# AWC vs CEC



## spaceman_spiff (Feb 5, 2021)

I've got a Rachio 3 controller, and my lawn zones are set up as:
- Clay loam
- AWC: 0.18 in/in
- Root depth: 6 in
- Allowed depletion: 50%

I recently received a soil analysis on all 3 of my lawn zones. 
For CEC, my 3 zones show:
- Front: 7.5 meq/100g, 6.0 pH
- Back: 6.4, 5.6 pH
- Pool: *4.1*, 6.4 pH

After reading this article about CEC (https://www.canr.msu.edu/news/what_is_your_soil_cation_exchange_capacity), it got me thinking. It basically states that soils with low CEC don't hold onto _anything_, nutrients or water, very easily.

Right now, my front yard (highest CEC) is still all completely green at this point in the California summer, even the portions that get 7-9 hours of sun per day. My back yard (mid CEC) is green in the shady areas, and green/brown in the sunny areas. My pool area has the lowest CEC out of the 3 zones, and easily contains the most brown/dormant grass of the 3.

Since lower CEC soils don't hold water as easily, does it make sense to drop my AWC of my back yard zone a bit, and my pool zone a decent amount, to try to counter-act this? It's still putting down the same amount of water per week, but does so less at one time, but more frequently. If I'm dropping 0.18 in/in of water on my pool zone, wouldn't a low CEC mean that a portion of that is wasted since trickles below the grass root zone quicker than a higher-CEC zone?

Notes:
- I've performed multiple irrigation audits and my in/hr settings for my zones are right on the money
- I am looking into organic matter to try to raise the CEC of my back/pool zones. This is just a temp fix for the next year until I get my next soil analysis.


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## bernstem (Jan 16, 2018)

Soil composition can be inferred from CEC, but it isn't always 100% accurate as some fine soils can have low CEC, so keep that in mind. In general: 
Sands are 3-10
Loams are 10-30
Clays are 30+
If you really want to know your soil composition, most soil labs can tell you the sand/silt/clay percentages. The test is ~$20.

Based on the CEC numbers from your soil test, you likely have a sandy soil (maybe a sandy silt in the front) and may benefit from switching to a sand profile. You may also want to pull some cores and check root depth and look at the soil to see how different the three areas are.

Adding organic matter will increase both CEC and water holding, but it is a years long process even with core aeration before top dressing unless you want to till the organics into the soil. It just takes a very long time for the organics to work their way down the soil profile.


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## spaceman_spiff (Feb 5, 2021)

bernstem said:


> Soil composition can be inferred from CEC, but it isn't always 100% accurate as some fine soils can have low CEC, so keep that in mind. In general:
> Sands are 3-10
> Loams are 10-30
> Clays are 30+
> ...


Yeah, so that's the weird thing. I've done multiple irrigation audits, so the amount of water that Rachio is laying down is pretty solidly consistent with the settings, but when I pull a soil plug (with the soil set as sandy loam, 0.12 AWC), only the top 1-1.5" is moist shortly following a watering, hence why I have my AWC set to a more clay loam value. I was pretty shocked when I saw that clays are 30+, as that's what my city told me to set the Rachio up as.


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## bernstem (Jan 16, 2018)

spaceman_spiff said:


> bernstem said:
> 
> 
> > Soil composition can be inferred from CEC, but it isn't always 100% accurate as some fine soils can have low CEC, so keep that in mind. In general:
> ...


There are some clays that can have low CEC. You can check the soil with a jar test as a quick estimate of soil composition.

AWC of 0.12 is consistent with a sandy loam. That may be a bit finer than your soil, but probably isn't too far off. FYI, clay soils have an AWC of 2.0. Most sandy soils are some sort of sandy loam and have AWCs of 1-1.5. Coarse sands come in at 0.5. You probably don't have a coarse sand soil. If I had to guess, you probably have a fine textured sandy or fine textured sandy loam.

To come back to your original question, it seems that the calculations are working well for the front. You don't need to change anything. For the other two areas, it seems to me it would be reasonable to lower the AWC. With the root depth unchanged, that will result in more frequent watering with less water per irrigation cycle.

I like that you are pulling cores to check soil moisture. I would pull cores from the three areas and compare texture and moisture levels/moisture depth to see what is actually happening.


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