High Potassium Levels

SoHotRightNowMay 29, 2013

I have a ornamental border with trees, shrubs, grasses, bulbs etc. that was planted last year. The area used to have a concrete pad about 3 inches thick. The concrete was removed and soil amended with 6 inches compost along with cottonseed meal, bone meal, and kelp at the recommended amounts. The ph was tested and lime was added. Fast forward to this year and my plants are displaying signs of micronutrient deficiencies so I ordered a complete soil test. Micronutrient levels came back as normal with the ph at 6.7. The phosphorus tested at 92ppm with the potassium at 630 ppm, very high on phosphorus and off the charts high on potassium. After doing some research I found the high potassium levels can effect the plants ability to use micro nutrients and that is probably the reason the plants are showing those symptoms. What I haven't been able to find is a solution to the problem. Does anyone have any suggestions?

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The cottonseed meal, Bone Meal, and Kelp all helped bring the levels of P and K to where they are, one reason to not add those things to your soil without a good reliable soil test. There is really not much you can do now except wait for those nutrients to leach out as they will over time.
Excess levels of Phosphorus in the soil will interfere with a plants utilization of Zinc, Iron, and Cobalt and excess levels of Potash can prevent a plant from properly utilizing Nitrogen.

    Bookmark   May 30, 2013 at 6:46AM
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Would you any nitrogen this year?

    Bookmark   May 30, 2013 at 10:43AM
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macthayer(z9a NV)

Relax. There is something you can do about this problem. Check out the link below. It is well known that 'weathering' will lower soil potassium. You can speed that up by leeching the soil yourself. Please check link. I know this works because I've done it myself.

You should also understand that there are 3 kinds of potassium found in the soil. Two of them are available to the plant, and the third is 'bound' to another mineral, like clay, mica, etc. This bound form of potassium is not available to the plant until natural weathering breaks it down and unbinds it. Since you are having problems, it's obvious that the potassium level is too high. You should know that labs are highly variable when it comes to potassium levels. If the lab shows a high potassium level but your plants aren't suffering, it may be the lab. The next time you do a soil test, make sure that you give them lots of samples from a variety of sites. Also make sure that you do 'core' samples, which means you're not taking the soil right off the top 3 inches, which are the most variable. Also make sure that you are taking the samples from a uniform depth around the plot. Go down until you are essentially taking samples that are near the root system. This is the potassium level that will most affect your plants; and as I said, work hard to get a uniform depth.

There are other ways to reduce potassium over a season, and that is to plant cover crops that 'suck up' a lot of potassium. Alfalfa is a good example, although it does take micronutrients with it. This shouldn't be a problem with your highly enriched soil. Another suggestion would be grass, which needs a lot of potassium. Keep the grass going strong, clip it regularly and make sure you don't allow the clippings to stay on the soil to decompose. In these cases, you may need to add nitrogen to keep the cover plants growing strong, especially with the grass. Both of these cover crops will utilize a lot of potassium. But beware! If your soil sample was off the surface, your nitrogen may be a lot higher just a few inches down! Might want to recheck that before adding nitrogen.

My motto is to always go low and slow, especially in gardening. It is much easier to ADD nutrients to the soil, and adjust levels, than it ever is to reduce levels in the soil. For instance, if you actually need nitrogen, plain Blood Meal is just the ticket. Check container for its specific dispersion rates.

I'm sure there are other cover crops that might be more acceptable to you in terms of what your garden looks like, or what wil grow well in your area. Off the top of my head, I'm blanking on those, but they are easily 'googled'. Or perhaps it's enough just to know you only need to put up with a cover crop for one, m-a-y-b-e two seasons to achieve your desired goal.

I did note that your soil is alkaline, and I don't know if that's typical for your geographic area (it is in mine), but you can use sulfur to make it reduce pH. Again, check the container instructions for rate of dispersion. Just as a note, I tend to use less than the container says - perhaps a third less - and then retest and repeat if needed.

Please let us know how you do!

Here is a link that might be useful: How to lower soil Potassium.

    Bookmark   June 2, 2013 at 2:33PM
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A pH of 6.7 is hardly alkaline and cover crops are pretty much useless in an ornamental planting bed. Also the kelp meal and cottonseed meal - if applied according to package directions - had no effect on phosphorus or potassium levels since they contain virtually none of these nutrients. It is likely these levels were similar before any amending took place as both of these tend to be adequate to high in PNW urban soils.

These levels will decline in time - I'm not sure I would be overly concerned. And you can tie up excessive phosphorus by adding a mixture of other mineral fertilizers, like magnesium sulfate or iron sulfate. Iron sulfate would likely be my choice as a soil pH of 6.7 tends to bind iron. And yes, I probably would add some nitrogen to offset the stunting or limited growth that comes from excessive levels of P&K and because nitrogen is typically lacking from our soils.. And I would use the cottonseed meal again - first because it is a great organic source of nitrogen and second because it is a natural acidifier. I'd be a lot happier seeing that pH down to about 6.3-6.4.

FWIW, liming in the PNW is virtually unnecessary. Most often folks want to bump up their acidity. And for future reference, amending soils for new plantings here requires little more than just adding compost. I would avoid adding any other amendments or supplements until a soil test indicated a need.

    Bookmark   June 2, 2013 at 4:10PM
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macthayer(z9a NV)

Alfalfa is a pretty plant with beautiful blue flowers. See link. I have 3 different ornamental grasses growing in my flower beds, and they're beautiful. You can choose to skip even considering cover crops because they "are useless in an ornamental border", or you can think outside the box, be creative and imaginative, and find plants that heavily take up potassium and are still beautiful.

If you read the link I sent you, you'll see that it's not a difficult process. The Seattle rain should help you on this. All you're doing is speeding up a natural process.

IMHO, it is better to follow or speed something that nature would do eventually anyway.

Here is a link that might be useful: Photos of Alfalfa

    Bookmark   June 2, 2013 at 6:34PM
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First, your pH @ 6.7 is very slightly acidic, and in a range I like for all-purpose planting. Unless you're planting things with unusual needs (azealas, camellia, blueberry, etc.), you pH should be fine.

Both high P and high K can and will affect plants ability to use micronutrients. High K and especially high P will take time to resolve. P is not very mobile in soils, and K while more mobile tends to be a bit lasting.

You will still need to provide regular source of N - that disappears comparative quickly. Depending on what you grow, climate, and how you provide that N once a year may be enough. If you want to stay with an organic source bloodmeal would be a good choice - 11-12% N, and typically no P or K. Stay away from the bonemeal (typically ~ 11%P), kelp (typically ~ 2.5% K) and cottonseed meal (~ 2%P and 1% K).

As far as correcting the P and K, you can help the K along a bit by leaching as much as is prudent. But I doubt you will be able to do much about the P. Both will move better in more acidic soil, especially the P, but there is a limit to that too. A bit of garden sulfur to try to edge the pH down a tad might help a bit, but not much.

I think a better approach would to try to compensate by boosting those micronutrients that correlate with the plant deficient symptoms you're seeing. Just go easy - you can always add a bit, and if the effect is positive add some more. But once you add, it's going to be in there and not much you can do.

    Bookmark   June 4, 2013 at 8:40AM
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