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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 Cliff, Was the wheat research you mentioned published? Glenn Brown Associate Professor Biosystems Engineering Oklahoma State University Stillwater, OK 74078 (405) 744-8425 gbrown@okstate.edu >I think Len Ornstein and I have a communication problem - what we are saying >is not substantially different. >Most basic soil physics texts describe the process quite well. > >I will try for a 'potted version' in different words.- >All saturated porous materials have a 'bubbling pressure'. >This is the air pressure needed to force water out of the largest pores (or >the suction in the soil water to drain the largest pores - same thing) >In terms of the relationship between water content and suction, it means that >an initially saturated material when subjected to a steadily increasing >suction (starting at zero) will hold all its water until the bubbling pressure >is exceeded. > >In materials with a wide range of pore sizes - like loams, there are usually >enough very big pores that this point is close to zero suction - say 0.01kPa. > >In a sorted, uniform pore size sand, such as exist in beach or dune sand >deposits, the bubbling pressure is often as high as 1 to 5 kPa (corresponding >to hydraulic heads of 10 to 50 cm suction) Until this pressure is reached, >there is no drainage at all. When this pressure is exceeded, nearly all the >water drains. This produces a water retention curve , and a hydraulic >conductivity curve which approximate step functions. > >I agree with Len that some water remains on the grain surfaces and grain >contacts which drains slowly but this usually represents only 1-2 % of the >saturated water content. A typical sand might go from (say) 30% water content >at saturation to 2% when its bubbling pressure is exceeded. > >An interesting side effect of all this came to light on a survey of the water >use characteristics of wheat plants across southern Australia. The survey >used the varieties selected by farmers for optimal production in their soil >and climate conditions We found that the varieties bred on sandy soils >grew much greater root lengths than those bred on clay soils - even when >rainfall was the same. > >This makes genetic sense. If a plant is to survive in a sand where the only >water is held tightly to grain surfaces (most of the time) with poor >hydraulic conductivity (most of the time) then it will need to have a roots >separated by as small a distance as possible. In a clay or loam soil, the >hydraulic conductivity is much higher even at quite high suctions so roots can >be further apart and retain the same water gathering capability. > >Cliff Hignett >Soil Water Solutions >45a Ormond Ave >Daw Park >South Australia 5041 >pH 61 (08) 8276 7706 >WWW.SOILWATER.COM.AU >