[Objective] This research aimed to explore the existing forms of ammonium nitrogen adsorbed in a sandy soil with different particle sizes by extraction experiments and provide references for investigating the transport and transformation of the ammonium in the vadose zone.[Method] Sandy soil sample was collected from a landfill and sieved into coarse sand and fine sand.The three kinds of samples were soaked in NH4Cl solution with different initial concentrations,respectively.Then,ammonium adsorbed in soil samples were extracted by three kinds of extraction agents with different extraction capacity,including water,KCl and CaCl2.[Result] The order of extraction capacity of different extraction agents was：water KCl CaCl2;when the concentration of ammonium was low in solution,the ammonium preferentially adsorbed in the exchangeable positions of minerals and mainly existed in the form of exchangeable ammonium;with the increase of concentration,ammonium entered inside the 2：1 clay minerals with enough driving force of the concentration differences and existed in the form of fixed ammonium;little fixed ammonium was observed in coarse sand samples,which was mainly existed in 2：1 clay minerals with strong extraction capacity.[Conclusion] The existing forms of ammonium were closely related to the mineral compositions in soil and the initial concentrations of ammonium.
