Topsoil stratification in ridged soil and implications for the measurement of average wetness and estimation of change in profile wetness by the neutron probe method are discussed. Defining the topsoil of a Haplustalf as the ridge plus the 12.5 cm of soil below the furrow elevation, four strata in terms of bulk density and water retention were identified: ridge, ridge base, furrow, and 10-cm depth. The count ratio was sensitive to ridge geometry and density variables in addition to weighted average wetness. In terms of precision and simplicity, the best linear calibration model included wetness and ridge cross-sectional area as independent variables. The optimum depth of measurement was found to be 3 cm. Failure to recognize the importance of ridge geometry for the top-soil calibration, especially for shallow profiles in uniform soil, may introduce a bias in estimates of change in profile wetness and significantly increase the number of access tubes required for a given level of precision. It is concluded that tillage-induced stratification and geometry of the topsoil must be considered when investigating and modeling the water balance of ridged soil.