Testing biostimulant effects of composted marine materials

Summary in the original language of the document

The present report describes two experiments conducted within the project «Sustainable utilization of MARIne resources to foster GREEN plant production in Europe» (MariGreen,2021-2024). The aim was to improve the currently poor utilisation of some marine materials from blue value chains such as captured fish, blue mussel, and macroalgae, for application as fertiliser and biostimulant in organic growing. Composting of substrates from seaweed, captured fish and blue mussel has been a central project activity, and the testing of biostimulant effects was conducted with compost tea from marine composts. Compost tea is a water extract from compost prepared over several days at room temperature, where biological activity leads to a fermentation process.
We conducted one experiment with lettuce seedlings grown at sub-optimal and optimal temperatures, to study if amendment with compost tea could give a relatively better growth at the sub-optimal temperature. A relatively smaller difference in growth at the sub-optimal and optimal temperatures with compost tea compared to the mineral fertiliser control could indicate a biostimulant effect of the tea, showing that the tea could support plants to overcome cold stress.
The next experiment was conducted with cuttings of blackcurrant. We hypothesized that compost tea could support the development of roots. Root development is another process than plant growth driven by photosynthesis and uptake of nutrients.
Whereas residual materials from both macroalgae, blue mussel, and captured fish have demonstrated positive effects on the growth of crop plants in pot and field trials, we did not find any biostimulant-effect of the amendment of lettuce seedings or blackcurrant cuttings with fermented teas from composts made with marine materials, or from teas made with these materials. The best results were achieved with a windrow compost where residues of macroalgae and captured fish were mixed with woodchips and bedding material from cattle (straw + dung). This compost windrow was established outdoors, on soil, in July 2022, turned twice and sampled in
February 2023. A thorough thermophilic decomposition process was achieved. Even if the windrow was covered with fleece, the humidity was maintained due to significant precipitation. Further, the outdoor conditions supported a high biological activity compared with the compost produced in containers indoors. Tea from the windrow compost increased the growth of lettuce seedlings significantly, but not as much as the same amount of nitrogen applied with a liquid mineral fertiliser. The relative difference in growth between low to high temperature was less with compost tea than with mineral fertiliser. This could be interpreted as a positive biostimulant effect, where the compost tea treatments performed relatively better at the lower temperature. However, we believe that the result is rather due to some negative characteristics of the tea, such
as a high salt content, and that this had a larger effect at the higher temperature. This windrow compost tea gave a quite satisfactory development of roots on blackcurrant cuttings, but not as good as cuttings amended with only water. For the cuttings, we mixed the compost tea with 95%,
90% or 80% water, and we found a clearly negative effect of reducing the water dilution. This is likely due to a high content of salt in the marine composts.
The experiments provided useful knowledge on how to conduct trials to study biostimulant effects. This is a new field of research at NORSØK, that requires a lot of development of appropriate methodology.