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Rick Allen's idea to put the 'shootout' papers on the WWW is a good one. The paper of ours to which Rick referred is available as a PDF file at: http://www.cprl.ars.usda.gov/programs We compared Troxler and CPN neutron probes with the Troxler capacitance probe (Sentry 200). The methods used for field calibration may be of interest. We have found that we can get excellent calibrations with a few simple precautions. 1) Make sure there is a wide spread in the water content data by finding or creating (eg. by growing a crop of sunflowers) a dry site, and then creating a wet site adjacent to it by berming an area and flooding it until the profile is wetted to the depth desired. Let drain to 'field capacity'. 2) Ensure adequate numbers of samples by installing at least three access tubes in both the wet and the dry sites, and by taking four samples around each tube at each depth that is read with the neutron probe. This typically gives enough samples that calibration equations can be broken out by soil layers or horizons and the slopes can be shown (reliably) to be equivalent or not between layers. (The 10 cm depth always requires a separate calibration equation due to loss of neutrons to the atmosphere.) 3) Ensure that samples are good ones. We do this by trenching alongside the access tubes and sampling horizontally around the tube with a Madera probe. This probe has a small cross sectional cutting area and compresses samples very little. Also, after driving in the probe one can see easily if the sample is compressed, by comparing the soil surface inside and outside the probe body. Likewise, one can see if the sample is shattered, which would result in bulk density being too low for that sample. Bad samples can be discarded on the spot and replacement ones taken. Because this probe gives a 60 cm^3 sample volume the volumetric water content can be determined directly and the heterogeneity of bulk density and water content assessed at each depth. Note that the Madera probe was developed by the SCS in the US for sampling downhole as access tubes were installed. Having used the probe extensively in this way I have concluded that the downhole method is less desirable for two reasons. First, only one sample per depth is obtained. Second, despite the best care samples may be compressed and there is no way to directly assess this with a downhole sample. 4) Ensure that the probe is at the correct depth for each reading. We take readings at 10-cm depth and in 20-cm increments below that. We have built stands that slide over the access tubes and keep the gauges a constant height above the soil surface (in our case 81.5 cm from gauge base to soil surface). We then set cable stops to give the desired depths of measurement. With this system we always get reading depths referenced to the soil surface, not to the top of the access tube. In normal field use the user can march through the field quite readily with gauge in one hand and stand in the other. An added advantage of the stands is that the user can operate the gauge while standing, avoiding the back strain incurred when the gauge is set directly on top of the access tube. 5) Ensure that standard counts are not influenced by soil water content. This is another advantage of the stands. We set up the stand on a base plate to take standard counts in the field away from vegetation. Previous to this we saw that standard counts varied depending on whether the soil was sopping wet from a heavy rain or dry (this with the gauge set on its case for the standard count). I'll get some pictures of the stands and Madera probes up on our WWW site for anyone interested. Best regards, Steve -------- Steve Evett srevett@ag.gov http://www.cprl.ars.usda.gov/ USDA-ARS, P.O. Drawer 10, 2300 Experiment Station Road Bushland, Texas 79012. Tel:806.356.5775 FAX:806.356.5750 ---------------------------------------------------