Ash from the thermal gasification of pig manure—effects on ryegrass yield, element uptake, and soil properties

Abstract

Effects of thermally gasified pig manure ash (GA) and lime-free gasified ash (LF-GA) on properties of an acidic soil (pH 4.5) and the growth and elemental uptake of ryegrass (Lolium rigidum Gaudin) were investigated. The GA was an effective liming agent (2% addition raised soil pH from 4.5 to 7.9); both GA and LF-GA increased soil electrical conductivity and bicarbonate-extractable phosphorus (P). Soil fertilised with LF-GA supported slightly higher plant dry matter (DM) yield than GA (1.5–1.7 v. 1.2–1.5 g DM/kg soil) for the first harvest, due to greater initial P availability at pH <5 than at pH >6. However, plant yields for the subsequent two harvests were similar, as soil acidity dissolved lime in untreated ash (GA) over time. Maximum yields for ash-treated soil and soil treated with mono-calcium phosphate (MCP) were similar. Relative agronomic effectiveness of P sources for three harvests, based on plant P content compared with values for MCP, were 6, 11, and 12% for GA and 19, 10, and 33% for LF-GA. Internal efficiency of P utilisation was similar for all three P sources for each harvest, indicating that differences in yield were mostly a consequence of differences in P supply. Heavy metal concentrations in plants fertilised with ash were minor and within regulatory limits. In general, application of ash did not systematically affect the concentrations of elements (Al, B, Cd, Mg, Mn, Fe, Pb, S, Se) in plants.