Advances in Soil Mechanics of Structured Agricultural Soils - The North American Perspective.- Strength of Structured Soils Due to Loading: a Review of Processes on Macro- and Microscale; European Aspects.- Soil Deformation and Fracture under Tensile Forces.- Soil Deformation under Compressive Forces.- Effective Stress in Structured Soils.- Strength of Low and Variable Charge Soils.- Effects of Dynamic and Static Loading on Compaction of Structured Soils.- Soil Cohesion Changes.- Unslaked Lime Versus Organic Soils Conditioners to Improve Soil Mechanical and Physical Properties of a Loamy Soil.- Interface Between Implements, Tillage and Soil Structure.- Modeling Soil Disturbance Due to Tillage and Traffic.- Modeling Soil Mechanical Behavior During Compaction.- Root System Responses to Soil Structural Properties: Micro and Macro Scale.- Root Activity Related to Shallow and Deep Compaction.- Tillage and Compactive Effects on Soil Hydraulic Properties and Water Flow.- Tillage and Compactive Modifications of Gaseous Flow and Soil Aeration.- Assessing the Suitability of Different Soils for New Cropping Systems in Terms of Rates of Change in Soil Structure.- Poster Paper Abstracts.- Workgroup Reports.

Agricultural soils are subjected to many applied mechanical stresses that influence their behavior. Stresses arise from tillage machines, seeders and chemicals applicators, tractors, and equipment for harvest and transport. Applied stresses may compact or loosen the bulk soil. Micro sections of soil (aggregates or clods) may be compacted during tillage while the bulk soil is loosened. Because most granular soils are combined into structural units of varying sizes with different strengths and properties, prediction of the effect of stresses on the behavior of bulk soils is difficult. The basic strength properties of soil are determined by many fac­ tors: the size distribution of particles, chemical and mineralogical properties of inorganic clay fraction, the organic matter content and composition, the water content and the stress history. These factors combine and interact to produce many possible behavior patterns. Changes in structure of the soil from applied stresses and biologi­ cal reactions may profoundly influence storage and transmission of water, heat, and air, and the mechanical resistance to penetration by plant roots. They may also affect the traction of vehicles. Manipulation of structural properties of soils by tillage implements is a major cost under most crop production systems. Reduced energy use is also a significant conservation objective. Improved management systems are very dependent upon a better understanding of soils' response to applied stresses. The content of this book resulted from a NATO Advanced Research Workshop held in St. Paul, Minnesota, U.S.A. September 13-16, 1988.