Soil Tensiometer Sensor Network

[hek]

@aur said:

From your experience(and expertise!) do you foresee any difficulties/incompatibilities in implementing this?

Nope

[epierre]

@aur how is your project by now ? anything to share ?

[NeverDie]

I looked into this once. Those gypsum sensors are easy to use but not very accurate, not even the ones you buy. Plus, they do eventually fail, and how do you know when that has happened unless you dig them up every year to inspect them? Two failure modes:

1. The gypsum (or granular matrix or whatever) dissolves away, exposing the electrode metal. Corrosion ensues, and accuracy plummets.
2. Similar to 1, except sensor loses contact with the surrounding soil. This is why they tell you to dig them up every year and replant them. It's supposed to help ensure good sensor to soil contact.

Checking soil moisture levels using a circuit that measures capacitance seems like it would be more robust.

[epierre]

@NeverDie this looks like what Vinduino says.

are you using 3 sensors at one place ? are the sensor auto powered ?

[NeverDie]

@epierre said:

@NeverDie this looks like what Vinduino says.

that's the first II can remember hearing about it.

are you using 3 sensors at one place ?

yes

are the sensor auto powered ?

yes

[epierre]

good, yes Vinduino is the only ... and has been awarded for this !

How do you auto power ?

and how deep do you make each sensor ?

[NeverDie]

@epierre said:

good, yes Vinduino is the only ... and has been awarded for this !

???

How do you auto power ?

Not sure what you mean by "auto power." Each soil moisture sensor runs from batteries.

and how deep do you make each sensor ?

2", 4", and 8" deep. I picked those depths because the purpose is to monitor the soil moisture content for my lawn. Probably 80%+ of the grass roots are within the top 8 inches or so of soil, so that's where most of the water gets absorbed and used by the grass to drive photosynthesis.
Here's a real-time plot:
https://plot.ly/~WhiteRabbit/997
As you can see, there's little change in soil moisture at night, when there's effectively no photosynthesis happening.

[epierre]

@NeverDie auto-power I mean sustainable energy, solar pannel... nothing to change and dispose of batteries too fast...

I have the same issue with lawn, and I would like to have something to look for because it is not flat but that doesn't explain my problem in whole.

[NeverDie]

These look like they would be interesting sensors, but it would be expensive if the foundation for a large number of soil moisture sensors deployed in a home network:. http://www.vegetronix.com/Products/Soil-Moisture-Sensor-Probes.phtml I believe it may use some sort of high frequency capacitance measurement, in which case I wonder whether the Arduino's capsense library would yield results which are just as good? The high frequency apparently has some advantages.

Adafruit sells this: http://www.adafruit.com/products/1965?gclid=CjwKEAjw6IauBRCJ3KPXkNro1BoSJAAhXxpyN6VAQf445eQ7-3EGkxjcmui4-FUFiQGbd73y2PEBrxoCu_7w_wcB
which is meant to be hackable, and it's a lot more affordable too. However, it's not fully waterproof, like the vegetronix is, so that's a major negative. On the other hand, it is open source, and the details are here:
http://wemakethings.net/chirp/
It even gives Arduino code for reading it. It actually uses an attiny44 to do the capacitive measurement!
If you could make the entire thing waterproof (or at least the probe), wireless, and "auto power," then you'd have a very useful sensor!

This document: http://dspace.library.colostate.edu/webclient/DeliveryManager/digitool_items/csu01_storage/2012/02/14/file_2/120523 compares a number of soil moisture sensors. It's worth noting that the watermark wasn't deemed accurate enough to simply use out of the box but instead required a correction factor to be developed. However, if you need to do that anyway, maybe make your own or use a different technology altogether--one that won't dissolve on you and require annual digging up and re-implanting.

It probably does make sense to get a tensiometer to calibrate whatever you end up buying or making. That's something I'd like to do, maybe next year.

[epierre]

@NeverDie we have the same reading and objectives, so far I didn't finished the sensors because I was stuck with solar and LiPo/LiOn powering, but we may task up to make something interresting !

[NeverDie]

I'm presently using soil moisture sensors that measure soil conductivity, which might be OK if I never fertilized my lawn. However, when I do fertilize, it seems to gradually affect conductivity, and the measurements lose calibration. Definitely not ideal.

Apparently each method has its own limitations, but I'd like to try capacitance and see if it maintains calibration better.

