Soil structure evaluation by using a fiber-optic sensor

Soil structure is an important measure of soil quality that significantly a ffects crop production, water-use efficiency, and soil erosion. More effect ive techniques to measure soil structure are needed to determine if a tilla ge operation achieves the desired result and whether soil management practi ces are improving or degrading soil structure. A fiber-optic displacement s ensor was used to scan the surface of bulk samples of soil aggregates. The soil was air dried and sieved to aggregate size fractions of < 0.18, 0.18-0 .25, 0.25-0.5, 0.5-1, 1-2, 2-4, and 4-8 mm. The sensor following a blade to level the surface at a constant distance beneath the sensor tip, was moved at 5.08 mm/s by a universal testing machine. Voltage output of the sensor was recorded with data acquisition software at sampling rates of 10, 100, a nd 200 Hz. Mean sensor output voltage decreased significantly (P = 0.05) as aggregate size increased. Values measured for the aggregate size fractions , in the order listed above, were 4.6, 3.1, 2.5, 1.5, 0.8 and 0.2 V, respec tively. Data sampling rate had negligible effect on mean output voltage. Th e number of peaks in the sensor output signal was affected by aggregate siz e, but was comparatively independent of the gap between sensor and soil sur face. Data sampling rate should be greater than 20 readings/mm in order to use the number of peaks in the sensor output signal for discriminating amon g aggregates of the sizes evaluated in this research. The results indicated that the fiber-optic soil structure sensor may have potential for use in e valuating soil structure.