S-LAP (Soil Leucine Aminopeptidase) refers to a class of enzymes that hydrolyze peptide bonds at the N-terminal leucine residues and are secreted by soil microorganisms. Changes in S-LAP activity are closely associated with certain pathological states in organisms. This assay kit provides a simple, convenient, and rapid method for detecting S-LAP activity in soil samples. The detection principle is based on the hydrolysis of L-leucine-4-nitroanilide by S-LAP to produce p-nitroaniline, which has a maximum absorption peak at 405 nm. S-LAP activity is calculated by measuring the rate of increase in absorbance.