Residues of pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine], trifluralin ([alpha], [alpha], [alpha]-trifluoro-2,6-dinitro-N, N-dipropyl-p-toluidine), and oryzalin (3,5-dinitro-N⁴, N⁴, dipropylsulfanilamide) in soil organic matter