Recycled Phosphorus from Biomass Ash: Fertilizer Performance Across Crops

Biomass ashes represent a promising secondary phosphorus (P) source, yet their agronomic performance depends on feedstock origin, processing, and crop-specific interactions. This study evaluated the P fertilizer efficacy of raw and processed biomass ashes derived from cereal straw and paludiculture biomass, compared with triple superphosphate (TSP), using two sequential greenhouse pot experiments with maize, amaranth, and blue lupine. Processed ash products, particularly compacted ashes and ash–straw mixtures, increased plant biomass and P uptake to levels comparable to or exceeding those achieved with TSP. The cumulative P uptake of the three crops reached up to 250–300 mg pot−1 under processed ash treatments, exceeding the uptake under TSP (≈150–180 mg pot−1) and the unfertilized control (≤80 mg pot−1). However, crop-specific differences were observed: amaranth benefited most from the ash products, whereas combinations of ashes with lupine were less favorable. Beside acting as a P source, processed biomass ashes also increased soil pH by about 0.5 units, improved soil aggregation by increasing macroaggregates (>2 mm) to up to 20% compared with only about 7% in TSP and the control, and promoted favorable shifts in Hedley P fractions. Soil enzyme activities were governed primarily by crop species, with amaranth stimulating phosphatase activity the most. Further research should aim to refine crop-specific application strategies for processed biomass ashes and to elucidate their impacts on soil structure and P dynamics.