Changes in penetration resistance of Ultisols from southern China as affected by shearing B. Zhang a,* , R. Horn b , T. Baumgartl b a Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, PR China b Institute of Plant Nutrition and Soil Science, University Kiel, Olshausenstr. 40, 24118 Kiel, Germany Received 14 March 2000; received in revised form 4 July 2000; accepted 10 August 2000 Abstract Shear stresses and soil properties modi®ed due to stress play an important role during formation of seals in a series of rainfall events and during tillage. The objectives of the study were to evaluate the effects of the penetrometer geometry on the penetration resistance as affected by shearing under different initial soil conditions and to use the information on soil strength to elucidate shearing process. Nine homogenous air-dried soils (<2 mm) were sprayed and stored so as to obtain equilibrium soil water contents. The moist soils were sheared by horizontal displacement of layers of soil particles/aggregates in between hands in one direction. The soil cores were prepared with comparable bulk density before the measurement of maximum penetration resistance (P max ) with a small ¯at tip and a cone tip penetrometers. At a wide range from 0.05 to 6.2 MPa, P max was linearly correlated between the small ¯at tip and the cone tip penetrometers. The conversion ratio was higher under the saturation condition irrespective of the shearing effect. The penetrometer with the small ¯at tip was more sensitive for the weak soils. Shearing generally increased P max in most cases, but it decreased P max for some sandy soils under both saturated and unsaturated conditions and for a clayey soil under the saturated condition. The soil consisting of swelling clay exerted a decrease in P max . Rearrangement and/or sliding of particles/aggregates and increase in soil suction during shearing were attributed to the increase in P max . Increase in porosity due to the aggregation during shearing was ascribed to the decrease in P max . In addition, it was shown that agricultural cultivation resulted in a reduction in soil strength. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Shearing process; Penetration resistance; Soil structure; Ultisols 1. Introduction Surface sealing induced by rainfall results in trans- formation of the surface of a structured soil into a layer with higher bulk density, lower porosity and lower hydraulic conductivity compared with the underlying soil (Moore, 1981). The process results in an increase of soil strength (Bradford and Huang, 1992). In addi- tion to the deleterious effects of reduced water in®l- tration and increased runoff and erosion, it can lead to poor seedling emergence, drought stress, yield reduc- tion due to poor in®ltration, off-site environmental problems and even threaten the sustainability of agri- culture (Sumner, 1995). Shear stress is an important property in formation of seals (Bryan et al., 1989). During rainfall the Soil & Tillage Research 57 (2001) 193±202 * Corresponding author. Permanent address: PO Box 821, Nanjing 210008, PR China. Tel.: 86-25-3369284; fax: 86-25-3353590. E-mail addresses: bzhang@ns.issas.ac.cn (B. Zhang), rhorn@soils.uni-kiel.de (R. Horn), tbaumgartl@soils.uni-kiel.de (T. Baumgartl). 0167-1987/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0167-1987(00)00148-3