Silty Loam vs. Sandy Loam What to Set?


I have conducted a soil test. I am RIGHT on the line between sandy loam and silty loam (using the soil type pyramid). I am running a Flex Daily schedule and so I want to be particularly certain I preset the best option for my soil type. Should I expect some minor challenges due to my programming with soil type - “sandy loam”? There is no “silty loam” option. However, even if one existed, I don’t know that it would be especially wise to choose one over the other…

I guess what I am asking is, if I shouldn’t need to worry because these soil types are similar enough that by selecting the option that is available (sandy loam), the system will effectively do the same job as if the other option (silty loam) were available and I chose it.

Thank you!


Select sand loam to be on the safe side. This has a slow absorption rate so smart cycle will be more cautious, BUT, make sure your available water is set correctly. Use this link to compute it,

If you need help let me know


@883 I couldn’t agree more with @plainsane. Getting close to right AWC regardless of the soil type is key.


Thank you. Not sure how to understand the AWC from that page (been there many times though). I have attached the data I can get from the page but I do get warning about potential inaccuracy for my area…


On. That post, you will see available water storage, divide those inches by 60. You have multiple profiles there so you will need to average those together.


So you are saying to: 3.5+5.7=4.6 then 4.6/60=.077 and .077 is what I should enter in for available water? Do I have that right?

How does entering a lower number vs. a higher number affect the watering cycle? I’ve been reading all day trying to understand this stuff an really appreciate your help.


yeppers round it up though.


As in .08 or .1?

Also, how does .1 vs. .15 etc. affect the watering cycle? Again, trying to understand what it all means.


So available water indicates how much your soil column can hold.

So if your root depth is 6 inches, flex knows that it needs to apply .48 inches of water because 6 * .08 = .48.
Now flex can compute how much is lost each day to et and water when needed. The cool part is that if your irrigation waters at .5 inch per hour, rachio knows they need to water at .48 / .5 = .96 hours, so 57 minutes.


Where did the 60 come from when I divided by 60? — So I round to .08 then?

So grateful for your answers! This is wonderful to understand.


Everything is in velocity, we removed the gallons and left with time, so convert our fraction of an hour to minutes.


So it is like saying the soil can absorb .08 inches of water per minute then — yes? By entering this, the system knows how quickly the moisture can evaporate as well as how much can be applied in x number of minutes without runoff etc. I think I have it. :-D! I got more answers on this page (THANK YOU AGAIN!):


Had a related question - based on this soil survey data, the water storage value is 7.4, so my AWC should be 7.4/60 = 0.12, correct? This lines up with Rachio’s “sandy loam” setting. However I also noticed in the soil survey that the top 12 inches is loam, and the recommended AWC for “loam” is 0.17.

Should I ignore the soil composition profile and just use 0.12? Thanks!


@jleh I get to the info. from a slightly different path, which allows you to pick the depth that you’re looking for and it returns a weighted average. I think the table from your post above lists the data based on the assumption of 60 inches of depth, which is why you end up dividing the number in that table by 60 (to get it ‘per inch’). I would think the most accurate method would be to choose the depth of your deepest roots in that zone, but I’m not positive on that. Since my AWC doesn’t change much with depth it’s of less consequence to me.

To see how I get to this part of the WSS website see this post: Moving from flex monthly to flex daily

Here’s an example of how my AWC changes from 0.12 to 0.13 when I change my depth from 20 inches to 60 inches.

Help: Soil Type Selection

Thanks for sharing that alternate method.

For my geography, I believe my lawn is a cool season grass, which to my understanding has a RZD of 6 inches. So if I follow the steps you outlined and set Top Depth = 0 and Bottom Depth = 6 inches, I do end up with a value of 0.16 - which is closer to the “loam” setting.

In fact, that’s if I’m using the default aggregation method of “Dominant Method”. If I change it to weighted average, it becomes 0.17, which is exactly what Rachio mentions is the industry standard for just “loam”. So I’m back where I started :slight_smile:


@jleh That’s interesting info. I’m not an expert, so I’m not 100% sure if it’s valid to use 6", or something deeper. I think 60" is somewhat standard for the agriculture industry based on this article.

This is the comment that I keyed in on that leads me to believe that one should try to match the root depth in their zone of interest.

Available water capacity is often stated for a common depth of rooting (where 80 percent of the roots occur). This depth is at 60 inches or more in areas of the western United States that are irrigated and at 40 inches in the higher rainfall areas of the eastern United States.”

My guess is that you’re close enough for your lawn. I would use the settings that you’re comfortable with here and, understanding the range of AWC that you have underneath, keep an eye on runoff. If you are watering too often you may see runoff, and that might lead you to try a slightly higher AWC. If your soil gets significantly dry between watering maybe you need to drop it a tad.

For your other zones, consider the same analysis. If you have trees and shrubs the roots will go much deeper, so you should likely change the AWC for those areas to match the deeper root depth. Thanks for asking these questions, it gets me thinking, researching, and above all learning!


If setting the available water is better done through on using the Web Soil Survey to generate depth appropriate data (as you have provided instructions for) is more accurate — This dramatically changes my settings. I go from the .08 earlier mentioned in the thread to .16 — for grass at 6"!! Big difference there.

Any thoughts in the collective trust of expertise here? My area is especially tricky since as plainsane pointed out – I have types listed for my area. I averaged them but then realized that one of them makes up 60%+ while the other is obviously much less. So… a simple average gives the lesser soil type more influence in my calculations than appropriate. Fortunately the Web Soil Survey (when using the depth setting page) allows for a “weighted average” and/or other customization.

What a great community here!