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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