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Dr. Clinton C. Shock writes to the sowacs list: ===8<============== Mike, We have monitored GMS in extensive replicated grids to the sides and beneath drip lines for several years in silt loam soils. We have more limited experience in other soil types. Leonard has it right that the GMS response should be OK if the soil has not separated from the sensor. How does the soil behave? A key point for predicting irrigation scheduling is to place the sensors in areas which are regularly rewetted upon irrigation, dried by plant water use, and away from the wetting front where salts can accumulate. GMS responses are generally consistent in fine textured soils from -10 kPa to -80 kPa if they maintain good contact with the soil. Calibrations may be needed for a given soil. Wishing you the best, Clint -- Dr. Clinton C. Shock (home) 1059 SW 2nd Ave. Ontario, OR 97914, USA Telephone (541) 889-7057 Cellular phone (208) 739-2674 http://www.shockfamily.net Email ccshock@fmtc.com (work) Malheur Experiment Station, Oregon State University, (541) 889-2174 Email is clinton.shock@orst.edu www.cropinfo.net Leonard Ornstein wrote: > > Len Ornstein writes to the sowacs list: > ===8<============== > Mike: > > The problem is whether water will flow more rapidly around an > air-filled space as liquid, by capillarity, or across the space, by > diffusion, as vapor. Vapor diffusion is usually much slower than > capillary flow over distances greater than a few micrometers. And > liquid flow is slower, the smaller the pores. Also, the thinner > the layer of water on the surface of a pore, the slower the flow. So > for rapid flow and quick equilibration over large distances, large > pores are preferred near saturation, but smaller pores work more > efficiently as the soil dries. > > In dry soil, the difficulty usually arises when the pores of the soil > are much LARGER than the pores of the block, and there are too few > capillary contact points between the soil and the sensor surface. > Therefore, PROVIDED THAT IT DOESN'T SHRINK AWAY FROM THE SENSOR > DURING THE DRYING CYCLE, THUS LEAVING VERY FEW CONTACT POINTS, > performance and response times of granular matrix blocks can be > better in clay than in coarser soil. > > Hope this helps. > > Len Ornstein > > At 6:35 PM +1200 3/20/03, Bruce at Findasensor wrote: > >Mike Schulz writes to the sowacs list: > >===8<============== > >greetings, > >As pore size has a major impact on soil moisture movement i have a query > >regarding whether granular matrix resistance blocks could potentially have > >problems rewetting in clay soils. > >Its apparent that gypsum blocks have very fine pores as they loose little > >water before ~60kPa, ie. they don't work in this range. Granular matrix > >sensors are therefore made of coarser material in order to allow them to > >function in the >10kPa range. That is they have larger pores. > >In a clay soil that has been rewet after drying to say 250kPa, can water > >move from the wet but not saturated clay into the dry but larger pored > >matrix of the granular matrix block. > >If the soil around the block is saturated at each irrigation or rain > >rewetting should proceed rapidly. My query is what happens when the block > >is positioned on the edge of say a dripper wetted ball and the soil texture > >is finer than that in the block. > >Comments on your experiences would be appreciated. > > > >Regards > > > >Mike Schulz > >Serve-Ag, Victoria. > > > >PO Box 615, Mooroopna, Vic, 3629. > >Tel: (03) 5825 4783 > >Mobile: 0408 395 076 > > > >Serve-Ag provides a range of services to the agricultural and environmental > >communities. > >For additional information consult our website (www.serve-ag.com.au), call > >me or, call our head office (03 6427 0800) > > ===8<============== NOTE: To get off this list, send an email to list@sowacs.com with the subject line: unsubscribe_sowacs For full instructions see this page: <http://www.sowacs.com/subscribe/index.html>