Interested in soil moisture sensors and irrigation control? Start with the UF/IFAS virual extension series

I've been having ... "fun" ... trying to find a way to build an affordable network of soil moisture sensors that don't require too much looking after.

It's harder than you'd think, but this UF/IFAS Virtual Extension series on soil moisture and irrigation has made it a lot easier to understand the different approaches and sensor types. It'll help you understand the differences between resistive and capacitive soil moisture measurement, introduce alternatives like tensiometers, etc. This is important whether you plan to DIY your sensors or buy off the shelf.

Unfortunately, the article series says very little about how to get your hands on working soil moisture sensors without your wallet screaming for mercy. I'm still working on that bit.

Promising options that need further investigation include:

• Vegetronix (USA) VH400: Capacitive probe (?), US$35 each, shipping cost to non-USA addresses is extortionate. Not yet tested.
• Honeywell HCH-1000 Hydrogrometer: At $5 to $7 ea, promising. Available cased or uncased. See HCH-1000 datasheet. They're even on pins for our soldering sanity. As these seem to be designed for air humidity I'm not sure they're going to be suitable, but at this price it's worth a go. To test.
• DIY DC resistive soil moisture sensor: Sensitive to impurities in the water, like salt levels, and quite temperature sensitive. Prone to electrolysis causing corrosion of the electrodes. Tested successfully but too crude for real-world use.
• DIY AC resistive soil moisture sensor: As the DC version, but less prone to corrosion in exchange for being a bit trickier to build (or using up extra I/O pins for the lazy-man's version). Yet to test.
• DIY capacative soil moisture sensor: These are less sensitive to soil composition and water impurities, but appear to be very hard to get right. Yet to test.

Less promising or totally non-viable options include:

• DIY neutron probe: Just kidding. Don't even think about it.
• Senviro sensors. Senviro claims to be focused on low-cost sensors, but their website is useless. When I called them (Jan 2102) I was told that the company has been essentially mothballed for the last year and a half, though they're looking at picking it back up to focus on long range wireless sensors for park/golf course/industrial use, focused on high durability and long life at a price of "hundreds not thousands". Still not really in our range; not testing.
• Holman WS7880 433MHz wireless soil moisture sensor: A Western Australian supplied sensor, presumably a rebadge of something made overseas. Available as sensor and base station bundle or as stand-alone sensor, since each base station supports 3 sensors. At AU$17 (Bunnings) these aren't cheap but they're not that expensive compared to many of the other options. They require near-line-of-sight and won't work through multiple walls (as specified, even through one). The sensors take 2xAA batteries. They're a big and white and might be a theft/vandalism risk, plus they're not that pretty, but at least they'll be hard to accidentally mow over if installed in lawn. I think they're using resistive metering. I bought a couple for testing and they work well when used as designed. Being able to use these from a microcontroller - either by repurposing the base station or by directly receiving the 433MHz RF signals from the base stations - would be exciting and fun, but is beyond the ability and resources I have at the moment without at least some documentation. I have some but won't be testing them with my microcontroller unless I can find out more about their radio protocol.
• Honeywell HIH-6131 series Hydrogrometer (eg HIH-6131-021-001 @ AU$27 ea for 10+): i2c-enabled sensor with on-chip signalling, optional temperature sensor integration. See also this product page. I'm not sure even the filtered versions can cope with immersion, they're more designed for air humidity. Not that cheap ($28 and waaay up) or suitable, so untested. Pity, as i2c bus support would be great.
• Honeywell HIH-4000/HIH-4100/HIH-4200/HIH-4201 series Hydrogrometers: Simple voltage-output sensor. Still not less than $25 ea in 10-unit quantities, filtered variants over $35. No tested.
• HIH-4602: Thermisisistor. Over $100 ea, so totally unsuitable, not further investigated.
• Sensirion SHT10/SHT11/SHT15/SHT71/SHT75: The "low cost" SHT10 is still over AU$60 anywhere I can find it, so I have no plans to test this for home irrigation control. Not tested.
• Silicon Labs humidity sensors don't actually appear to exist. Their site lists the category but no datasheets, and none of their catalog vendors carry them. Not tested, possible vapourware.
• Various scientific/industrial integrated soil moisture measurement systems: Seriously expensive, usually hundreds or thousands of dollars. When looking for sensors, this is mostly what you find. Obviously not tested.
• Various Tensiometers: Not only are soil tensiometers expensive, but they don't work well in dry soil and require a lot of babying. They're totally unsuitable for irrigation control where they'll be left in place for long periods. Not tested.

I'm planning follow-up posts as I try different DIY meter designs. I'll include lists of articles I found during research and used as references, so hopefully it'll help others doing similar work down the track even if they all get given the big red X at the end of testing.