Effect of anaerobic-digested and lime-stabilized dairy processing sludge on phosphorus dynamics in grassland soils with varying textures

Dairy processing sludge (DPS) is a bio-based alternative to chemical phosphorus (P) fertilizer in agriculture. However, there is limited information on the effect of bio-waste on P dynamics in soils with varying textures amended with treated DPS. Changes in P availability and supply potential, were measured in mineral soils amended with anaerobic digested (AD-DPS) and lime-stabilized dairy processing sludge (LS-DPS). Grassland soils with varying soil textures namely, sandy loam, loam, and clay were incubated with triple superphosphate (SP), AD-DPS and LS-DPS at a rate of 40 kg ha−1 P and 100 kg ha−1 P, respectively for 6 months. Changes in chemical properties including available P (Morgan's P), P sorption properties and pH, were observed. The LS-DPS significantly (p < 0.05) increased soil pH across all soil textures, unlike the AD-DPS. At an application rate of 40 kg ha−1 P, the LS-DPS increased P availability in only the loam soil from P deficient to agronomic target P levels. There was no change in P availability for all soils amended with AD-DPS at this rate. Applications of 100 kg ha−1 P in LS-DPS increased P availability across all soil textures and LS-DPS amended loam and sandy loam soils recorded lower P binding energies at both 40 kg ha−1 and 100 kg ha−1 rates, compared to AD-DPS and SP amended soils. The increase in soil pH coupled with a lower P binding energy in LS-amended soils increased plant-available P in all soils. Thus, the LS-DPS is a feasible replacement for synthetic fertilizer in P deficient grassland. However, at an application rate of 100 kg ha−1 care must be taken to avoid overloading soils with P using the LS-DPS. In conclusion, further research should evaluate the feasibility of alternate application of LS-DPS and AD-DPS for the build-up of P across soils of varying textures.