A Dynamic Cone Penetrometer for Measuring Soil Penetration Resistance

ters to characterize both agricultural and rangeland A DYNAMIC CONE PENETROMETER soils, however, has been limited by concerns about (i) FOR MEASURING SOIL instrument cost, (ii) measurement repeatability, (iii) limited ranges of soil resistance that can be measured by PENETRATION RESISTANCE a single penetrometer, and (iv) difficulties in comparing Jeffrey E. Herrick* and Tim L. Jones data collected using penetrometers designed for different soil resistance ranges (Fritton, 1990; Vyn and RaimAbstract bault, 1993). The dynamic penetrometer design described addresses these concerns. Recognition of the importance of soil compaction is increasing, but instrument cost, measurement repeatability, and data interpretation limit its measurement on agricultural and rangelands. The dyStatic Penetrometers namic penetrometer described here follows American Society of AgriA number of static designs are commercially available. Most cutlural Engineers standards, but replaces the proving ring with a strike plate, a shaft extension, and a sliding hammer. The penetrometer consist of a rigid, cone-tipped rod attached to a pressure meacone is pushed into the soil by successive hammer blows. Penetration suring device. The measuring device is usually a load cell or resistance is calculated as the work by the soil needed to stop cone strain gauge coupled with an analog dial or pressure transducer movement divided by the penetration distance. The work by the soil for readout. The force exerted by the operator (either average is defined as the kinetic energy of the hammer when it impacts the or maximum) is normalized to the basal area of the cone to strike plate. Construction cost is approximately $100 to $150. The form a parameter called the cone index (i.e., pressure applied standard drop height and hammer mass ensure measurements are to the cone), usually reported in kilopascals (American Society consistent between operators. of Agricutlural Engineers, 1992). A manually operated, static penetrometer developed by the U.S. Army Corps of Engineers WES (Waterways Experiment Station, 1948) is endorsed by the ASAE (American Society of Agricutlural Engineers, 1992) I interest in the effects of soil compaction and is commonly referred to as the “Corps of Engineers” or on soil quality has created a demand for tools which “COE” penetrometer (Bradford, 1986). This design is widely measure soil penetrability or penetration resistance on used in agricultural soils (Radcliffe et al., 1989; Clark et al., a routine basis (Romig et al., 1995). It has long been 1993; Vyn and Raimbault, 1993; Mullins et al., 1994). A variarecognized that compaction affects both root growth tion on this design, found in pocket penetrometers, uses a blunt and soil water and air availability to roots, and that tip and nonrecessed shaft to measure unconfined compressive increased penetrometer resistance is correlated with strength (Bradford, 1986). compaction when all other factors are held constant Manually operated static penetrometers suffer from several limitations. They (i) are relatively expensive, (ii) must be (Baver et al., 1972). The most common method for meamoved through the soil at a constant velocity, (iii) must be suring compaction is to determine cone index values recalibrated on a regular basis in order to generate consistent, using static penetrometers. Static penetrometers are derepeatable measurements, and (iv) are designed for a relasigned to measure the force required to push a probe tively limited range of soil resistance. The cost for a standard (usually a cone or blunt tip) through the soil at a constant Corps of Engineers instrument equipped with a strain gauge (static) velocity. Dynamic penetrometers form a second is ≈$600. While not unreasonable when compared with other general class (Perumpral, 1987). These probes rely on research tools, this puts the instruments out of range of most one or more discrete applications of kinetic energy to extension workers and crop consultants who are seeking a advance the probe (Table 1). rapid, reliable indicator of soil compaction. More recently, Cone indices, computed from static penetrometer lower-priced strain gauge-based instruments have become data, have been used to characterize soil compaction available, but these appear to be less durable and lack a recalibration option. Manually operated penetrometers often yield and resistance to root growth (Barber, 1994; Mullins variable results when used by the same operator and especially et al., 1994), tillage effects (Vyn and Raimbault, 1993; when used by different operators because of differences in the Busscher et al., 2000), wheel traffic effects (Sharratt et rate of insertion. Correct interpretation of static penetrometer al., 1998), and hard pan resistance (Radcliffe et al., data also requires insertion into the soil at a constant velocity 1989). The values (Fritton, 1990) depend on cone prop(i.e., probe acceleration equal to zero), so that the soil resistive erties (i.e., diameter, height, and included angle), as force can be assumed equal to the total force applied to the well as soil properties (e.g., bulk density, shear strength, penetrometer. If penetrometer velocity changes, then the soil water content, and texture). Use of existing penetromeresistive force will be either more (negative probe acceleration) or less (positive probe acceleration) than measured by J.E. Herrick, USDA-ARS Jornada Exp. Range, MSC 3JER, and T.L. the operator. Constant probe velocity is difficult to maintain Jones, Dep. of Agronomy and Horticulture, New Mexico State Univ., in manually operated penetrometers. Box 30001, Dep. 3Q, Las Cruces, NM 88003-0003. Joint contribution In addition to variable penetration velocity within a single from the USDA-ARS Jornada Experimental Range, and the New measurement, different operators generally develop different Mexico Agric. Exp. Stn. Received 8 Jan. 2001. *Corresponding author average penetrometer velocities because of different physical ([email protected]). strength and leverage. Laboratory studies have demonstrated that differences in average penetrometer velocities alone Published in Soil Sci. Soc. Am. J. 66:1320–1324 (2002).