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We may trust the sensor manufacturers for their test bench calibrations, and even for acceptable uniformity of the product. The variability of of calibration seems to be inherent to all types of sensors which read low volumes of soil water content, like TDR and FDR, for another reason: Water moves and equilibrates within the soil by potential gradients, not by water content. Soil pore volume distribution (hence soil water retention characteristics) is quite variable on the micro-scale even in undisturbed soils, not to say after all what we do to the soil during the process of sensor insertion. The chance to preserve the same water retention characteristics around the sensor as in the surrounding native soil is quite small. In fact what we read with the TDR&FDR sensors is the soil water content of the disturbed soil around the sensor, which is equilibrated with the soil water potential, but not the soil water content of the bulk soil around. So under similar bulk soil moisture content in a field we may read a wide variety of values on the TDR&FDR sensors, because the measurement is on the micro-scale, with a tremendous variability of pore size distribution. Add to this the natural variability in soil water content, and soil water potential on the micro-scale in the soil. For instance lately obtained (and not yet published) results by Martin Shmitz from the FAL-Braunshweig, Germany, (SCHMITZ@BT.FAL.DE) showed similar, and quite sizable, variability for TDR, FDR and granular matrix sensors readings in a very small grass plot. Common to all of those sensors was the measurement of a very small soil volume. It looks like the key of useful soil water content / potential measurements is at large volume measuring devices, or alternatively at numerous (and cheap...) small soil volume measuring sensors. There is still some place for technological advances in soil water measurement. Moshe Meron ===================================================================== MIGAL Galilee Technology Center - Crop Ecology Laboratory Kiryat Shmona, PO Box 90 000 Rosh Pina 12 100 ISRAEL Phone: +972-6-6953559 Fax:+972-6-6944980 Email:MERON@migal.co.il ==================================================================== .