Soil Moisture FAQ - What does it mean? What moisture should I target? etc

[homedigy-lawrence]

A lot of people are very excited + curious what soil moisture numbers to set their GeoDrops to.

Many people are learning about soil moisture values for the first time!
While others have learn A LOT of misinformation online (please ONLY trust universities articles, especially form Agricultural and Physics departments).

So.. here’s an FAQ.
Let me know in the comments below if you have more questions, and/or have something to contribute too based on your experience, and I’ll do my best to update this FAQ post regularly!

Thanks,
Lawrence – GeoDrops Founder/CEO
(Ex-Google Platforms Chief Computer Architects, with Advanced Engineering Physics degree)

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[Q] What is Soil Moisture? What does “50% soil moisture” mean?

It depends on the “measurement unit”. There are two industry standard measurement units:

• “Volumetric Water Content” (VWC) used by agricultural-grade soil sensors (typically >$500 per sensor), and
• “Managed Allowed Depletion” (MAD) used by commercial-grade and Smart irrigation controllers (including Rachio)
• (“Unitless” for any cheap consumer soil sensors you can find on Amazon under $200. Their “moisture percent” doesn’t mean anything since there’s NO scientific calibration against your actual soil)

The difference between the two standard units above are summarized in the picture below. We designed GeoDrops so that its moisture value can be translated between these two units without the need for hours long scientific soil calibration.

(To learn more about Scaled VWC, see also this community discussion)

It’s very important to know what unit you’re working with. 50% of max moisture depleted on your Rachio irrigation controller may be roughly 67% of max soil moisture percent on the GeoDrops App, but can be very different from an agricultural grade sensor – only 20% of max soil moisture content.

ps - it’s our longer term goal to allow GeoDrops AI to automatically translate GeoDrops reported measurement unit between any of the three units above. We need more soil data to train our AI to make this happen!

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[Q] Why can’t soil sensor just report MAD for my irrigation system?

MAD is a proxy into soil moisture that is based on the same unit as mm of rain precipitation (something that people have been accustomed to for >100 years).

It’s not a measurable unit, so it has to be calculated via an extensive scientific soil measurement and calibration process, see video below:

How to calibrate METER soil moisture sensors

We’ve found that “Scaled VWC” is actually relatively close to MAD in reality (as shown in example above), so people can mostly use it “as is” without having to go through the hours-long soil characterization process shown above. (except heavily compacted or clay soil, see more below)

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[Q] I don’t care about physics. Just tell me what soil moisture target to set

For most people who are simply interested in setting a “target watering moisture” for GeoDrops to automatically schedule your irrigation to, we’ve found the best approach is to simply observe how your plants respond as you adjust your watering schedule slightly, over a few weeks. Once you find the “sweet spot” for your lawn and garden without stressing your plants out, GeoDrops will do an incredibly accurate job helping you maintain that ideal moisture level through every season.

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[Q] Ok… Maybe tell me (just a bit more) soil physics

In summary, GeoDrops measures percentage of moisture (actual water, or H2O particles, filling up micro-pores) in your soil. This means:

• 100% means soil is fully saturated (at field capacity)
• 0% means soil is completely dry (ie, your soil would crumble and be like sand or dirt, not able to hold together since there’s zero moisture)

Somewhere within this 0% to 100% range is the permanent wilting point — the minimum amount of soil moisture that the plant requires to not wilt. This point varies by both soil composition and aeration.

The following table can be used as a very rough rule of thumb for the permanent wilting point (ie, absolute minimum soil moisture you must maintain):

• 10~40% for sandy soil
• 30~60% for aerated loamy soil
• 50~80% for compacted loamy soil
• 70~90% for aerated clay soil
• 85~95% for compacted clay soil

IMPORTANT: Finally, set your target moisture to approx half way between the permanent wilting point and 100% max moisture for most plants.
Set this value lower for drought tolerant plants (such as roses), and higher for plants that need plenty of water, such as tomatoes.
For lawns, you can try varying this value. You can set it a bit lower to conserve more water (and your lawn will look slightly more yellow), and a bit higher to make it more green. Do NOT go overboard setting this value too high, or else you’re going to suffocate your grass roots, and have fungus and other problems.

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[Q] Really? So I should keep my moisture above like 80~90% if I have clay soil?

Yes, you’re reading the above table right.

Clay soil particles are very dense, resulting in very high water potential. This means it has a very high permanent wilting point in terms of soil percentage moisture.

Think about playing with Play-Doh. There’s actually quite a bit of moisture in the Play-Doh modeling clay when you opened it brand new, but clay compounds hold onto the water so well that your plant roots won’t be able to absorb any of the water within the clay.

[kredmore]

Helpful FAQ, thank you.

[Mark]

This is great background, thanks! I use Rachio and have been wondering about this correlation. By the way I requested for Rachio to support integration with Geodrops…got a reply that they do not currently support integration with cloud services.