Using Web Soil Survey to define your soil settings

I found through my own experience that setting an incorrect soil type can have a dramatic impact on the watering duration and frequency of flex schedules. I’d lived in my home for 22 years and always assumed we had a lot of clay in our desert soil. Through these forums I found that was not the case, and instead we have sandy loam. Changing that setting really had a big impact on my scheduling.

Finding out your soil type can be confusing, but I personally found the USDA Web Soil Survey site to be incredibly helpful. The first few times through it wasn’t intuitively obvious how to find all of the information that I needed. The document that I put together here is an attempt to make it easier for new users. Please let me know if there are mistakes and I’ll try to keep this post up to date with the latest instructions.

I hope this helps !

Web Soil Survey Instructions for Rachio Iro

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Great job, @azdavidr!!

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That’s a great guide @azdavidr They definitely need to find a way to pin the information. I ran through your directions and I get this…

So by those numbers, my AW 0.12 that I’ve been using is too high. Granted, for my grass I think my number is closer to 0.12 because it was sod so there is some non-local soil for a lot of that, but I believe the 0.09 number for the rest of my yard. My yard’s soil seems a lot more sandy than the soil below my grass, so I’m willing to bet the AW is closer to that 0.09. Plus, my plants seem to show a bit of wilting days before the watering schedule is set to kick in so I’m guessing that lowering my AW will help to compensate for that.


Thanks @Modawg2k. I wonder how the USDA gathered so much data. There’s a ton of detail and it’s crazy to think of how much effort must have gone into collecting and publishing it. Thanks for the feedback!


@azdavidr, thanks for putting the hours into create this great and very useful guide. Really great job!
Same here, everyone told me clay clay clay, so I before you started the soil subject I never doubted local knowledge… only to find that my house sits on 3 different soil types! Below is the overview (golf course) and the zoomed in picture, my location circled.

The good news is all is sandy loam of some kind with both light blues in the backyard at an AW 0.16, the red front yard at an AW 0.11. For comparison (only on the overview chart) the dark blue around me is AW 0.19 often flooded), and the amber AW 0.13, all sandy loams. I have not yet done a personal soil analysis as you suggested before :(.

@Modawg2k, I suspect I am in the same situation as you are: the yard soil feels a lot more sandy (as my gardening wife confirms) than the soil under the lawn. To make things more complicated, the banks on two sides of the house have a plastic perforated liner on them to reduce erosion. However (less when smart cycling) with a good rainfall, the lawn close to the bank often turns into a swamp, since rainfall doesn’t come with smart cycle around here ;). As the season went along, I kept reducing the amount of watering the area near the bank, which in turn dries out the central portion with the most sun exposure. Can’t win this one without redesigning the irrigation layout - and I am not up for this yet.
my question is how much of an AW difference do you generally allow for sod on a sandy loam base?

Ok, in summary and barring a big AW difference, I plan to switch to 0.11 (red, front) and 0.16 (blue, back), banks set to clay and 0.09 (to account for liner). Makes sense to you guys? Feels a bit over-engineered, but what the heck, since we have the parameters to play with.

Again, thanks for all the terrific advice on this forum. Makes sprinkling actually fun…

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@hgugger I’m glad you found the guide helpful! Your situation is way more complex than mine. Honestly, I’m not sure what I’d do. You can always have soil testing performed where they test AW, but there’s the associated expense, which would be increased in your case if you were getting multiple samples analyzed I guess I’d probably go by the numbers given, but watch the vegetation carefully for signs of stress and tweak as necessary. Larger AW numbers will create schedules that have less frequent watering.

My guess would be to just started out with the stated AW and then just manually check out the area, both how is the grass doing and how moist is the soil. Play around w/ the numbers from there.


This is amazing and on our integration new software list :wink:



Glad that you like it. This sounds pretty cool, but what does it mean ? :grin:

This is probably unique to my area, but it seems like when they build new neighborhoods, they grade everything by covering the original soil with a few feet of a different type of soil that gets trucked in. Once the house is built, they’ll do a final grade by adding even more different soil. I’m imagining the end result isn’t like what the USDA originally surveyed. I’ve got the mason jar test on my list to do after the catch cups.


This is definiately an issue for a lot of people. I know they truck in dirt to make these neighborhoods in my area, but I’m not exactly sure where the dirt is coming from.


Great guide!

What soil type should I choose based on the below profile? AW was showing 0.15
H1 - 0 to 8 inches: stony fine sandy loam
H2 - 8 to 31 inches: clay
H3 - 31 to 60 inches: bedrock

@rwabel Glad that it was helpful for you. With regards to your question, take a look at step 16. It gives you an idea of how to handle situations where the soil type isn’t an obvious choice.

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Thanks azdavidr I was however more curious if we need to look more at the H1 or H2 in regards of what soil type to put in Rachio. Guess it depends on the depth of the roots which is a bit difficult to know unless we go with what comes standard

@rwabel As long as you use the same AW number after picking either of the soil types your watering duration and frequency will be the same. That leaves only the Smart Cycle feature to think about. However, cycle soak doesn’t engage for drip systems, since the PR is assumed to be so low that it never gets higher than the inflation rate. If you are setting up a non-drip system choosing clay will be the more conservative route as Smart Cycle will engage with shorter run times, and you’re less likely to get runoff.

Feel free to experiment with both settings and look at your scheduled run times. They should be the same for the total watering amount, but you might see that the clay setting will break up the run times into shorter durations than the Sandy Loam setting. Just be sure to override the AW number each time as you experiment. Here’s a nice article on Smart Cycle:

Let me know if you have more questions!


This is a fantastic guide. I spotted it the day you posted and discovered that while I had correctly determined my soil type, I had to change my Available Water setting from 0.17 to 0.13 according to the USGS survey. This seems to have made a noticeable difference to my watering times and I’m much more confident that my system is making better calculations based on more accurate data now.

Thank you @azdavidr !


That’s awesome @DSNJ, and you’re welcome. Thanks for taking time to write your note. It’s great to know that the guide was helpful !

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I want to echo what other posters here have been saying about finding non-local soil in their lots. Also want to note that for some 20+ years, it’s been a law in California to compact the soil in new construction to up to a 98 percentile, an almost stone-like consistency, to provide seismic stability so seismic waves pass through it quickly, producing less shaking. This kind of compaction doesn’t drain, at all. Folks living in new construction in earthquake prone states may also have this law.

Determining soil is hard stuff! Even my civil engineer neighbor, who’s taken numerous courses in soil science can still get fooled now and then by a soil that doesn’t test out like he thinks it will.

According to my soil survey, even at 24 to 31 inches down, it says I should have “gravelly sandy clay loam:joy: and at 0-11 inches, I’m supposed to have “gravelly sandy loam”, which would suggest a really well-draining soil. I wish that were true, but a percolation test absolutely suggests an heavy, compacted, nasty clay-like subsoil. This, and other basic visual tests and soil texture “Feel Method” confirm this.

Over the years, constant mulching has made the soil in parts of my yard more friable and has started to build a nice black layer of top soil. I think one should use the web soil survey has a general guide along with various other basic visual tests, mason jar test, or a soil test if you can. Probably the best advice is what @azdavidr said, watch your plants for signs of stress and tweak values if necessary.


If only 90278 were on the survey!

@Toomanybarts Are you not able to use the survey? I’m assuming that’s your zip, I often use lat/long because it’s more accurate. Let me know if you would like me to look into it for you :slight_smile:

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