Field evaluation of a boron recycling fertiliser

Abstract

Boron (B) is a plant nutrient and a limited mineral resource. Therefore, secondary B sources such as end-of-life cellulose fiber insulation (CFI) should be preferred for B fertiliser production over primary borates. In addition, crop B fertilisation is challenging because B is only weakly adsorbed in soils and prone to leaching in particular if the soil pH is below 7. The objectives of this study were to assess the effect of pyrolysed CFI (B-Biochar) on crop B uptake in the field and on B leaching in a lysimeter study. B-Biochar was pyrolysed at 600 °C and tested (1) in a field experiment with maize (Zea mays L.) and sunflower (Helianthus annuus L.), and (2) in a lab microlysimeter experiment to study B leaching under simulated rainfall. In the field experiment, B concentration in plant tissue increased by up to 100% with B-Biochar compared to the control (from 29.6 to 61.6 mg B/kg in young sunflower leaves) and was only slightly lower (–10% to –20%) than with water-soluble Na-tetraborate (Borax). This lower uptake was attributed to the slow-release properties of the B-Biochar. In the lysimeter experiment, 41% and 55% of added B through B-Biochar was leached below 16 cm depth when fertilised with 1 and 2 kg B/ha, respectively, but B concentration of the leachate remained below the 1 mg B/L threshold value for drinking water in the European Union. In conclusion, CFI has a strong potential as a secondary B source for fertiliser production, and pyrolysis appears to be a suitable process for that purpose. During the processing of CFI to fertiliser, more focus should be given to slow B release in the future in order to reduce losses by leaching.