Soil Test results from Exstension- teach me about P

timmytwoSeptember 11, 2013


I am getting ready to prepare for new plantings of apples and blueberries. I sent samples from each of two planting sites to the U of Mn and got back the results.

I had expected that I would get a report back that showed I had average to good growing soil and that I would end up digging out and replacing the soil for the blueberries.
That seems to be what I got.

But I am really confused about the Phosphorous - The report shows it as Very High on the scale. I am curious as to
why it might be so high, and also what it means to me for my growing plans.

At least for the apple site I am pretty sure that the soil has been pretty ignored/undisturbed for decades. It is a grassy area near oak trees.

Also- does the organic matter tell me anything?

I am as much interested it learning about the measures as figuring out what they mean for me.

Apple site:
Organic matter: 3.5%
Ph 6.9
P: 36 ppm
K 110 ppm

Blueberry Site:

Organic Matter 4.8%
Ph 6.7
P 45 ppm
K 151ppm


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One of the issues with test results is that the numbers depend on the method used, and different labs use different methods based upon their predominant regional soil characteristics. Those test results are then assessed based upon decades of correlations studies for the regional soils and crop results.

The reason for the preamble is those P numbers look low to me by what we would expect in TX. TX uses a different extraction method (Melich III) than U. Mass (Modified Morgan) because our soils are quite different. In TX the "critical level" for P is 50 ppm for general vegetable culture as well as turfgrass (don't know what they would use for apples or blueberries - not big crops in TX). Anything below that is considered deficient and would call for supplementation. At the same time, we want to be below 100 ppm because of P toxicity issues that start becoming significant at 150 ppm and above.

So one question is, are you in MA, if not where are you? Did U. Mass provide critical levels for P along with your analysis?

On soil OM - you look pretty decent. An ideal range is generally 5-8%. So you have some room for more, but it's actually pretty decent if it's native soil.

For your blueberry site you'll want to lower the pH.

Did they send you the paper linked below? Some good basic info in there.

Here is a link that might be useful: Interpreting Your Soil Test Results

    Bookmark   September 11, 2013 at 8:17PM
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As long as your soil has adequate iron + zinc, excess P isn't going to be an issue is the long-story-short as far as what you need to watch out for.

Calcium can also be a concern, but with a pH below 7.3 you should be just fine. Above that, some Ca becomes fixed/unavailable in the soil...though that's only a concern if you don't have a lot available in your soil to begin with.

There is not really a "P toxicity" that's not a concern. The biggest danger is competition with Fe/Zn...and that's mostly not a concern unless you notice deficiency signs in the leaves.

P is not very mobile in soils...plants will have to uptake what's there to use/remove high levels. Avoid P fertilizers (or mixes with P fertilizers) for a while if possible.

All in's not much of an issue...keep an eye out for Fe/Zn deficiency signs in leaves and if your pH creeps high, you might want to look for Ca deficiency signs, too.

Also, as mentioned, you're going to want to lower the pH of where you're planting your blueberries. 4.5-5.5 is ideal, but that may be unobtainable over a short period of time.

    Bookmark   September 11, 2013 at 8:40PM
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The gentleman from NC is correct, and I improperly characterized excessive P issues as "toxicity". It's not really toxicity, it is as he describes it. The linked paper from TAMU may help explain better (and in the future I will avoid calling it "P toxicity").

Here is a link that might be useful: Aggie - Excess Soil P Levels

    Bookmark   September 11, 2013 at 8:52PM
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Either way, about 40 ppm P is not high. It is within the norm. I think that soil, assuming other micronutrients are OK, only needs a bit of organic matter, and some elemental sulfur by the blueberries.

    Bookmark   September 11, 2013 at 9:17PM
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Thanks for that great info.

I am in MN. So the test was done by the University of Minnesota

According to th report they use Bray 1 method to test

    Bookmark   September 12, 2013 at 10:06AM
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Sorry - I misread as MA, not MN. The I would say your test are appropriate for your region.

For your soil pH Bray is a good test and I believe will agree with Melcih III quite closely. What constitutes high vs. low in part depends upon soil texture. I just looked at MN's nutrient Management recommendations (published via link below - P is on page 14), and they are quite different than what is common in TX on our heavy, alkaline calcerous clay soils. By MN's correlations you're in the high to very high range. For your situation you'll probably want to avoid adding any P containing fertilizers for a while. And as nc-crn pointed out, watch for telltale signs of chlorosis. If you haven't already done so, you might consider springing for the micronutrient test (iron, zinc, copper, manganese) at some point.

Here is a link that might be useful: MN - Nutrient Management

    Bookmark   September 12, 2013 at 10:41AM
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BTW - With a bit more time I just looked further into the Nutrient Management guide I linked above and see the fruit recommendations start on page 24.

Specific to your interest see tables 31, 33 and 35. By those tables for your apples as a new planting you are medium in soil OM and the recommendation would be 45 lbs N per acre, they recommend you add P as P2O5 at a rate of 75 lbs/acre, and K as K2O at a rate of 200 lbs/acre.

