In conventional tillage systems, uniform depth tillage is applied over the entire the field, however, the depth of hardpan layer varies greatly within the field. Site-specific tillage, which modifies physical properties of soil only where the tillage is needed for crop growth, could achieve significant savings in tillage energy. Site-specific tillage can be implemented either through a pre-tillage map technology would be a two-step process in which a sensor such as a soil cone penetrometer would be used to develop maps showing hardpan existence and depth. This map would then be used in the site- specific tillage equipment control system to control tillage implement location and depth. The real-time sensor would provide a one-step system to control tillage implement location and depth. In this research, a GPS-based tractor-mounted cone penetrometer for developing geo-referenced soil resistance to penetration map was constructed. GPS-based variable-depth subsoiler was developed and evaluated. On the go, the controller system of variable- depth subsoiler based on the soil compactiom map, control the tillage depth. The variable-depth subsoiler was first calibrated in the workshop. The response time of the electro-hydraulic actuator was measured. The precision and accuracy were 1.2 and 0.83% respectively. Then, the variable-depth subsoiler was evaluated under actual field conditions. The time required for the implement to reach to set depth under two ground speeds (2.6 and 3.6 km h -1 ) was measured. In the third, part of this work, the spatial variablility of mechanical resistance to penetration (PR) and gravimetric moisture (MC) at a depth of 0-50 and 0-40 cm, respectively, in a silty clay loam soil was studied. Two- and three- dimensional PR and MC maps of one hectare field on the 10 m
