Agronomic evaluation of bone char as phosphorus fertiliser after five years of consecutive application

Abstract

Bone char (BC), pyrolyzed defatted bones from slaughterhouse waste, may be a promising secondary raw material to produce phosphorus (P) fertiliser, but its agronomic value has not yet been shown in longer-term field experi­ments. Phosphorus in BC (15 % P) is mainly bound in a structure similar to hydroxylapatite (HA). Generally, P in HA is of low solubility and, consequently, it can be expected that bones or BC have a low fertilisation value. Better solubility of P is expected from BC enriched with sulphur (BC^plus). In 2013, a field experiment was established to test the fertilisation potential of BC and BC^plus compared to a control (P0) and triple super phosphate (TSP) treatment. According to their mean P_CAL concentrations (0–30 cm) plots were assigned to the initial soil P-test class (iSPTC) A (11 mg kg^–1 P_CAL), B (21 mg kg^–1 P_CAL), and C (47 mg kg^–1 P_CAL). After a first crop rotation an increased cumulative yield was determined for iSPTC-A depending on the fertiliser treatment with means of relative yield of P0 (90 %) < BC (94 %) < BC^plus (95 %) < TSP (100 %). Effects on the relative P uptake of grain in iSPTC-A depending on the fertiliser treatment were in the order of P0 (81 %) < BC (88 %) < BC^plus (91 %) < TSP (100 %) and in iSPTC-B of P0 (84 %) < BC^plus (90 %) < BC (92 %) < TSP (100 %). Fertiliser treatments had no effect on mean yield and P uptake in the first crop rotation if initially sufficient soil P (iSPTC-C) was available. An increase of P_CAL concentrations was only achieved by TSP fertilisation. Future experimental years will show, if BC fertilisers are able to maintain sufficient P availability to crops in the long term.