Re: Stewart---67%

Thanks Rick,
I think I will be able to modify the coffee can experiment to obtain a
barnyard type result.
Stewart ACT Aust
sdeans@pcug.org.au
-----Original Message-----
From: Richard J. Noffke <collinsr@execpc.com>
To: pumpkins@mallorn.com <pumpkins@mallorn.com>
Date: Tuesday, 16 December 1997 1:06
Subject: Stewart---67%


>The 67% rule has nothing to do (as far as I know) with the 2/3 moisture
>rule.....however its quite suspicious.  Anyway the key to using the 67%
>field capacity rule is to know the field capacity of your soil.  The
>measure here is not a volumetric measure, it is relative to and calibrated
>to an individual soil.   For example I have three major soil types on my
>farm.  One is a fairly heavy clay loam, one is a silt loam, and the other
>is a sandy loam.  At field capacity each of these soils will hold a
>different amount of water per inch of soil.  So the rule is that optimum
>plant growth is achieved at 67% of full field capacity.  You need to first
>determine what field capacity is and then calibrate a moisture meter
>(tensiometer) to that amount.   100% field capacity is the point at which
>the soil is completey saturated BUT- ALL THE MOISTURE THAT CAN BE DRAINED
>OUT BY GRAVITY HAS BEEN DRAINED OUT.  This would not be applicable but an
>example would be a coffee can with holes in the bottom.  If you filled the
>can to the top with loose soil, then filled it to the top with water, and
>let all the water drain the gravity would drain out, you would be at 100%
>FC.
>
>Practical application of this:  what I do is install my meters during or
>after a major (1-3" rain) event.  When I see water standing in places where
>it normally doesn't stand, I'm fairly confident that the soil is saturated.
> Then I wait till the soil has drained (usually a day or so on the sandy
>and silt loams) an then I take a few readings.  For barnyard science that
>is close enough to 100% FC for me.
>
>The reason why you can't use a coffee can experiment for establishing a FC
>baseline is that water drainage by gravity is a function of soil depth.   A
>deeper soil will drain more than a shallow one.
>
>I hope this helps clear the 67% thing up.  I'd be happy to answer any other
>questions about it if I can.
>
>Also there is some written standards about field capacities and soil types.
> If you question your FC baseline you can check these to see if your close.
> For example a typical sandy loam should have a 100% FC around 13-18
>centibars.
>
>Rick
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