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Rick Allen writes Marcus indicated that he is seeking neutron probe calibrations that are based on count ratio for the whole range of soil textures. I concur with Clint Shock (and most likely many others) in that back ground elements, organic matter, probe type, and neutron access tube material, thickness, and diameter all impact the calibration. The American Society of Civil Engineers -- Committee on Irrigation Water Requirements -- conducted a neutron "shootout" back in 1992 in Logan, Utah to compare 1) methods for sampling bulk density, 2) impacts of access tube type/size, and 3) impact of neutron gauge type on calibration. Three soil types were tested. These were a sandy loam, a silt loam, and a silty clay loam soil. The results of the "shootout" were written up in seven papers that were presented in 1993 at the ASCE National Conference on Irrigation and Drainage Engineering, in Park City, Utah. They are included in the proceedings from the conference, on pages 1096 to 1153. The proceedings can be purchased from ASCE, 1801 Alexander Bell Dr., Reston, VA 20191-4400 or in the U.S.A., by toll-free telephone at 1-800-548-2723. The ISBN of the proceedings is 0-87262-919-8. It was the intent of the committee to republish the results of the shootout in a user's manual on calibration and operation of neutron meters, to be published by ASCE. However, work on the manual has stalled out. It is uncertain whether the work will be completed due to the current trend to transition to TDR and other automated measurement systems and away from neutron meters. If there is still a large need in the soil water, irrigation, and hydrologic communities for such a manual, I would appreciate hearing about it, since this might provide the motivation necessary to complete the work. I hope to have the seven Park City papers scanned and placed on the web, in case the proceedings are not accessible to interested readers. I will alert the Sowacs group when this happens. In summary, the shootout concluded that the type of method used to determine bulk density (Giddings boring machine, hand-operated tubes (NRCS Madera and USU soil samplers), Drive cylinder sampler, and gamma gauge) all produced accurate, working measurements of bulk density IF the operators utilized precise care and thoughtfulness. As to neutron meter calibration, we found, to no surprise, that the gauge and access tube material affected the slope of the calibration curve more than did the soil type (soil texture varied widely, but all soils were from the same mountain valley). Even two neutron meters manufactured by the same company produced significantly different calibration curves. Various vintages and types of CPN and Troxler gauges were tested. Calibration curves for aluminum access tubes, as expected, showed higher neutron counts at a specific water content than did steel tubes, and steel tubes showed higher counts than did tubes of polyvinyl chloride. A similar study in 1990 detailed the differences in calibration curves due to the thicknesses and diameters of PVC and aluminum access tubes. The results were presented by Allen, R.G., and D. Segura (1990), "Access tube characteristics and neutron meter calibration" in the Proceedings of the 1990 ASCE National Conference on Irrigation and Drainage (Durango, CO), p 21-31. ------------ I believe that the primary conclusion of the ASCE group was that each guage, access tube, and soil need to be calibrated specifically for one another. It was also concluded that "down-hole" bulk density/soil water content sampling produced sufficiently accurate data for the calibration of the neutron meter (in the same hole), provided that a wide range of water contents from the same or different, adjacent holes were obtained. I believe that Steve Evett, USDA-ARS, Bushland, Texas, has done similar studies and has published writeups on the results. I'm sure that there are others, also. Rick Allen Assoc. Professor Dept. Biological and Irrigation Engineering Utah State University, Logan