ABSTRACT

Natural and machinery traffic-induced subsoil compacted layers (soil hardpans) that are found in many southeastern US soils limit root growth with detrimental effects on crop productivity and the environment. Due to the spatial variability of hardpans, tillage management systems that use site-specific depth tillage applications may reduce fuel consumption compared to the conventional uniform depth tillage. The success of site-specific tillage or variable depth tillage depends on an accurate sensing of the hardpan layers, field positioning, and controlling the application of real-time or prescribed tillage. The over arching objective of the work was to understand and advance the art and science of soil compaction analysis and prediction with an eye towards compaction management in precision farming. The specific objectives were (1) To investigate the influences of soil parameters (soil moisture and bulk density), and the soil-cone frictional property on the interpretations of cone penetrometer data in predicting the magnitude and depth of hardpans, (2) To determine spatial variability for creating hardpan maps and (3) To investigate a passive acoustic based real-time soil compaction sensing method.

The soil cone

penetration problems were also modeled using finite element modeling to investigate the soil deformation patterns and evaluate the capability of the finite element method to predict the magnitude and depth of the hardpan. Laboratory experiments in a soil column study indicated that the soil cone penetration resistances were affected by soil moisture, bulk density and cone material type. Soil drying increased the magnitude of soil cone penetration resistance and slightly decreased the predicted depth of the hardpan. The small difference (approximately 2 cm) of the predicted depth of the hardpan due to soil drying may imply that cone index measurements for prediction of the depth to the hardpans are less sensitive to soil moisture variations. Cone index readings varied due to the type of cone material. The cone index obtained with Teflon cone material was less as compared to using a stainless steel (ASAE 1999a). By coating dry powder Teflon on the stainless steel cone, the cone index values were between the stainless steel and Teflon.

The spatial variability and the theoretical semivariogram model parameters for Kriging prediction were significantly affected by soil drying in determining the magnitude of soil strength contained in the soil hardpan. The spatial variability of the predicted depth of soil hardpans, however, showed less spatial correlations at dry soil moisture conditions. The magnitude of the soil hardpan was more strongly affected by soil drying than the predicted depth of the hardpan. Precision tillage that varies across the field according to the spatial structure of soil hardpan attributes can be prescribed on soils of the southeastern US to improve the sustainability of crop production.

Besides soil cone penetrometer based hardpan characterization, real-time acoustic compaction layer detection system was developed using a microphone-fitted cone mounted on a tine. The detection of the location of hardpans was carried out on the highest frequency range of

the acoustic signals (termed "detection edge"), which was band filtered on the fast Fourier transformed acoustic signal. High levels of agreement were found between cone index measurements and associated sound levels, which clearly demonstrated the methods' potential to detect hardpans.

INDEX WORDS: Soil compaction, Soil Hardpan, Soil Cone Penetrometer, Finite Element Method, Acoustics-soil compaction, Precision Tillage.