Improving soil test P assessment at plot scale is essential for productivity in conservation agriculture systems. We characterized the distribution of Mehlich-3 P (P-M3) concentrations at the decimetric scale with depth on either side of the sowing row in no-till (NT) and moldboard plow (MP) plots fertilized with 35 kg P ha(-1) every 2-yr in a corn-soybean rotation (20-yr). A total of 996 soil samples (83 samples x 2 depths [0-5 and 5-20 cm] x 6 plots [3 blocks each MP and NT]) were collected at corn harvest in 2012. The average PM3 concentrations in the 0-to 5-cm layer were 35.7 and 63.4 mg kg(-1) in MP and NT, respectively. The PM3 concentration in the 5-to 20-cm depth was similar between MP and NT and averaged 32.0 mg kg-1. The horizontal distribution of PM3 concentrations in these plots was less sensitive to extrinsic factors including tillage, P fertilization and soil depth. High coefficients of variation were associated with PM3 data in both MP (77 and 63% at 0-5 and 5-20 cm, respectively) and NT plots (46 and 66% at 0-5 and 5-20 cm, respectively). It is possible that this strong overall variability overshadowed any PM3 pattern that could have been introduced by NT management. Geostatistical semivariance analysis indicated a predominance of random spatial dependence in most plots, except two plots (one MP and one NT) with moderate spatial structures. The 2-D geospatial model related to tillage was not detected by the sampling grid used at this experimental site. Therefore, a similar sampling strategy would be appropriate and could be recommended for these two tillage systems in this long-term corn-soybean rotation.< Réduire