Managing Nutrient and Pathogen Leaching: Impacts of Slurry pH Adjustment on Sandy Soil

This study aimed to evaluate the leaching of nutrients and pathogens following the surface application of pH-modified slurry on sandy soil. Three slurry pH modification strategies—mineral and biological acidification (pH 5) and alkalinization (pH 9.5)—were tested using mineral acids or bases, paper-industry by-products, or combinations of additives. We hypothesized that: (i) acidification increases nitrogen (N) and phosphorus (P) leaching through nutrient solubilization, and (ii) effective sanitization reduces the risk of pathogen leaching. A 24-day column leaching experiment was conducted with slurry applied at 240 kg N ha−1 and four weekly irrigation events. Results indicated that nitrate (NO3−) leaching accounted for less than 15% of the total nitrogen applied; however, acidified slurry significantly increased ammonium (NH4+) leaching by 72%. The combination of H2SO4 with sucrose reduced NH4+ and NO3− leaching, although P leaching exceeded 35% of the total P applied. Sulphur (S) concentrations in leachates ranged from 42.3 to 112.8 mg S kg−1 soil, particularly in treatments involving H2SO4 or SO42−—rich additives such as spent acid. Faecal coliform leaching declined throughout the study, with acidified slurry consistently maintaining levels below the threshold for irrigation water (<100 MPN/100 mL). Regarding nutrient leaching, pH-modified slurry may present a higher risk of N, P and S leaching compared to untreated slurry, which could also be interpreted as an increase in plant nutrient availability.