End-of-life of organic growing media: assessing the residual nutrient concentrations in mineral and organic growbags of tomato

The composition of a horticultural substrate for soilless greenhouse cultivation directly affects the sustainability of the cropping system but has also an indirect effect through the end-of-life of the spent media. These spent media can be an important source of nutrients. Reducing the nutrient accumulation in spent growing media at the end of soilless cultivation is crucial in the framework of circular horticulture and environmental sustainability. Understanding the impact of the growing medium composition on the carbon (C) stability and the nutrient accumulation in growing media blends allows to optimize the nutrient use efficiency, i.e., minimize the losses by nutrient accumulation in soilless cultivation. Horticultural substrates at the end of tomato cultivation were compared under variable conditions of cultivation length or fertigation scheme in six trials at commercial scale. Mineral wool and 100% coir slabs, being the conventional substrates, were compared with 100% peat-free, organic blends with variable doses of biochar. Besides plant growth and yield, the accumulation of nutrients in the spent growing media was measured to assess the effect of the growing medium on the nutrient use efficiency. Spent rockwool was four times higher in total Ca and Mg but was also characterized by higher P and K concentrations than the peat-coir and peat-free blend. In comparison with the virgin peat-free blends, there was a clear decrease in K content during use as growing medium for tomato for the peat-free blends, which indicates K leaching and/or crop uptake. An increase in biochar concentration in the peat-free blends up to 10 g L^-1 did not affect the nutrient concentrations in the spent growing medium.