The increased demand for food due to population growth requires that agricultural practices become increasingly more efficient, including the proper use of agricultural chemicals. If improperly applied, nutrients may lead to soil and groundwater contamination through their leaching from the soil root zone. They may be responsible also for increased soil salinity in arid and semi-arid regions such as northeastern part of Brazil. Since many transient physical and chemical processes affect their fate and transport in the subsurface, mathematical models have become popular tools to understand and improve water and fertilizer management practices. This research aimed at using the HYDRUS-1D software package to simulate water and potassium movement, under laboratory conditions, in unsaturated saline soil columns containing two tropics soils: An Ultisol and an Oxisol. Comparisons were made with experimental data. Our studies involved: 1) Measurement of potassium breakthrough curves (BTCs) during steady water flow, 2) Application of similar potassium solutions to initially unsaturated soil columns containing the same saline soils, 3) Simulation of water and potassium transport using HYDRUS-1D, and 4) Performance evaluation of the numerical model. Results indicate that the experimental BTCs for estimating the potassium transport parameters were sufficient to describe the potassium transport during infiltration in the soils, if used in conjunction with estimated soil hydraulic parameters. HYDRUS-1D predictions of the wetting front and potassium concentrations closely followed observed data. Estimated solute retardation factors were found to be less than unity, indicating exclusion of the potassium cations from the positivity charged surfaces of the two tropical soils.