Abstract: Background: Most highly weathered soils in the tropics contain high concentration of (hydroxide) minerals, which are responsible for phosphorus adsorbed and consequently not available for plants. In practical manner Phosphorus (P) adsorption on acid mineral soils is commonly inhibited through addition of organic matters. Materials and Methods: For understanding the importance of organic matter addition on reducing P adsorption, this study was conducted to measure P adsorption on goethite as affected by humic acids and compare the result with P adsorption on acid mineral soil. Phosphorus adsorption to the goethite was studied by a bath method. A serie of phosphorus solution of 0.1-0.75 mM phosphorus and 0.1 and 0.3 g L–1 humic acids was added to 1 g L–1 goethite suspension in background electrolyte of 0.01 M NaNO3 in which its pH was adjusted to certain range of pH with adding acid or alkaline solution. Results: In the absence of humic acids P adsorption to goethite was more pH-dependent than in the presence of humic acids. Conclusion: This implies that there is a change in charging behaviour of goethite due to the concomitant or previous adsorption of humic acid on goethite. Humic acid reduces P adsorption on goethite much more pronounced at pH<6, suggesting that in the normal pH, humic acid is very important to make phosphate more available for plants. The result was consistent with increasing P adsorption on acid soil from the top soil layer previously leached by rainfall.