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NOTE: To get off this list, send email to majordomo@aqua.ccwr.ac.za with the body of the message containing the line: unsubscribe sowacs I think my comments probably started this rumour so I had better comment. A neutron moisture meter generic (or ANY) calibration is likely to have LESS error in working with water changes than with water content because clay content in the soil is a major cause of the error in both the calibration and the measurement. The change in counts of a point in the field as it dries is unaffected by field variability (of course). If you combine two or more such pairs of points to produce a calibration then field variability is (of course) introduced, BUT if you plot change in counts against change in water content then (in a field with variable clay or OM) the error in the slope is much less than if you plot EXACTLY THE SAME RAW DATA as counts against water content. The reason is quite straight forward - the NMM responds to Hydrogen. While there are a number of other factors affecting its response, the MAJOR source of error is changes of soil H which are not reflected in change of soil water. the main sources of non water H are in clay and organic matter. Soils with highly variable clay (eg duplex soils) or variable OM have a large risk of a biased calibration slope. (where the water content is correct but the slope is wrong) Just because a calibration has less error (ie less variability) does not mean it is more accurate. It could still have the wrong slope hence you will get a very precise answer but it will be WRONG! My advice is to calibrate - or at least check the calibration with a few measured points. One useful shortcut is to use gravimetric sampling to get water contents and a few soil density measurements to convert them to volumetric. This is fast and cheap. MUCH better than never knowing if you have the right answer or not. owner-sowacs@aqua.ccwr.ac.za wrote: > >Claims have been made that a 'generic' calibration equation will yield the > same CHANGE in water content/ depth of a summed profile as a site specific > calibration curve but will have different absolute values. This means that > a generic equation is just as helpful in determining such measurements as > plant water use as a site specific equation. ... > < < < > > > > To SOWACS: > The use of a generic calibration to determine relative change in soil > water content from one period to the next does not require an accurate > absolute water content - > PROVIDED THAT ANY ERROR IN INDICATED SOIL WATER CONTENT DUE TO THE > GENERIC CALIBRATION ERROR IS CONSTANT OVER THE MEASUREMENT RANGE. > > My own experience has been a lot less lucky, that is, calibration errors > varied from dry to wet and were greatest in the wet end. The calibration > also shifts with a change in salinity. > > I would recommend checking the generic calibration over the expected > soil water content measurement range gravimetrically and then (since you > have the calibration data on hand) develop your own calibration > equation. > > Thanks > > R. Louis Baumhardt, Soil Scientist > USDA-ARS Conservation and Production Research Laboratory > P.O. Drawer 10 > Bushland, TX 79012 > > Phone:(806)356-5766 > FAX: (806)356-5750 > email: lbaumhar@ag.gov -- Cliff Hignett Soil Water Solutions 45a Ormond Ave Daw Park South Australia 5041 pH 61 (08) 8276 7706 WWW.SOILWATER.COM.AU