Recycling pyrolyzed organic waste from plant nurseries, rice production and shrimp industry as peat substitute in potting substrates

Organic waste from greens of tomato plants, gardening substrate, rice husks and shrimp-derived chitin were pyrolyzed at 400 °C and 500 °C for 3 h, with the aim to elucidate the feasibility of using such products as replacement of peat in soilless gardening substrates. Characterization of the carbonized organic matter (COM) and the gardening substrate indicated that neither the peat nor the COMs provided the recommended levels of nutrients for the cultivation of tomato plants, although improvements could be obtained using COM/substrate mixtures. The toxicity thresholds for Zn were exceeded significantly by the COMs of the tomato greens and high boron levels were found for all the COMs except for those derived from chitin. In a 40-days pot experiment, germination and development of tomato seeds and plants (Solanum lycopersicum L.) were tested on COM/peat mixtures at 30%, 60% and 100% COM substitution rate. The lack of seed germination on the mixtures with COM from tomato greens is best explained with the high salinity of the COM. Best plant growth was obtained with COM from chitin at 60%, most likely because its high N content satisfied best the N-needs of the growing tomato plants without increasing the pH of the growing media. Moreover, an increase of water retention was evidenced for COM/substrate mixtures. Although the use of COM from chitin and rice husks showed promising results, the proposed recycling of organic waste from agriculture or fishery as soilless gardening substrate requires the development of formulations of COM/peat/and added nutrients with ready-to-use characteristics to increase its feasibility.