Peanut Shell Biochar as a Standalone Soilless Substrate for Komatsuna (Brassica rapa var. perviridis): Growth Response and Reuse Potential

Biochar is increasingly being explored for soilless cultivation; however, high pH, elevated electrical conductivity (EC), and heterogeneous particle size often limit its use as a standalone growing medium. This study evaluated peanut shell biochar (PS-BC) as the sole substrate for Komatsuna ( Brassica rapa var. perviridis ) and assessed its short-term reuse across two consecutive crop cycles. Pot experiments examined the effects of particle size and PS-BC conditioning (unwashed vs. prewashed), rinsing frequency and reuse, and acid treatments using pyroligneous acids (PAs) and citric acid (CA). Prewashing markedly improved plant growth, and leaf area and dry weight increased by up to approximately four-fold across particle sizes; however, pH and EC decreased by approximately 7% and 89%, respectively. Changes in substrate chemical properties were also observed. Particle size had a lesser effect on plant performance, with the greatest response observed for particle sizes of 5 mm and >7 mm. Interestingly, the rinsing frequency did not significantly affect plant height or dry weight within individual crop cycles; however, reuse of the preconditioned PS-BC was associated with improved plant height and leaf area as well as further shifts in substrate chemical and nutrient statuses relative to the original unwashed substrate. Both PA and CA treatments reduced pH dose-dependently, but only PA treatments increased EC. The best plant performance was observed with 5% CA and 14% rice-derived PA, whereas bamboo PA inhibited growth despite acceptable pH levels. Overall, preconditioned PS-BC effectively supported Komatsuna growth and maintained performance upon reuse across two consecutive crop cycles, demonstrating its potential as a practical standalone substrate for short-term leafy vegetable production.