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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 Dear Demetrios, I would like to make some comments on measuring soil water potential using gypsum blocks, watermark granular matrix sensors (GMS), or psychrometers/hygrometers techniques. All techniques have some advantages and disadvantages depending on the application conditions (soil and plant type, accuracy, etc.). 1. In fact, Watermark GMS and gypsum block work based on the similar principles. The difference is that the material used in the sensors to make them more durable and buffer against soil salinity etc.They measure the soil matric potential Arial by measuring changes in electrical resistance with changing in soil water status (either soil water content or water potential). The Watermark GMSs are very sensitive and can be used to estimate soil matric potential (SMP) from the measured sensor resistance (kohm) continuously and successfully. However, they do not respond to the potentials higher than approximately -10 kPa in sandy soils (I'm sure that this lower limit of the SMP will greatly depend on the soil type). In addition Watermark GMSs need to be calibrated for a specific soil under consideration to obtain a reliable and accurate representation SMP. If you are planning to use potted plants, in the potted plant medium (substrate) I think the SMP will rarely exceed -10 kPa. 2. I certainly agree that the hygrometers/psychrometers do not couple with the liquid phase of water. Thermocouple hygrometers/psychrometers have been developed to measure the relative humidity of air in equilibrium with soil and plant materials. They can be used to measure SMP even at very high potentials. Many other instruments are not as sensitive and accurate as hygrometers/psychrometers. However, since most of the hygrometers/psychrometers require small amount of soil/plant samples to measure potential (except in situ ones), you need to take enough samples to obtain a reliable and accurate representation of the water potential. In potted plants, in a 1 gal or 2 gal containers, I suggest 3 to 5 samples to be taken then averaged. 3. Irrometer Company produces mini tensiometers to be used in container-grown (potted) plants. They measure SMP successfully. I don't see any problems to measure SMP at wilting point, but in potted plant substrate you never reach saturation point. Well, you can assume that it is saturated but the maximum value of the water content in the substrate would actually be less than saturation point. I think it is your call between options 2 and 3. Sincerely, Suat Irmak At 07:46 PM 7/31/00 +0000, you wrote: <excerpt>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 Dear Demetrios, You said: > I am a research plant pathologist working at the present time on the > epidemiology of a cucumber disease caused by a fungal parasite. In my study > I would like to investigate the effect of osmotic and matric potential of > water on fungal growth, survival and disease development. I would recommend psychrometry for your type of study (and not just because we make them). Psychrometers are used by plant physiologists and pathologists more than gypum blocks and Watermark sensors primarily because of their extended range; a psychrometer will measure water potential in the range of about -0.1 to -7 MPa. Psychrometers do not need to be coupled to the liquid phase of water, as do gypsum and Watermark sensors (someone recently posted that this was not entirely true; comments?). Psychrometers measure water in the vapor phase, hence the extended range. The same technology is used for vapor pressure osmometers, which output in osmolality units (mmol / kg). They measure the depression of the dew point temperature due to water being bound by solute (osmotic) and / or particulate matter (matric). For more information on psychrometry, see the Sowacs sensor page: <<http://www.sowacs.com/sensors/psychrometry.html>http://www.sowacs.com/sens= ors/psychrometry.html and the link to Wescor. John Manwaring Applications Scientist Wescor Environmental Division Logan, UT 84323 (435)753-8311 (435)753-8177 fax </excerpt> Suat Irmak Agric. & Bio. Eng. Dept. University of Florida, Rogers Hall P.O. Box 110570, Gainesville, FL 32611-0570 E-mail: aysu@grove.ufl.edu WWW: <http://www.agen.ufl.edu/~isuat>