Agronomic Performance of Surface Application of Polyferric Sulphate and Tanfloc-Treated Manure Solids to Pastures Under Temperate Australian Conditions

Ensuring the agricultural use of chemically treated manure is not detrimental to pasture productivity or soil health is key to integrating coagulation–flocculation into dairy effluent management systems. This study evaluated the agronomic performance of manure solids treated with polyferric sulphate (PFS) or a tannin-based coagulant/flocculant (Tanfloc SG®) in south-eastern Australia. In two independent experiments using PFS (2022) and Tanfloc (2023), fifteen perennial ryegrass plots (1.5 × 2.5 m) were assigned to treatments: Control (no fertilizer), untreated liquid manure (96 t ha−1), treated solids (96 t ha−1), or urea (100 kg N ha−1). Over nine months, pasture dry matter (DM) yield, soil properties, plant chemical composition, and nutrient uptake were measured. Short-term measurements of CO2, CH4, N2O, and NH3 concentrations were also undertaken. In both experiments, treated solids, urea, and untreated liquid manure achieved a higher pasture DM yield than control, with no statistical difference among treatments. PFS increased plant S and soil Fe, while Tanfloc increased plant Cl and soil P; relative to untreated liquid manure. Treated-solids application reduced N2O and NH3 concentrations at sampling times; however, further research is required to quantify gas fluxes. Our findings demonstrate PFS- and Tanfloc-treated solids are not detrimental to pasture growth and nutritive value or soil properties.