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To Save Water ...Measure Soil Moisture

This time of year, every California gardener becomes an irrigation specialist.  What doesn't get irrigated doesn't live - it's as simple as that.  Here in San Mateo, we had 4 1/2 inches of rain for the entire season!  There is not a lot of moisture to go around - so our use of water in the garden must be optimized.

We learn so much about the garden just by putting our hands in the soil - but our fingers aren't long enough to really tell what's going on in the root systems of our plants.  For the past month, I've been experimenting with three different Soil Moisture Meters to help configure our irrigation control settings.  Here's what I've found:

1) The Optimal Level of Moisture: You Decide

There are so many different variables that go into soil moisture: organic composition, soil grain type (ie. sand, clay), amount of daily sun exposure, type of plant.  In starting to measure moisture, I quickly realized, for example, that I was watering all of my grow bags at the same time - which was resulting in my pepper plants being overwatered.  And by moving the grow bags to a potting mix from a garden soil mix, I could see in the meter readings that the bags were retaining more moisture.   The meter has given me valuable feedback on my watering habits and the gardening materials I use.

Remember that as you change variables in the garden, your moisture levels will also change - and the best way to gauge that is through actual measurement.  The meter won't tell you what the optimum moisture level is.  That's for you as the gardener to decide.  

2) Favorite meter: REOTEMP 15" ($32)

REOTEMP 15" Moisture Meter

The REOTEMP Moisture Meter allows you to calibrate it to your optimum level of soil moisture.  So when you measure the level of moisture in the tomato bed, a reading of "5" tells you it's exactly where you want it to be.  
A reading of "4" tells me the bed is a little drier than optimal.

This is accomplished by hydrating the soil to the optimum level, inserting the REOTEMP meter into the bed and turning the calibration screw on the back of the meter to have the meter read "5".  Going forward, all of your readings are based on a comparison to the optimum level that you've pre-determined.  This is a game changer in terms of being able to consistently maintain an optimum level of moisture.

The REOTEMP calibration screw

At 15 inches, the REOTEMP probe is also twice as long as the other meters I tested - which allows you to measure deeper into the root zone.

The REOTEMP probe is about twice as long as the others.

Because the REOTEMP meter can be calibrated, everything you measure is based on the conditions to which the meter is calibrated.  In my case, I calibrated to the tomato bed, which means that, as I measure all of the other beds in the garden, the readings are a comparison to the optimal tomato bed moisture level - which isn't ideal.  This is where the other meters become useful.

3)Always carry one of the less expensive meters in the garden

The other two meters I tried were the KINCREA ($11) and XLUX soil moisture meter ($13).

You'll find these meters under a variety of brand names.  Both do not use batteries (the REOTEMP uses one AAA battery) and operate somewhat similar to a temperature thermometer.  Of the two, I prefer the XLUX meter simply because of the rounded shape.  It just feels better to carry around.  

I've found these especially useful in monitoring grow bag moisture, as they are small and easy to carry and their smaller length isn't as much of an issue with grow bags.  After a few times inserting the probes, you do start to get a sense for what the meter considers "Dry" or "Wet".   

Most important, carrying one of the meters with you in the garden goes a long way to prevent you from making soil moisture determinations just by looking at the soil surface (I'm guilty of doing this all the time).  It only takes a few seconds to get valuable information that can prevent you from overwatering - which saves H2O.

4) Location of Drip Emitters is Key

Because drip irrigation can be targeted to a single spot, you really have to keep an eye on the location of your emitters.  With a soil moisture meter, you'll see where the irrigation is soaking in and, importantly, where it isn't.  It's so easy to put the irrigation system on a "set & forget" mode - which could mean your plants aren't actually getting the water you're expending.

Hope you all have a successful 2021 Growing Season!

-Bill S.


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