Waste products as alternative phosphorus fertilisers part II: predicting P fertilisation effects by chemical extraction

摘要

Knowledge of the relative agronomic P efficiency (RAE) of waste products compared with mineral fertiliser is essential for establishing reliable fertilisation plans. This creates the demand for an evaluation of laboratory methods that can predict RAE of waste products. In this study we compared eight chemical extraction methods in terms of their ability to predict readily-available P and late-season P fertilisation effects of a wide range of waste products (two biomass ashes, meat bone meal, fish sludge, catering waste, two food waste-based digestate products) at two soil pH levels, approximately pH 5.5 (pH level 1) and pH 6.9 (pH level 2). Extractable P was correlated with RAE determined in a bioassay with ryegrass (Lolium multiflorum) and a nutrient-deficient sand-peat mixture as model soil. At pH level 1, P extraction with 0.005 M CaCl2 and simultaneous adsorption to iron-oxide impregnated filter paper predicted RAE best at harvest 1 and neutral ammonium citrate predicted RAE best as the sum of harvests 2, 3 and 4. At pH level 2, 0.5 M NaHCO3 (pH 8.5, Olsen P) was the only extraction method that was significantly correlated with RAE at harvest 1 and the sum of harvests 2, 3 and 4, because all other extractions overestimated RAE of waste products containing stable calcium phosphates. The optimum extraction method thus depends on whether readily-available P or late-season P fertilisation effects are being predicted and on the pH of the target soil.