Nutrients in circular horticulture: blending peat with biochar alters interaction with fertigation solution

Biochar can be a sustainable alternative to peat in growing media for horticulture. Partly replacing peat by biochar may change interaction with nutrients in the fertigation. Detailed information on accumulation or release of nutrients by biochar can be obtained by column studies. One-liter columns filled with 100% peat and with 90% peat and 10% holm oak biochar were subjected to irrigation with a nutrient solution (24 days) followed by demineralized water (39 days). Analyses of the leachates revealed that concentrations of Ca, Mg, Na, Fe and Al increased while K, P, NH4-N and Mn concentrations decreased during percolation of the nutrient solution through pure peat. The accumulation of P, NH4-N and Mn by peat was only temporary given that the accumulated amount was subsequently leached out during the water irrigation phase. By adding biochar to the peat, the pH of the leachates was increased by one unit, and most nutrients showed an increased release compared to pure peat. This increase in leachate concentrations was most obvious in the first phase of the column experiment, but after a total of almost 4 L percolating solution, concentrations of Fe, Mn, Ca, K and P were still significantly higher in the peat with biochar compared to the pure peat leachates. Biochar addition caused a temporary sorption of nitrate and a non-temporary accumulation of NH4-N. These interactions between biochar and nutrients is crucial for optimizing the fertigation strategy when replacing peat by biochar in growing media.