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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