Optimized analytical protocol for quantifying small microplastics in greenhouse agricultural soils

Intensive greenhouse farming in Almería (Spain), Europe’s largest concentration of plastic-covered cultivation, has raised concerns about microplastic contamination in agricultural soils. This study aimed to develop and optimise an accurate analytical protocol for the efficient extraction and reliable identification of the most common microplastics (MPs) found in agricultural soils, particularly polyethylene (PE) polypropylene (PP), polyethylene terephthalate (PET) and polybutylene adipate terephthalate (PBAT). To this end, we evaluated multiple extraction agents including two inorganic salt solutions (NaCl and CaCl₂), four surfactants (SDS, CTAB, Tween 20, and Triton X-114), and two oxidative reagents (Na₂S₂O₈ and Fenton’s reagent) to determine the most effective combination for maximising microplastic recovery. The method was validated for reference microparticles as small as 50 μm and achieved recovery rates above 75% for most particle-type and polymer combinations. Application to real greenhouse soils revealed microplastic concentrations ranging from 1,133 to 8,500 particles/kg. Fragments dominated the morphological profile (62%), predominantly in the 100–250 μm size range. Chemical analysis identified PP as the most abundant polymer (70%), followed by PET (16%) and PE (6%). This study presents the first comprehensive chemical and morphological characterization of MPs in agricultural soils from plastic-covered greenhouse systems in a critical region for protected agriculture globally. The validated protocol provides a robust analytical tool for monitoring microplastic contamination in intensive agricultural environments.