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Soil moisture meters for lawn irrigation


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I'm starting a new project to use soil moisture sensors to control lawn irrigation.  I have installed some Davis tensiometer sensors in the ground, and have them hooked up to a hobby boards 1-wire system.


Right now, I'm just starting to get data from the sensors, so I don't know much yet.  However, I'm curious if maybe anyone else has fiddled with such things.  Most of my questions revolve around what the proper values should be.  The Davis manual seems to indicate that you should water when the readings get around 25-40cb. But then water until they reach what?  zero?


Zero seems pretty wet.. So I'm really trying to just get a handle on where the right butter zone is for a lawn.   Any thoughts would be appreciated.

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I don't know much about Tensiometers but I am intrigued.  I did a quick search and one paper I found said "they indicate when to irrigate but no HOW MUCH to irrigate".  That statement indicates to me that you can use the sensor to trigger the system to turn on but it won't indicate when to stop watering.

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The tensiometer basically measures how much force is needed to draw water out of soil, replicating how hard it is for a root to draw in water.


I found a nice paper online that kind of describes the process of setting it up.  Basically it said that you need to find the point of "vegetative stress".  Apparently grass shows some kind of outward symptom when the soil gets to a specific dryness.  Then you back off that number a bit, and water just before it stresses.


Yeah.. how much to water.  That part is unclear.  I'm going to have to play with the sensors and try to gather data..  Also, yes, there is about a 30 minute delay between watering, and registering a change on the sensor.  I think that mostly has to do with the fact that the sensors are buried about 5" below the surface.  So obviously I can't water until it's saturated, or it would be a flood.

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I think the question of how much to water will take care of itself as you gather data.  For example, the tensiometers will tell you that you need to water.  Once the tensiometers are triggered disable monitoring the tensiometers for 24 hours (or some other time that will prevent false triggers or double triggers)  Fire up the sprinkler system and do a thorough watering making sure you stop before run-off occurs.  In 24 hours start monitoring again since the watering you did the day before will have had time to distribute itself.  Repeat.


I really like this method of monitoring soil moisture and want to implement it on my system.  I actually want to slightly stress the grass which I believe encourages better root development and, since the soil dries out, discourages disease.


Could you give more information on the tensiometers you are using?  Where did you get them?  Cost?




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I'm using a hobby-boards 1-wire moisture sensor reader.  The sensors themselves are from Davis (the weather station guys) and are normally shipped with their soil moisture system. They are "granular matrix" style.  About $50 a pop.  I have 4 setup right now.


On watching the system over the last week or so, what I have seen, is that at a roughly 5" depth of sensor, the probes register water in under 30 minutes.  (I largely assume this specific value is soil, placement, and depth dependent).  What I've done initially, is set up a routine that checks all 4 probes every 30 minutes.  If a probe is over a specific value (still fiddling with that), I do a semi-heavy watering of the three rows of sprinklers that are near that probe.


The idea is then, that 30 minutes later, it will check again, and in theory, with the spread of the water, some of the probes will have partial moisture, and not require a watering, but if any still need it, it will fire off and handle those.


So far, I have promising results, however, I'm still learning the probe values.  I feel that to make this work, you kind of have to muck with it a bit and just watch the numbers, maybe manually run valves here and there, and see how things react.  I think it will take about 2 months to tune this out.  My long term goal here, is that in the hot summer, the watering will self-adjust to happen more often.


I'm also thinking of throwing in a call to the windspeed, to not water when the wind is high.  And finally, I also have a pulse sensor attached to the feed water pipe, that counts the gallon usage.  I'm monitoring this right now, with the idea that I could detect broken heads, by noting a difference in the gallons per hour usage of each line.


To your message about the stress, I have been reading many papers online (research grants, etc) regarding this sort of thing lately.  I found 2 things:


1) They highly recommend not allowing the grass to stress, but to water just prior to stress.

2) When you do water, they recommend a heavy watering, like you said.  The science behind it is interesting, apparently weeds have difficulty sending down roots to lower depths, while grass does not.  Therefore, if you water heavy, the grass will grow long roots, and take advantage of the lower water, and outcompete the weeds.


This, I did not know, at all. I've been watering alot in short spurts.  Apparently this is all wrong.  Fun stuff!

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