Availability of potassium in biomass combustion ashes and gasification biochars after application to soils with variable pH and clay content

Abstract

The expansion of the bioenergy sector and adoption of novel thermal conversion technologies produce increasingly large amounts of biomass ashes and biochars. Before returning such products to agricultural soil, the plant availability of nutrients when mixing with soil should be assessed. The objective of this study was to evaluate the potassium (K) availability in various types of biomass ashes and gasification biochars (GBs) derived from straw, wood, sewage sludge and poultry manure when mixed with soil. A 16-week incubation study was conducted with three contrasting soils of variable pH (5.8–7.8) and clay contents (3–17%). Exchangeable K in the product-soil mixture was determined, and the K recovery rate from the applied products varied from 31 to 86%. The relative recovery compared to applied KCl was used to indicate K availability and was 50–86% across all soils, but lower for two sewage sludge-based GBs in the alkaline soil. Most of the K release occurred within the first week, with insignificant release thereafter. Wet storage of GB did not affect K availability. We conclude that the tested biochars and ashes can be used as K fertilisers with slightly lower short-term availability than KCl.