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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 Reply to Glen Brown, We did not publish this particular piece of data. But the overall experiment was published as 'Water balance under wheat modelled with limited soil data' Greacen E L and Hignett C T Agricultural Water Management 8, (1984) 291 - 304 Have a look at that and if you are still interested I am happy to supply the remaining data. cliff.hignett@soilwater.com.au owner-sowacs@aqua.ccwr.ac.za wrote: > 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 > > -- Cliff Hignett Soil Water Solutions 45a Ormond Ave Daw Park South Australia 5041 pH 61 (08) 8276 7706 WWW.SOILWATER.COM.AU