Variability of soil physical and chemical properties in different locations within a field is generally regarded as the main causes contributing to variability in crop yield. Soil strength/compaction has been recognized as an important factor in crop growth and yield. By measuring spatially continuous estimates of bulk soil strength by means of sensors; the soil compaction map of a field can be obtained. Soil mechanical resistance measures, such as cone penetration resistance, can be used as an indicator of soil strength and compaction. Measurement of soil mechanical resistance by cone pentrometer is basically a stop-and-go insertion method which is not fast enough to obtain valid data in intensive sampling situations. Generally, this problem has been addressed by obtaining a large number of measurements, a process that is time-consuming and labor-intensive. In this research, a system capable of rapid determination of soil strength at discrete depths was designed, constructed and field evaluated. The sensor is equipped with a 30° prismatic tip and has a cross-sectional base area of 324 mm 2 . The prismatic tip attached horizontally to an S-shape load cell housed inside a shank. Applied force to the load cell is recorded in a data acquisition system with a rate of 10 Hz. Soil mechanical resistance can also be affected by water content. For measuring soil water content simultaneously, a capacitance-base moisture sensor is designed and developed. The sensors were tested in a field with silty clay loam soil in three depths of 20, 25 and 30 cm. Cone Index and soil gravimetrical water content were measured in the same plots. Results showed that the measured soil mechanical resistance depends on soil failure type in front of prismatic tip as well as soil compactness. Average soil horizontal resistance indices (SHRI) for both depths of 20 and 25 cm were similar due to the same failure type which the sensor shank induced in front of prismatic tip. However, when the tip was operating at 30 cm depth, the value of SHRI was increased by three times as compared to 20 or 25 cm depth values. This was due to change in failure type from brittle to compressive
