Re: Measuring Moisture Levels

Rick,
67% sounds suspiciously like 2/3 to me, perhaps it was derived from a 2/3
moisture rule?
How is this percentage determined? e.g. If you got a cup of dry soil and
added 2/3 cup of water would that lead to a moisure level of 67%?
What does 100% equate to, is it equal volumes of water and soil or is it a
measure of how much water the soil will retain?
If it is a volume ratio then I could mix up a 67% soil solution and put a
moisture meter in and thus calibrate the meter for future measurements on
that soil.

Stewart ACT Aust
sdeans@pcug.org.au
-----Original Message-----
From: Richard J. Noffke <collinsr@execpc.com>
To: pumpkins@mallorn.com <pumpkins@mallorn.com>
Date: Saturday, 13 December 1997 0:26
Subject: Measuring Moisture Levels


>There are a lot of techniques out there for measuring soil moisture.  One
>can develope a pretty keen eyeball approach and get fairly close.  There
>are also moisture blocks and various other meters.  The one technique that
>I have found to be the best and that has been proven to be the most
>accurate is the tensiometer.  You may have seen them in garden catalogs,
>and they aren't real cheap but when used properly they are very accurate.
>They indirectly measure soil moisture content by measuring how tightly the
>soil particles are hanging onto the water.  The instrument is a ceramic
>tipped plastic tube equipped with a vaccume gauge.  It is filled with a
>water solution and has all the air removed from it.  Then when a soil has a
>certain amount of "dryness" it will suck water through the ceramic tip and
>out of the instrument thus leaving behind a vaccume which you read on the
>gauge.  The readings are in Centibars.  The key to using these accurately
>is first to install them properly and then to calibrate them to your soil
>type.  To calibrate them you just need to find out what reading on the
>vaccume gauge is 100% of field capacity.  An average soil may have a field
>capacity somewhere between 10-25 centibars.   Then you do the math and the
>rest is easy.
>
>On the farm we install several of these at a given site and use it as error
>control.  Also we use various depth instruments to tell exactly where the
>water is.  For example in tomatoes I'll put 2 gauges at 12" and 2 at 18".
>Shallower rooted crops would be shallower like 12 and 6".  I have one of
>our highschool summer employees go around and take readings on the gauges
>and then plot the info on a handy graph.  You can then see exactly what is
>going on in the soil.  When the moisture content gets below 67% field
>capacity I turn on the irrigation and then I turn it off when the deeper
>gauge gets close to  67%.   This way you know you have watered enough to
>water deeply.  Watching these charts is a real easy way to manage your
>water its better than guessing.   I must say though that I'm not one to put
>all my reliance on an instrument.   I keep a watchful eye on the plants and
>soil just in case one of the instruments fails.
>
>I can give anyone some sources if they are interested in looking into these
>meters,
>
>Rick
>
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