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Container Gardening

why are crfs not recommended for self-watering containers?

14 years ago

I've read a number of posts over the years that CRFs like Osmocote are NOT recommended for use in SWCs. Can someone give me a simple answer as to why this is? Common sense that a 4 month controlled release would seem to be the natural thing to use.

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It would seem to me that the NPK of 19-6-12 would be ideal (diluted in half for tomatoes) to what is recommended here of a 3:1:2 NPK ratio:

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So folks, what am I missing??

Raybo

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Comments (5)

justaguy2
14 years ago
Nothing. I believe the source of info that says not to use CRFs is EarthBox.

I will say to be careful with CRF though. A 4 month formula is based on an average temp inside the container of 70F. Warmer and it won't last as long.
rnewste
Original Author
14 years ago
justaguy,

Thanks. Yes, I now recall it was the EarthBox folks (or a posting on their Forum) where it said not to use CRFs.

My methodology (to test now), is when mixing in the 3 cups of Dolomite Lime (remember, the EarthTainer holds twice as much potting mix as the EarthBox - about 3.3 cu. ft.), I want to add half a cup of Osmocote that will be down 5 or 6 inches.

I will still plan to use Tomato-tone or other vegetable type fertilizer to the 2 one cup strips in furrows just below the surface. The goal is to get a longer term CRF "presence" down toward the lower portion of the 'Tainer, and not rely totally on a "top-down" movement of the fertilizer to the lower roots.

Remember, in a conventional Container, top-down percolation works, but in a bottom-up capillary fed SWC, the reverse may not be true.

Raybo
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justaguy2
14 years ago
The nutrients (salts) will still diffuse.

I am not trying to talk you out of using the CRF, but if you will also be using granular fert on/near the surface I would wonder why you would want the CRFs at all.

It's basically doubling up.

Is the granular fert on/near the surface not doing the trick for you?
rnewste
Original Author
14 years ago
justaguy,

My "visual" observation is that the tomato plants appear to slow down in growth at about the 90 day mark. My theory is that the fertilizer applied along the surface strips has been expended by that time. I could alternatively, use a soluble application of Miracle-Gro but I thought it would be better to have the CRF deeply buried in the lower section of the potting mix, and have it release slowly (over 4 months, as the package says).

I'm just trying to simplify things since adding in the CRF when mixing in the Lime would eliminate the need for mid and late season soil drenches of a soluble fertilizer.

Raybo
justaguy2
14 years ago
I think you will find the 4 month formulations don't actually last 4 months. You might wish to purchase from a wholesaler/nursery supply place as I find it is easier to get 8-9 month or 6-7 month formulations that way.

The 4 month claim is based upon a constant 70F under lab conditions. I strongly suspect your tomato containers get much warmer than that since I know mine do even in a mild Wisconsin summer.

This is a link to an excellent series of articles on controlled release fertilizers. Note that the link takes you to the first article. To find the others you need to click on the archive link on the left. The year of the article in 2007 and the month for the first one is July. So, for the next one look under August 2007.

Once you read through the series I think you will know more about CRFs than you ever thought you would want to ;)

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