Technical Notes: A Comparison of Three Soil Surface Moisture and Bulk Density Sampling Techniques D. S. NeSmith, W. L. Hargrove, E. W. Tollner, D. E. Radcliffe ASSOC. MEMBER ASAE ABSTRACT T HE objective of this study was to compare a moisture-density gauge, a small core sampler, and a large core sampler for making soil surface moisture and density measurements. This was conducted in an on- going tillage study. The moisture-density gauge used was a Troxler model 3411-B. The small core sampler was a Uhland-type sampler with core dimensions of 5.4 cm diameter and 5.9 cm length, and the large core sampler was a custom-made device with core dimensions of 14.6 cm diameter and 10.1 cm length. Seventy-two observations were made by each method. For soil water measurement, all methods were well correlated, but the value obtained with the neutron probe was generally greater than either core method. Thus, the factory calibration does not appear adequate for all conditions, and calibration on site appears necessary. The gamma probe did not appear adequate for bulk density measurements of the upper 10 cm of the soil profile. In this study, the small core sampler was the more efficient method for measuring bulk density because of the higher correlation with the large core sampler and because of more convenient handling. INTRODUCTION Soil surface (0 to 10 cm) water content and bulk density are often parameters of interest, especially in tillage studies. Many methods have been suggested for measuring these soil properties, with each having advantages and disadvantages (Blake, 1965; Erbach, 1982; Gardner, 1965). Soil core sampling is considered the traditional method. This technique offers the ability to obtain bulk density and water content from the same soil sample. Generally, the equipment is relatively inexpensive and simple to operate. Disadvantages of this method include intense labor and time consumption. Sampling is somewhat destructive such that repetitive measurements in the same location are not possible. Destructive sampling can present problems on small experimental Article was submitted for publication in March, 1986; reviewed and approved for publication by the Soil and Water Div. of ASAE in August, 1986 as a "Technical Notes" contribution. Contribution from the University of Georgia Agricultural Experiment Station. Supported by State and Hatch funds allocated to the Georgia Agricultural Experiment Station and grant funds from the Georgia Commodity Commission for Soybeans. The authors are: D. S. NeSMITH, Graduate Research Assistant, Crop and Soil Science Dept., Michigan State University, East Lansing (Former Graduate Research Assistant, Georgia Station, Experiment, GA): V^. L. HARGROVE, and E. W. TOLLNER, Associate Professors, Georgia Station, Experiment, GA; and D. E. RADCLIFFE, Assistant Professor, College Station, Athens, GA. plots. Hillel (1980) further suggested that core methods may be subject to errors through excess handling in the form of sampling, transporting, and repeated weighings. Nuclear devices have gained popularity, especially in measuring soil water content. Van Bavel (1958 and 1963) and Greacen (1981) gave a very comprehensive and basic discussion of this practice. This technique is rapid and non-destructive, and offers the possibility of repetitive sampling from the same location. Disadvantages include relatively expensive equipment and possible radiation hazards. Also, installment of access tubes (necessary for many nuclear devices) is ofter laborious, and care must be taken in filling around the tubes to ensure good soil- to-tube contact. Organic matter, soil density, clay content, and stones can affect measurements with nuclear devices (Huser, 1984; Greacen and Schrale, 1976). Measurement near the soil surface with most nuclear devices presents special problems because neutrons escape to the atmosphere and because large differences in water content often occur over a short vertical distance. Several methods have been suggested to overcome some of these problems (Black and Mitchell, 1968; Hanna and Siam, 1980). Farah et al. (1984) suggested measuring soil water content with a neutron moisture meter designed for detecting leaks in roofs. Troxler manufactures such a device which also has a gamma source allowing the measurement of bulk density as well. Greene and Stuart (1984) used a similar instrument to measure bulk density and water contents of a forest soil and found it to be adequate for moisture predictions, but somewhat questionable for determining bulk density. The objective of this study was to compare a moisture-density gauge, a small core sampler, and a large core sampler for making soil surface moisture and density measurements. MATERIALS AND METHODS The gauge used was a Troxler model 3411-B* surface moisture-density gauge (referred to as *probe') and is described in the instrument's manual (Troxler Electronic Laboratories, Inc. 1984. 3400-B Series. Instruction Manual. Research Triangle Park, NC. 50 pp). This model contains a micro-computer which stores all calibration constants and algorithms to compute and directly display density and moisture. Table 1 provides some specifications for the probe. The small core sampler was a Uhland-type sampler (Uhland, 1949) with core dimensions of 5.4 cm diameter and 5.9 cm length. The large core sampler was a custom-made device with *Trade names mentioned are for the benefit of the reader and do not imply endorsement by the University of Georgia. Vol. 29(5):September-October, 1986 © 1986 American Society of Agricultural Engineers 0001-2351/86/2905-1297$02.00 1297