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I am interested in making a wicking bed for typical vegetable growing. I have read that the height of the soil should be no higher than 300mm. I suspect this number is experimental or simply copied from other sources.

I am a little more curious regarding the mechanics of such a system.

Please follow me on this brief journey and correct me if I divide by zero.

If I where to Engineer It, I would be tempted to look at soil granule size and from that derive an average, and approximate, pore size. From the average pore size I would want to consider capillary action and determine a theoretical height. At that height I would try to determine a possible water pressure and linearly plot it through the height of the soil. On this plot I would mark the field capacity and permanent wilting point, if possible.

A few assumptions;

The soil consists of only one grain size, no organic matter and pure water is used.

Pore size is calculable from the grain size.

Pore size is similar to tube size in capillary action.

Field capacity is about -33kPa and the permanent wilt point is about -1500kPa.

Another assumption would be that all this is theoretical, however feel free to question my sources.

Sand granule size would be determined from available data: Medium Sand - 0.25mm to 0.5mm, with a 38% porosity

irrigation.wsu.edu/Content/Fact-Sheets/Soil-Monitoring-and-Measurement.pdf

To calculate average pore size: granule size * porosity = pore size * (100% - porosity)

Therefore: pore size = granule size * porosity / (100% - porosity)

For 0.25mm: pore size = 0.25mm * 38 / 62 = 0.15mm

For 0.5mm: pore size = 0.5mm *38 / 62 = 0.31mm

www.newton.dep.anl.gov/askasci/env99/env201.htm

Assume pure water in a clean glass tube for simplicity in calculating capillary action:

capillary height = 2 * surface tension of water / (radius of tube * gravitational acceleration * density of water)

For 0.15mm: capillary height = 2 * 72.75mN/m / (0.15mm / 2 * 9.8m/s/s * 1000kg/m/m/m) = 0.198m

For 0.31mm: capillary height = 2 * 72.75mN/m / (0.31mm / 2 * 9.8m/s/s * 1000kg/m/m/m) = 0.096m

books.google.co.za/books?id=ltVtPOGuJJwC&pg=PA69&lpg=PA69&dq=capillary+water+tension+height+soil&source=bl&ots=XJrUhlTTGj&sig=DeSQVPf4aXnKIwFKaid_IYUNoTQ&hl=en&sa=X&ei=Z6LtU7zCMYKp7AaPpoDACA&ved=0CCsQ6AEwAg#v=onepage&q=capillary water tension height soil&f=false

water pressure = -density of water * gravitational acceleration * height

For 0.198m: water pressure = -1000kg/m/m/m * 9.8m/s/s * 0.198m = -1.94kPa

For 0.096m: water pressure = -1000kg/m/m/m * 9.8m/s/s * 0.096m = -0.94kPa

I have read field capacity could be -10kPa for sand, but I suspect it depends on the water retention curve.

For this thought experiment the sand is below a field capacity of -33kPa (And -10kPa) and for a field capacity of -33kPa a height of 3.3m (And -10kPa from a height of 1m) is required. This would suggest that there is more water than air which is less than optimal (Dependent on water retention curve).

It would also suggest that sand is not useable to 300mm.

Lets see what results sandy loam gives with the following specs;

Sand at 60% with a granule size of 0.175mm.

Silt at 30% with a granule size of 0.020mm.

Clay at 10% with a granule size of 0.002mm.

The average granule size would be:

average granule size = sand granule size * sand percentage + silt granule size * silt percentage + clay granule size * clay percentage

average granule size = 0.175mm * 60% + 0.020mm * 30% + 0.002mm * 10%

average granule size = 0.111mm

The porosity of sandy loam is 43%.

irrigation.wsu.edu/Content/Fact-Sheets/Soil-Monitoring-and-Measurement.pdf

The pore size is therefore 0.084mm (See above equations).

The capillary height is therefore 0.354m (See above equations).

At this height water pressure is -3.47kPa (See above equations).

Water pressure at -1kPa occurs at 0.102m.

Funny how the equations suggest what is taken at face value; 300mm height.

What Im lacking is a representation of water pressure vs water content for specific soils (Water retention curves) so that I could determine at what height the soil experiences its field capacity (Which appears to be soil parameter dependent); at which pressure water content becomes less that at saturation (Air starts getting to the roots).

Have I befouled physics and / or can any one give me some more info, especially regarding water retention curves (Theoretical).

Thanks

dferasmus

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# Capillary action with respect to "wicking beds"

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