This study was undertaken to develop a method for the extraction of trifluralin (α,α,α, trifluoro‐2,6 ‐ dinitro‐N,N ‐ dipropyl‐p‐toluidine) from soil and humic acid. The adsorption‐desorption constants were evaluated through Freundlich isotherms. The studies were carried out with a Podzolic soil from Pindorama, SP, with the humic acid (HA) extracted from the same soil, with the same soil free of organic matter by calcination, and finally with the same soil fortified with 2% of humic acid. The results indicated that the quantity of organic matter was a key factor for the adsorption of the herbicide trifluralin in soil. The order followed by trifluralin with respect to its adsorption was: humic acid > soil spiked with 2% of humic acid > soil > calcinated soil.