The improper discharge of industrial and urban residues and the inadvertent use of fertilizers and pesticides can result in soil and water pollution and improve the potential of trace metals to enter in the human food chain. Adsorption reactions occur at the solid/liquid interface and are the most important mechanisms for controlling the activity of metal ions in soil solution. In a complex system with amphoteric behavior, the comprehension of the mobility, availability and fate of pollutants in the soil system is crucial for the prediction of the environmental consequences and for development of prevention/remediation strategies. Metal adsorption by variable charge soils depends on soil solution parameters, due to both the extent and signal of the surface charge, as well as the metal speciation in soil solution, changes in response to liquid phase parameters, above all the pH and ionic strength. A comparative study of cadmium (Cd), copper (Cu), nickel (Ni) and zinc (Zn) adsorption by highly weathered soils was conduct using quantitative adsorption parameters: (i) parameter D=%ads/(100-%ads), calculated and considered as log{[M]_ads/[M]_sol} in order to transform the S-shaped curves obtained from %ads vs pH (Kurbatov plots); (ii) pH₅₀, defined as the pH value at which 50% of the initial metal concentration is adsorbed, and; (iii) free energy of Gibbs variation (ΔG), a measurement of the reaction strength calculated from the thermodynamic molar relationship ΔG=RT(log[M]_sol – log[M]_added). Surface (0-0.2m) and subsoil (B horizon) samples were taken from a Rhodic Kandiudalf (RH), an Anionic “Xanthic” Acrudox (XA) and an Anionic “Rhodic” Acrudox (RA), located in brazilian humid tropical area. As the pH and the ionic strength are important environmental factors influencing the solution chemistry of heavy metals in variable charge systems, adsorption envelopes, in a batch adsorption experiment, were elaborated by reacting, for 24 h, soil samples with individual 0.01, 0.1 and 1.0 mol L^-1 Ca(NO₃)₂ aqueous solutions containing nitrate salts of the adsorptive heavy metal (Cd, Cu, Ni and Zn) at the initial concentration of 5 mg L^-1, with an increasing pH value from 3.0 to 8.0. In the simple correlation analysis, pH explained the majority of the variation in adsorption parameters for Cd, Ni and Zn. A sharp increase of adsorption density (adsorption edge) was observed within a very narrow pH range, usually less than two pH units. Kurbatov plots for Cd, Ni and Zn adsorption could be distinguished into three linear parts, correspond to regions I, II and III. The line slope was comparatively steep within both the low (4.0 to 5.0) and the high (6.0 to 7.0) pH range, while region II was relatively flat region within the middle pH range (5.0 to 6.0). Probably, in region I, specific adsorption was the major mechanism. In region II, because of the increase in negative surface charge, electrostatic adsorption contributes to a certain extent. In region III, where probably the degree of hydrolysis of metal ions increases, specific adsorption of metal species in the form of MOH⁺ becomes the dominant mechanism. Relevant curves for Cu adsorption exhibited only one region along the 4.0 to 7.0 pH range. Cadmium and Cu exhibited the highest and the smallest pH₅₀ values, respectively, while Ni and Zn presented intermediate pH₅₀ values. The negative values of ΔG approved the feasibility of the adsorption reaction and the spontaneous nature of metals ions adsorption. Independently of the sampling depth, metals adsorption and the free energy of the reaction increased with the pH increasing. pH₅₀ values for Cd, Ni and Zn increased as the ionic strength increased, but not for Cu. ΔG for Cd, Ni and Zn adsorption decreased as the ionic strength increased. This behavior indicated the weak electrostatic bonding mechanisms involved in the formation of outer-sphere complex between these metal ions and the soil adsorbents. The highest ΔG values for Cu adsorption, reached at a smaller pH value than Cd, Ni and Zn, and its smallest pH₅₀ values, indicated that Cu retention can preferentially occurs. Additionaly, as the smaller the pH₅₀, the more selective the adsorbent for the metal, and then, the follow affinity sequence was suggested: Cu>Zn>Ni@Cd. Commonly, the relative affinity of a soil for a metal cation increases with the tendency of the cation to form inner-sphere surface complexes. This may be caused by differences in extent of hydrolysis of Cu ions and in affinity of adsorption sites for Cu.