Nowadays, using soil amendments to improve physical hydrological properties is popularly employed in agricultural engineering. This paper at first reports an experiment to compare the effect of two different soil amendments for their effect on soil water retention capacity. They two agents are the natural clay and a conditioning soil retainer. Soil water retention curve (SWRC) has been selected to quantify their effect on a benchmark pure sand soil in full range of water saturation, i.e. from fully saturated to nearly dry. Both the classic van Genuchten model and a novel three phase soil water retention model have been adopted to characterize the effect of the two soil amending agents on soil water retention capacity. The research results demonstrate that the clay has a significant enhancement on soil water retention at low content of clay and high soil water content range, however its effect reduces considerable with increasing clay content. Meanwhile the conditioning water retainer shows little effect at high soil water content range but has significant effect on soil water retention at low soil water content range. The results indicate the conditioning water retainer can help the reduction of the surface water evaporation and the water reservation underneath. The modelling has shown that the three-phase model is able to effectively represent the soil water retention curve in full range of soil water content, which provides a convenient tool to efficiently characterise the effect of conditioning water retainer. In addition, the three-phase model also provides the functional analysis and help understand the working mechanisms of the agents.