Soil Drying Effects on Soil Strength and Depth of Hardpan Layers as Determined from Cone Index Data

Mehari Z. Tekeste, Randy L. Raper, Eric B. Schwab


Site-specific detection of a soil hardpan is an important step in precision farming. Different methods have been developed including the ASABE standard soil cone penetrometer to detect presence of hardpan layers. Most of the newly developed methods use results obtained by a soil cone penetrometer as a reference to validate their potential. Soil factors, mainly soil moisture and bulk density, may influence the cone index measurement and the prediction of the relative strength and depth of the hardpan layer. The effects of soil drying on hardpan characterizing attributes of peak cone index, depth to the peak cone index and depth to the top of the hardpan layer were studied for three compaction levels on a Norfolk sandy loam soil in a soil bin. The soil in the bin was wetted to near saturation and then subjected to four levels of soil drying. A multiple-probe soil cone penetrometer (MPSCP) was used to measure soil cone index. The results showed that soil drying had a significant effect on peak cone index for the single pass compaction (1.78 Mg m-3 within hardpan) and the double pass compaction (1.83 Mg m-3 within hardpan). The peak cone index increased two-fold and 1.3 times due to soil drying from ‘day-1’ to ‘day-4’ for the single pass compaction and for the double compaction, respectively. The depths to the top of the hardpan determined from the depth to the peak cone index and the depth to the top of the hardpan showed a statistically significant decreasing trend for the single pass compaction. The differences, however, were too small (< 2 cm) to justify varying prescription tillage depth due to soil drying.

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