The paper describes the filter-paper method as an indirect method for the estimation of the pF characteristics. In this method, no direct measurements of soil water potential (pF) are performed, but the latter is determined on the basis of the gravimetric moisture content of the filter paper, which is in contact with the analysed soil samples. Therefore, there is a need to establish a calibration equation that would express the relationship between the value of the soil water potential (pF) and the gravimetric moisture content of the filter paper. The calibration procedure was performed using standard measurement methods of determining pF curves (sand box, pressure plates) and filter paper (Whatman no 42). Based on the research conducted, calibration curves were developed for the filter-paper method, for the analysed sapric (drainic) histosoles, developed from moss and alder peats of medium (hemic) and strong (sapric) degree of decomposition, originating from the mid-basin of the Biebrza river. The proposed calibration curves were presented as 3-degree polynomial equations. The aforementioned equations can be used for the estimation of retention characteristic for sapric (drainic) histosoles, showing similar properties and comparable degree of secondary decomposition of the peat mass.