A voltage-based sensor was developed to electronically determine soil physical properties and soil penetration resistance. The sensor was designed to generate a voltage output when a change in one, or more, of the soil properties took a place, disrupting the balance state of the sensor resistance. The performance of the sensor was tested on two soil types, sand soil acquired from a field in Saudi Arabia and clay soil obtained from the delta valley of Egypt. Due to its low water holding capacity, only three levels of soil moisture content (MC) namely 4, 6 and 10% were used when testing the sensor with the sand soil, however, six MC levels (8, 12, 15, 20, 24 and 26%) were used when testing the sensor with the clay soil. For both soil types and for all MC levels considered, the soil bulk density was determined to range from 1.15 to 2.65g/cm3.
Results of the study revealed that the value of the voltage generated from the sensor was proportional to the soil MC. On the average, the sensor voltage increased from 1.98 to 2.42V when the sand soil moisture content increased from 4 to 6% at an average soil bulk density of 1.72gm/cm3 with a linear relationship determination factor (R2) value of above 0.99. For the clay soil, the average voltage increased from 0.38 to 2.12V when the MC increased from 8 to 26% at an average bulk density of 1.32 gm/cm3 with a value of R2 above 0.98. In addition, the sensor voltage was found to be proportional to the soil bulk density (ρ), where for all MC levels, the values of the R2 of the linear relationship between soil ρ and sensor voltage were above 0.90 for the sand and the clay soil, except for soils at MC of 4 and 15% where the R2 value was above 0.80. However, the rate of increase in senor voltage due to increase in soil ρ was observed, for both soil types, to decrease at higher MC levels. Results also showed that the sensor voltage output was proportional, at all MC levels, to the clay soil penetration resistance, where the R2 values were above 0.85. The rate of voltage change as a response to change in soil penetration resistance was higher at lower soil MC levels. The sand soil, on the other hand, did not show any resistance to penetration at all MC levels; therefore, no voltage output was observed from the sensor.
Key words: Sensor, voltage, circuit, soil, moisture content, bulk density, penetration resistance.