Agronomic Potential of a Biochar Derived from Tomato (Solanum lypcopersicum) Green Waste.

Agronomic potential of a biochar made from tomato (Solanum lypcopersicum) green waste

Intensive greenhouse horticulture industry produces large amounts of nutrient-rich plant residues in the form of green waste. This waste is usually left to decompose on the greenhouse floor or in the proximity of greenhouses, wasting valuable plant nutrients, increasing greenhouse gas emissions and acting as a potential pest and disease bank. Converting this greenhouse waste into biochar for soil application offers enormous potential to recycle nutrients, sequester C and reduce greenhouse gas emissions. The aim of this study was to evaluate the agronomic potential of a biochar produced from tomato green waste. Biochar, produced by slowly pyrolysing the green waste at 550 °C, had exceptionally high ash content (56%), very high – pH (12.1) and electrical conductivity (54.2 dS m^-1), and a high CaCO₃ equivalence (33 %). Agronomic value of the biochar was evaluated by growing hybrid sweet corn (Zea mays var. rugosa cv - Sentinel) in the greenhouse for 7 weeks. We used three levels of biochar (0, 5 and 15 g kg^-1soil) in factorial combination with three fertiliser rates (0, 50% and 100% of the recommended rate) applied to two contrasting soils (an Orthic Tenosol and a Red Ferrosol).

Biochar application to the Ferrosol significantly increased the shoot dry matter of corn but contrastingly decreased the yield when applied to the Tenosol. The positive effect of the biochar in the Ferrosol was possibly due to its beneficial liming effect and associated increased availability of most nutrient elements. Addition of the biochar that contains excessive salts and a very high pH to the Tenosol, a sandy soil, with very low buffering capacity, caused phytotoxic effects and reduced the shoot dry matter yield. Concentration of most nutrient elements increased in the corn shoot in the Tenosol and decreased in the Ferrosol, while the opposite corresponding effects were observed on the uptake of the nutrient elements in the respective soils; these effects were primarily driven by the shoot dry matter yield.

Pyrolysis of the green waste offers a contemporary and environmentally friendly solution to recycling nutrients in the tomato green waste. While the biochar produced is highly alkaline and has a high content of soluble mineral salts, given the right soil properties it has an exceptionally high agronomic value. The green waste from intensive greenhouses could be mixed with other feedstocks to create a biochar that could be applied to a wider variety of soils.  In order to assess the true agronomic potential of the biochar, field trials need to be conducted to confirm the greenhouse results obtained in this study.

Key words: Biochar, green waste, black carbon, soil carbon sequestration, pyrolysis