Use of soil cone penetrometers to detect the effects of compaction and organic amendments in claypan soils

The effective use of soil cone penetrometers to assess soil physical restrictions in claypan soils for crop growth has been limited due to the dependence of soil penetrometer resistance on several factors, such as soil water content, variable topsoil depth and soil texture, and differences in cone penetrometer design and operation. However, improvements in cone penetrometer design and capabilities, such as measurement of apparent soil electrical conductivity (ECa), may make it a more effective tool for assisting in routine soil management decisions in claypan soils affected by compaction or application of organic soil amendments. The objectives of this study were (i) to determine the effects of surface compaction and poultry litter on soil bulk density, penetrometer resistance and soil water content in a claypan soil and (ii) to compare the use of several types of currently available soil cone penetrometers to detect and manage compaction. A field trial was conducted on a Mexico silt loam claypan soil in north-central Missouri for 3 years (2000– 2002). Treatments consisted of two levels of surface compaction (0 and 2 passes of a filled water wagon with an axle load of 2.9 Mg) and two rates (0 and 15.4 Mg ha(-1) per year) of incorporated turkey litter in a split block design with four replications. All treatments were applied to a previously untreated area of the field each year. Surface compaction in unamended soils increased soil bulk density by an average of 0.17 Mg m-3 in the 0–10 cm depth and 0.08 Mg m(-3) in the 10-20 cm depth and penetrometer resistance by an average of 2.52 MPa in the 0-5 cm depth, 1.90 MPa in the 5-10 cm depth, and 1.00 MPa in the 10-20 cm depth over the 3 years of the experiment. Addition of turkey litter significantly lowered soil penetrometer resistance by an average of 0.75 MPa at the 0-5 cm depth from 2000 to 2002. A separate comparison of four soil cone penetrometers, selected to represent a wide range of costs and capabilities, was conducted in the early summer of 2002. The soil penetrometers that were compared were the Profiler 3000 (Veris Technologies), the Investigator Soil Compaction Meter (Spectrum Technologies Inc.), the Soil Compaction Tester (Dickey-john Corporation), and the proving-ring dial gauge cone penetrometer (ELE International/Soiltest Inc.). The Veris instrument had the additional capability of measuring soil ECa. The mechanical resistance values measured by the penetrometers were linearly correlated between all of the units with the exception of the Dickey-john unit. Higher soil ECa was measured for compacted versus non-compacted soils and in soil amended with turkey litter. The use of soil penetrometer resistance with soil ECa measurements may facilitate detection of changes in soil bulk density and soil water content due to compaction, natural variations in soil texture and possible effects of added organic amendments on soil salinity, soil bulk density or soil water content.