Evaluation of compost from seaweed and fish waste as a fertilizer for horticultural use

• The compost showed an important residual effect at rates above 50 t ha−1.
• A rate of 66 t ha−1 significantly increased the yield of two successive crops.
• Despite its marine origin, the compost did not increase soil salinity.
• The compost may be recommended as fertilizer for organic agriculture.

Composting may be an effective process for treating fish waste and drift seaweed to recover organic matter and nutrient resources and to produce fertilizers for organic agriculture. The aim of this study was to evaluate the use of this type of compost as a fertilizer in a horticultural crop rotation. A greenhouse trial was carried out to test the effects of the compost on a tomato crop and its residual effects on the succeeding lettuce crop. Different compost rates (C1, C2 and C3: 40 t ha−1, 50 t ha−1 and 66 t ha−1) were compared to single rates of a mineral fertilizer (M) and of a certified organic fertilizer made from dehydrated broiler litter (BL) as well as to the control treatment, which lacked fertilization (C). Tomato yield increased significantly (5.56 kg plant−1) with the highest rate of compost compared to the mineral and control treatments (4.54 and 4.58 kg plant−1, respectively). This increased yield was associated with an increase in the diameter and weight of the fruits. Moreover, the lettuce yield improved with compost rates C2 (395 g lettuce−1) and C3 (367 g lettuce−1), showing a strong residual effect of the compost. This compost is suitable as organic soil fertilizer and may be recommended for improving horticultural crop yields.