SOWACS: Granular matrix re-wetting in clay

[Leonard Ornstein <lenornst@pipeline.com>]

Len Ornstein writes to the sowacs list:
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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:
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>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)
>
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