Agronomic performance of a high ash biochar in two contrasting soils

• Our weekly leaf picking estimate about 133 Mg ha−1 y−1 of green waste in intensive greenhouses.
• Tomato green waste biochar produced at 550 °C concentrates all nutrient elements except N.
• The biochar had very high-pH (12.1), electrical conductivity (54.2 dS m−1) and ash content (56%).
• Biochar produced positive effects on corn shoot yield and nutrient uptake in a well buffered soil (an Oxisol).
• However, in a poorly buffered soil (an Inceptisol) negative effects on yield and nutrient uptake were observed.

Intensive greenhouse industry wastes large amounts of nutrient-rich green waste through improper disposal practices. Converting this greenhouse waste into biochar for soil application offers a viable option to recycle nutrients and long-term C storage. This study was carried out to evaluate the agronomic potential of a biochar produced from tomato green waste in two contrasting soils. We also estimated the amount of waste generated from intensive greenhouse tomato production in Australia. From weekly measurements of leaf picking over a 13-week period, we estimate approximately 133 Mg ha−1 year−1 of green waste on fresh weight basis. Biochar, produced by slowly pyrolysing the green waste at 550 °C, had very high-pH (12.1), electrical conductivity (EC, 54.2 dS m−1), ash content (560 g kg−1) and CaCO3 equivalence (330 g kg−1). Agronomic performance 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−1 soil) in a 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 and contrastingly decreased the yield in case of the Tenosol. The positive effect of the biochar in the Ferrosol was attributed to release of nutrients from the biochar and biochar's liming effect and associated increased availability of nutrients. However, in poorly buffered Tenosol the application of biochar produced phytotoxic effects due to excessive soluble salts and high pH. The uptake of most nutrient elements increased in the corn shoot in the Ferrosol and decreased in the Tenosol. Although the biochar produced from green waste was highly alkaline and contained excessive soluble salts, given the right soil properties it can be a good soil ameliorant. The true agronomic potential of the biochar should be further evaluated in different soil types under field conditions.