Most likely someone out there has already solved this using an arduino, or with a simple circuit connected to an arduino to get the required accuracy. These guys seem close to an answer like that: http://zerocharactersleft.blogspot.com/2011/11/pcb-as-capacitive-soil-moisture-sensor.html
Based on that discussion, it appears that high frequency is required to avoid salinity (such as from fertilizer) throwing off the readings.

In any case, there seems to be a loose consensus that 80Mhz is the fright frequency (e.g. http://www.surechem.com.my/products/901003-100105-PDF.pdf)

[epierre]

or you have the other way around: Infragram DIY Plant Analysis Webcam
http://store.publiclab.org/products/infragram-diy-filter-pack

As the vegetronix, I have in RFX433 the Imagintronix soil moisture sensor: http://www.imagintronix.co.uk/shop/4587637347/soil-moisture-sensor-check-water-level-in-your-pot-plants/9163185

This is already quite useful to see water propagation in a 2x2 kitchen garden.

[NeverDie]

@epierre said:

or you have the other way around: Infragram DIY Plant Analysis Webcam
http://store.publiclab.org/products/infragram-diy-filter-pack

Doesn't work. I've been down that path already. In actuality, it mainly just tells you if your grass is alive or dead. That's because dead grass doesn't have chlorophyll and isn't green. You'd get the same results looking at a color picture and noting how green or brown it is. Checkout http://www.publiclab.org/profile/WhiteRabbit and the follow-up discussion in those posts.

You could try looking for other indicators of grass stress. For instance, grass does change color the drier it gets. If you have the skills, you might try measuring that. It might be a fun science project, but in the end it's going to be more complicated and expensive. However, it also changes color based on fertilization, so that's going to throw off your calibration.

As the vegetronix, I have in RFX433 the Imagintronix soil moisture sensor: http://www.imagintronix.co.uk/shop/4587637347/soil-moisture-sensor-check-water-level-in-your-pot-plants/9163185

This is already quite useful to see water propagation in a 2x2 kitchen garden.

I'm pretty sure that one isn't capacitive but actually measures resistance. Take a look at the probe near the tip. Do you see exposed metal contacts, or is it all sealed up? Pull it out and have a look. I'll wager you'll see two small metal pads near the tip, and it's along that gap between them that soil conductance is being measured.

[epierre]

@NeverDie in fact I'm colorblind so I need something to tell me between green and brown...but that's a very interresting discussion !

[NeverDie]

In that case technology is your friend. :smile:

Regarding # of soil probes and depth, I'm finding that the deeper one is probably the most important, at least for established turf. You can keep the upper layer fairly moist by watering for too short a time, and interestingly, the water never makes it very far down. Here we have clay soil, and it may be different for sandy soil. Here the result is that the roots at the deeper layers keep pumping the water out, and it just gets drier and drier at the deeper layer. It's invisible: you can't know that from just looking at things or feeling the top of the soil. In fact, without a soil probe (or a shovel) I don't think there's any way to know that such a thing is going on.

On the other hand, most grass roots are shallow, and if push comes to shove, I'm not sure to what degree deeper watering saves water or is a waste of water. I think it might save water, though, because the shallower soil is more prone to evaporation. The standard advice seems to be to water infrequently, but deep.

[epierre]

@NeverDie in engineer school I couldn't go and grab resistors nor check the values... a pity...

you said about 2", 4", and 8" deep, but given the sensor size are 2 and 4 not too close this way given their own size ?

[NeverDie]

Not sure what you mean.

[epierre]

@neverdie 2'' and 4'' are pretty close in depth (unless you put them horizontally, not vertically).

in international units that makes 5, 10 and 20cm depths, my sensor is already 8 cm in height.

[NeverDie]

@epierre said:

@neverdie 2'' and 4'' are pretty close in depth (unless you put them horizontally, not vertically).

in international units that makes 5, 10 and 20cm depths, my sensor is already 8 cm in height.

At present, I'm undoubtedly using a different sensor than you are, and I mounted it horizontally at the depth indicated, not vertically, if that's what you're doing. At the time I installed it, my goal for each sensor was just to measure soil moisture at a given depth, not across depths.

[epierre]

@NeverDie have you gone to use the EvapoTranspiration calculus such as described below (example in the second part after the theory)

http://www.konza.ksu.edu/weather/ReferenceET.PDF