wrack composition and biomass, and its relationship with the anthropogenic impact were studied along a coastal area in Nuevo Gulf (south Patagonia) in front of Puerto Madryn city. Beach-cast macroalgae composition was sampled from 1992 to 1999 in the Puerto Madryn beaches and in several other nearby beaches in 1993, 1996 and 1998. Historical information was based on local knowledge and observations reported by marine biologists who worked in the area. The botanical composition of the beach-cast macroalgae in Puerto Madryn indicates a succession in the dominance from Codium spp. to Ulva spp. during the 1990s and from Ulva to Undaria pinnatifida since 1998, accompanied by a significant decrease in biomass of Gracilaria gracilis and Macrocystis pyrifera . The increase of the opportunist species such as Ulva may be supported by the continuous delivery of waste waters into the Nuevo Gulf while the dominance of U. pinnatifida may be associated with port activities. During the sampling period the highest wrack biomass values were recorded in spring and summer. The beach-cast seaweed biomass harvested by the municipality of Puerto Madryn during beach cleaning operation, ranged between 2500 and 12000 t year −1 ( 200 and 960 t dry weight). Wrack harvesting produces an environmental impact by removing sand from the beach and affecting coastal communities. Composting of wrack is proposed as one of the environmental alternatives to land disposal.

Large quantities of green seaweed, linked probably to eutrophication, are cast ashore every summer on the Puerto Madryn beaches (Patagonia, Argentina, 42 degrees 5, 65 degrees W). This algal biomass interferes with recreational uses of the beach, and therefore must be periodically collected and disposed. Part of this algal biomass was composted with the objective to produce an amendment to improve physical and nutritional characteristics of some local soils used in intensive horticulture, and at the same time to find a way to reduce environmental pollution. The compost was then biologically evaluated by determining the growth rate of tomato plants cultivated on various substrata (washed sand, sandy loam soil, and sandy loam soil plus inorganic fertilizers) to which different doses of compost were added. Results showed that in all cases the addition of compost increased water holding capacity and plant growth. The increase of tomato plants (Licopersicum esculentum var. platense) was proportional to the compost doses. Also, plants grown on sustrata containing at least ten percent compost had significant benefits compared to control plants, improving growth and water stress resistance. Although the quality of this seaweed compost was limited by excessive amounts of sand and low nitrogen content, it proved to be a good amendment that improved both physical and nutritional characteristics of local sandy loam soils used in intensive horticulture. According to these results, composting is a useful technology both to solve environmental pollution problems and to produce a valuable organic fertilizer for soils.