Assessing sewage sludge-made biochar and compochar (compost+biochar mix) as horticultural soilless growing media

Organic and mineral growing media are commonly used in horticultural soilless systems for vegetable cropping in greenhouses and net-houses. However, extraction of these materials adversely affects environmental quality. The objective of this study was to assess the horticultural potential of biochar and compochar (a mix of biochar and compost) growing media—both produced from human (domestic) sewage sludge—as an alternative to tuff/tephra (light porous medium formed by consolidation of volcanic ash). The study was conducted in the Israeli drylands, where lettuce and cherry tomato were grown in a net-house under deficit nutrient management (50% of the recommended rate). The biochemical properties of the three substrates ((i) 100% biochar; (ii) compochar (50% biochar+50% compost); and (iii) 100% commercial tuff) and productivity of the two crops were assessed. For the two crops, the nutrient status of biochar and compochar were similar, and different from that of tuff. In the lettuce, mean values of substrate general quality index (SQI_gen) and substrate quality index minimum dataset (SQI_MDS) were similar for biochar and compochar (0.550 and 0.541, respectively, for SQI_gen; 0.614 and 0.613, respectively, for SQI_MDS), and significantly greater than that of tuff (0.337 for SQI_gen; 0.310 for SQI_MDS). In the cherry tomato, a similar trend was recorded, with significantly greater means for biochar and compochar (0.499 and 0.497, respectively, for SQI_gen; 0.436 and 0.482, respectively, for SQI_MDS) than those of tuff (0.320 for SQI_gen; 0.192 for SQI_MDS). In terms of crop yields, mean productivity of lettuce followed the trend of compochar>biochar>tuff, whilst cherry tomato yields followed the trend of biochar>compochar>tuff. Thus, biochar and compochar can be recommended as viable, sustainable alternatives in soilless horticulture systems. Further, insights of this study highlight the climatic change mitigation and adaptation capacities of biochar and compochar when used as horticultural growing media.