For new planting blueberries you just fall in the high range on soil OM, so 10 lbs N per acre, you're into the add zero P range, but should add K as K2O at a rate of 50 lbs/acre.

So actually, considering the crops and new planting you look pretty good, save the pH for blueberries.

    Bookmark   September 12, 2013 at 12:42PM
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Just for comparison, I have 80 ppm P in alkaline soil in the garden, all due to compost , and after adding some inorganic P and about 40 tons of wood chips, i have 40 ppm in the orchard. TXEB has even more P IIRC.

    Bookmark   September 12, 2013 at 7:49PM
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Yep, I got over 300 ppm in my veg bed. And it's a problem. But in my lawn after 8 years of absolutely no P added, I went from over 100 ppm P to 10. I now need to start adding some P back. It's a good test for me - my target is to be at 70 ppm P in the lawn one year from now. But my soil is vastly different and the numbers really can't be compared to MN.

My take based on the analysis and the MN recommendations for MN soils is the only thing you really need to add for new planting is K , but a touch of N in the form of a compost topdressing around the new trees/plants would be a good idea. P is okay for now, but certainly doesn't appear to be a problem. I'd try to avoid specific P additions, but I wouldn't shy away from non-manure compost or wood mulch. I would stay clear of all manures, in any form.

The near perfect soil conditions except for pH in your blueberry plan has me really curious. Do you have any knowledge of the history of how the land was used previously? The respectable OM and high P has me wondering if animals (leading to manure) were kept there?

    Bookmark   September 12, 2013 at 8:45PM
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Soil tests find both the readily available amounts of P, which is what is reported to you, and the amount of P2O2, the more stable and not very available Phosphorus. What your soil test tells you is that for what you told the lab about the use of that soil you have more than ample amounts of P.
What the Organic matter tells you is the humus, residual amount of organic matter, is in that soil. Some research tells us that your numbers, 3.5 to 4.8, are adequate while many of us, through experience, have found 5 to 8 percent levels are better. More organic matter in that soil will mean higher levels of available P.
The soil pH of your Blueberry site is too high, should be in the 4.5 to 5.5 range.

    Bookmark   September 13, 2013 at 7:00AM
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P2O2 ??? What the heck is that. Never heard of such a beast.

If you want to understand how P distributes in soils, check out the link below.

P2O2? Really?

Here is a link that might be useful: The Nature of Phosphorus in Soils

    Bookmark   September 13, 2013 at 7:34AM
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7 am, perhaps a coffee related problem. :-]

    Bookmark   September 13, 2013 at 11:19AM
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Must have, inadvertently, hit the wrong key and the should have been P2O5. This from your UMN link above.
"The phosphate form (P2O5) is a chemical produced during fertilizer analysis, but does not exist in either fertilizers or soils"

    Bookmark   September 14, 2013 at 6:21AM
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Right. P2O5 is used as a convention for reporting, but it doesn't exist in soils or fertilizers. Further, P2O5 doesn't exist in the soil extract solution that is subsequently analyzed. The analytical results for fertilizers are reported as P2O5 as a matter of standard. It's nothing more than a calculation and a representation of how much P was measured for the fertilizer. Soil test results are most commonly reported as just P.

This post was edited by TXEB on Sat, Sep 14, 13 at 10:34

    Bookmark   September 14, 2013 at 7:44AM
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When comparing soil analysis results across labs there is another issue that needs to be considered, and that is the nature of the analytical method employed, even when extractions solutions (e.g., Mehlcih, Bray-1, etc) are the same.

In the recent past the method widely used in soil analysis was a colorimetric method. A soil extraction solution, after filtration, would be reacted with a series of standard reagent solution, a color would develop based upon the amount of orthophosphate in the extraction solution, and based upon the color the amount of P would be determined.

In recent years many labs have converted from the colorimetric method to the use of inductively coupled plasma emission spectroscopy (ICP). In this method the filtered extraction solution (same as the one used for colorimetric determination) is analyzed directly. ICP shows the not only the orthophosphate that colorimetery shows, but also any other form of extraction soluble P, such as some of the organic-based P (e.g., phytin, phospholipids, etc.) that may be present in a soil. Hence ICP methods typically report higher levels of soil P than does colorimtery. In soils with high amounts of organic matter the difference can be substantial (more than 50% difference).

What that leads to is different levels of what constitutes high or low levels of P in a soil is based upon the method. This leads to different crop response calibration curves for different methods.

I just looked this morning and found out, to my surprise, than MN still uses the colorimetric method for determination of P in soils. Hence MN's levels of P that correspond to low vs. high will be noticeably lower than those used in TX, for example, that uses ICP.

Converting from colorimetery to ICP, and developing new crops response curves is not a trivial matter as explained in the paper linked below. While challenging, many soil labs have made the change. MN has not.

Here is a link that might be useful: SERA-17 P in Soils by ICP vs. Colorimetry

This post was edited by TXEB on Sat, Sep 14, 13 at 12:55

    Bookmark   September 14, 2013 at 10:10AM
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