The objective of this experiment was to assess the specific water capacity (Cθ) for soil containing wide range of carbonate minerals (3.2 - 352 g kg-1). A loam soil containing 334 g kg-1 carbonate minerals was utilized to prepare seven soil materials containing 305, 251, 203, 152, 103, 51 and 3.2 g kg-1 by removal carbonate methods, and sample containing 352 g kg-1 was prepared by adding pure calcium carbonate. Therefore nine samples of soil were used in this experiment. The soil-water retention curve (θ(ψ)) was estimated. Computer program (RETC code) were used to determine the best-fit for experimental data of water potential verses volumetric water content which have nonlinear relation and to determine the parameter of van Genuchten equation [α, n and m with m=1-(1/n)]. The derivative of van Genuchten equation was used to measure the change in the slope of soil water retention curve Cθ for estimate the capacity of soil water retain or lost at deferent water potential. The capillary rise equation was used to estimate the effective pore diameter (D). The result show that Cθ values increased with increasing carbonate minerals and the maximums values of Cθ were found at 8 to 10 kPa water potential for all samples which reflected on the amount of water maintained or the amount of water lost. The relationship between D and Cθ showed that the maximums values of Cθ found at soil pore diameters of size about 100 μm, and the soil sample which has maximum Cθ will be the fastest or easiest to lose moisture content of the pores with a size greater than 100 μm. Soil samples showed that lose of moisture content followed the order 334> 352> 305> 203> 152> 103> 51> 3.2 gm carbonate minerals. kg-1 soil, which indicate that the soil’s ability to retain water decreased with increasing carbonate minerals content.