Soil & Tillage Research 76 (2004) 1–11 Assessment of spatial variability in penetration resistance as related to some soil physical properties of two fluvents in Turkey Kenan Kılıç a,∗ , Engin Özgöz b , Fevzi Akba¸ s a a Department of Soil Science, Faculty of Agriculture, University of Gaziosmanpa¸ sa, 60240 Tokat, Turkey b Department of Agricultural Engineering, Faculty of Agriculture, University of Gaziosmanpa¸ s a, 60240 Tokat, Turkey Received 17 January 2003; received in revised form 23 July 2003; accepted 8 August 2003 Abstract The objectives of this study were to determine the relationships between soil penetration resistance (PR) and some physical soil properties affecting soil PR, with classical statistical and geostatistical methods, and to observe the formation of a plow pan and its variability in the study area, which was conventionally tilled for a long period of time. Soil textures of fields studied were clayey loamy (typic Ustifluvent) and loamy (mollic Ustifluvent), respectively. Each field of 1.5ha was divided into 15 m × 15 m grids (6 lines on the north–south direction and 11 lines on east–west direction). Soil samples were collected at two depths (0–15 and 15–30 cm) of each grid center. Bulk density (BD) had the minimum variability in proportion to water content, clay, silt, and sand contents, and PR had the maximum variability in both soils. PR values for both fields ranged from 300 to 5000 kPa, having a mean of 1522 kPa at 0–15 cm depth and 3844 kPa at 15–30 cm depth of clayey loamy, and a mean of 2210 kPa at 0–15 depth and 4355 kPa at 15–30 cm depth of loamy. Both fields have been applied conventional tillage forming from mouldboard plough (about 20 cm depth), cultivator (about 15 cm depth) and disc harrow (about 10 cm depth) for a long a period of time. It can be concluded that the tillage practices caused the formation of a plow pan at a depth of 20 cm where the maximum PR values were obtained. The correlation coefficients between BD, clay content and sand content at clayey loamy, and water content and BD at loamy and PRs were significant. Soil properties with strong spatial correlations (low nugget variance/total semivariance ratio) were water content, clay, silt, and sand contents and PR in the clayey loamy, and BD and sand content in the loamy. Semivariograms of these variables with high spatial dependence were fitted into the linear model. Range values were greater than 107 m. Soil properties were considerably spatially varied at small distances at 15–30 cm depths of both fields. PR had higher spatial variability at smaller distances than the lowest separation distance at clayey loamy as compared to the loamy. The results showed that spatial variability in PR depends on BD, clay content and sand content at clayey loamy, and water content and BD at loamy fields. Continuous moldboard plowing at the same depth caused formation of a plow pan below the tillage depth. This plow pan can be alleviated by varying depth of tillage over time or by special tillage operations. © 2003 Elsevier B.V. All rights reserved. Keywords: Penetration resistance; Soil compaction; Soil properties; Spatial variability ∗ Corresponding author. Tel.: +90-356-2521616x2175; fax: +90-356-2521488. E-mail address: kkilic@gop.edu.tr (K. Kılıç). 1. Introduction Soil compaction is described as decreasing poros- ity or increasing dry bulk density (BD) as a result of firm-pack soil particles (McKyes, 1985). Compaction 0167-1987/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2003.08.009