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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 Susan, Gypsum blocks require good contact with the soil. They will have problems in soils that change volume on wetting (shrink-swell soils). They rely on movement of water between the soil matrix and the porous block. At equilibrium (or some reasonable approximation thereof), the soil water potential and the water potential in the block will be the same (or nearly so); and a valid reading is obtained. The blocks really respond to soil water potential, not water content, and that is why your system gives data in bars (this is not water content). Newer systems might give the data in kiloPascals. So that the block may remain nearly in equilibrium with the soil, the movement of water between block and soil should be fairly rapid. But, for any particular gradient in potential, soil water movement slows down as soil dries. This is why gypsum blocks do not work well in the dry range. Another reason for not working well in the dry range is the above-mentioned soil shrinkage in some soils. I suppose that when you wrote "arable" you meant "arid". If the soil is dry most of the time, then gypsum blocks are not a good measurement technology. Same thing if the soil is frozen most of the time. Another thing to consider is that the old gypsum blocks were variable enough to require individual calibration for accurate work. Some newer designs may be more uniform. Further comment would be helped if you could relay some information on soil texture (particle sizes), mineralogy, salinity, and organic matter content (if any). But, as a first guess, I think that TDR could be the best technology for your study. It could show you when the soil water melts and freezes (see papers by Eggbert Spaans, John Baker, and others), and should provide good water contents when the soil is not frozen. Calibration will probably not be required. Also, you can get data for quite thin (2 to 3 cm) layers if the soil above the permafrost is shallow. The CSIRO folks have a lot of expertise in this area (Steve Zegelin and Ian White are two knowledgable contacts). You can visit my web site as well at http://www.cprl.ars.usda.gov/programs/ Best wishes, Steve At 10:04 AM 2/18/2000 +0000, you wrote: >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 > >Can someone more expert than me in gypsum blocks please help Susan? Thanks. >Alan Robock > >---------- Forwarded message ---------- >Date: Thu, 17 Feb 2000 20:27:01 +0800 >From: Susan Ferguson <susan.ferguson@antdiv.gov.au> >To: robock@envsci.rutgers.edu >Subject: Soil moisture help > >Hello Alan, > >My name is Susan Ferguson and I'm interested in soil moisture and I was >hoping that you would be able to give me some friendly advice. I'm >currently at Casey Station, Antarctica, studying bioremediation of petroleum >contaminated soils. My initial finding have indicated that soil moisture is >one of the primary limiting factors to bioremediation in this region. We >think that what happens is that we get a fair bit of snow over the winter >which doesn't melt until spring and summer. This snow melts, flows though >the soil, hit's the permafrost and flows out into the bay and is thus >unavailable for microbial activity. > >I am hoping to measure the soil moisture in this region to verify this >hypothesis. I have obtained a data logger with several temperature probes >(8) and two soil moisture probes (gypsum blocks). My original thought was >to measure temperature at four depths (surface, 5cm, 10cm and 20 cm) as well >as a moisture at one depth (8cm) in two profiles. I'm actually more of a >chemist so I really don't understand that much about soil physics and such. > >My main dilemma at the moment is setting up the data logger. Having just >read your web page, I'm concerned that the gypsum blocks are unsuitable for >this area as it is a fairly arable environment. As they are the only thing >that I have available, I have to plough ahead needless. So, how do I go >about calibrating these probes and converting the result into a meaningful >value. The data logger measures water content in bars, which is pretty >meaningless to me. What is soil moisture usually measured in? I usually >solvent extract all moisture out of the soil and convert my chemical >analysis results into per dry weight of soil. Is it possible for me to take >a series of reading on the data logger and calibrate it with the solvent >extraction method. > >I don't know what standard my results will be, but I'm more than happy to >contribute to your web site if possible. Thankyou in advance for your time >in this matter. Any other suggestions that you have would be very welcome >at this point before I drive myself crazy with this. Thanks again > >Yours sincerely > >Susan Ferguson >University of Tasmania & >Australian Antarctica Division >Casey Station, Antarctica > > >[Susan, >for starters read the Gypsum Block introduction page >http://www.sowacs.com/sensors/gypsum.html and follow the links to various >more detailed sites, including the MEA feature page "Ressurecting the gypsum >block" : http://www.sowacs.com/feature/mea/mea.html > >You may also want to subscribe to the sowacs email discusiion group so that >you will recieve any replies from the group. >Bruce] > ___________________ Steve Evett, Soil Scientist USDA-ARS, 2300 Experiment Staton Rd., Bushland, TX 79012 USA 806-356-5775, FAX: 806-356-5750 srevett@ag.gov, http://www.cprl.ars.usda.gov/programs/