The repeatability of reusing peat as horticultural substrate and the role of fertigation for optimal reuse.

Direct reuse of horticultural substrates may facilitate the transition from linear to circular greenhouse cultivation. Characteristics of spent growing media from strawberry cultivation were compared to substrates from ornamentals, mushrooms, microgreens, and greenhouse vegetables. Direct reuse of spent peat as a stand-alone material was tested in three trials under commercial conditions. In the first trial strawberry cultivation on reused growing media with included crop residues resulted in similar yield as for virgin blends in a spring cultivation. In the second trial repeated reuse of spent growing media was tested in an autumn cultivation, either with direct reuse or with reuse after shredding and steam treatment for sanitation. Nutrient retention did not increase further during repeated reuse. Yield was similar to virgin peat for the direct reuse and steam sanitation treatments. Under reuse deterioration in terms of physical properties was not observed. Biological stability remained high for all media during reuse. Reusing peat for strawberry 3 times did not lead to any problems for yield or substrate. In the third trial the effects of fertigation regimes with reduced P and K input on nutrient retention during direct reuse in a spring cultivation were limited. In contrast, significantly lower N, P, K, Mg and Ca contents in the substrate were measured when fertigation was replaced by water in the last two weeks of the cultivation. Cation exchange capacity in the spent peat in all trials remained high during reuse. Ca was the dominant cation on the exchange complex.