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Old 02-19-2018, 09:03 PM #3961
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Didn't we talk about peat having a crazy high CEC but the test wasn't showing it?
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Old 02-19-2018, 09:16 PM #3962
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I brought it up on page 7^ way back!
Bulk Density. meq/100g vs meq container volume.
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Old 02-19-2018, 09:42 PM #3963
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When I run into a question like this on the farm, I always try to think backwards. How would a farmer pre-testing get an estimate of TCEC on the farm?

Paradox, Can you give us the info as to why CEC is correctly stated on test results?
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Old 02-20-2018, 01:57 AM #3964
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Originally Posted by reppin2c View Post
Didn't we talk about peat having a crazy high CEC but the test wasn't showing it?
the test is flawed or limited, like all tests.... Even if a test were "perfect" the process or individuals administering it along the way aren't
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Old 02-20-2018, 04:13 AM #3965
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So what am I missing? If 40% of your mix is organic material how can your cec for the total not be around 400 plus whatever other contribution?

This is something I have not understood since I started this.

Because I don't think anyone's mix is 40% organic matter. Most of the organic matter inputs are derived from a histosol, which is a soil that is defined as having over 12% organic matter. These soils have 100-200 meq CEC. So if your mix is 40% peat moss, that still has soil in it.

Also I think it also has to do with the fact that it has to do with the fact that soil particles just skew the CEC so much. Sand takes up relatively a large amount of space compared to clay and OM and has such a low relative CEC it skews the numbers.

1000 meq is also a high number in the range of CEC for OM. I'll give the range of 500 - 1000 meq CEC.
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Old 02-20-2018, 04:26 AM #3966
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the test is flawed or limited, like all tests.... Even if a test were "perfect" the process or individuals administering it along the way aren't

So the way the CEC test works is this:

You flood the CEC sites with NH4+. Than you take a solution with a known concentration of K+ with a volume greater than the volume of the soil porosity. Than you flood the soil with a solvent and measure the concentration of K+ that were held on cation exchange sites. This concentration will tell you the amount of cations held per unit volume of soil.

This is the basic jist of the ammonia CEC determination. Obviously there is a little more too it, it's been a while since I have done this technique.

The limitations of this are with alkaline soils and saline soils, as salts will start to form ionic bonds which have the same effect as CEC, but are technically not part of CEC. Maybe the spectroscopy technique might have an accuracy issue. If you're running Atomic Adsorption Spectroscopy, versus something like an Induced Couple Plasma.

Idk what spectrum does btw as far as spectroscopy. But probably Atomic Adsorption. That is the most cost effective, and accurate enough for agriculture's purpose.
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Old 02-20-2018, 07:57 AM #3967
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Originally Posted by Arnold. View Post
Guys,

Could you look at my reasoning here beneath?
My calculations says I need to use only this little gypsum to the potting soil. But Iím doubtful if I can trust them.

I tried to calculate how many gypsum (and TSP) I would need to add to my potting soil which I got tested. These calculations says to use about 17g of gypsum on 5 gallons of potting soil. This seems a lot less than I would have imagined after reading this thread
Iíve seen Slow mentioning 300g of gypsum for 7 gal. Or 1 cup for 5gal.

My reasoning (with fault??):
I want to add 1845ppm of Ca to this potting soil. With gypsum being 22% of Ca in weight I have to add 8380ppm of gypsum.
Since the K3 procedure measures ppm in weight. This means I have to add 0.008380g of gypsum for each g of dried and sieved soil (just like the sample I sent in).
Then I weighed a sample volume of soil, oven dried and sieved it before weighing again. This way I could calculate the conversion factor from the dry, sifted soil to the wetter soil straight out the bag.
5 gallons of soil would weigh 2kg, thus needing 16.7g of gypsum.

Am I overthinking this and does this soil indeed only needs a dust of gypsum?

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If nobody else responds to your questions, I will do so tomorrow when I have some free time.
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Old 02-20-2018, 11:59 AM #3968
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That would be greatly appreciated growingcrazy.
I've been pondering this question a few days now.
I would think that I misinterpret the ppm values of the lab. But then, how can you misinterpret parts per million? It is just 1/1.000.000...

I also thought about multiplying with the molecular weight of the element. But that would make for pretty large numbers.

So I'm clueless
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Old 02-20-2018, 01:08 PM #3969
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Quote:
Originally Posted by Arnold. View Post
Guys,

Could you look at my reasoning here beneath?
My calculations says I need to use only this little gypsum to the potting soil. But Iím doubtful if I can trust them.

View Image

I tried to calculate how many gypsum (and TSP) I would need to add to my potting soil which I got tested. These calculations says to use about 17g of gypsum on 5 gallons of potting soil. This seems a lot less than I would have imagined after reading this thread
Iíve seen Slow mentioning 300g of gypsum for 7 gal. Or 1 cup for 5gal.

My reasoning (with fault??):
I want to add 1845ppm of Ca to this potting soil. With gypsum being 22% of Ca in weight I have to add 8380ppm of gypsum.
Since the K3 procedure measures ppm in weight. This means I have to add 0.008380g of gypsum for each g of dried and sieved soil (just like the sample I sent in).
Then I weighed a sample volume of soil, oven dried and sieved it before weighing again. This way I could calculate the conversion factor from the dry, sifted soil to the wetter soil straight out the bag.
5 gallons of soil would weigh 2kg, thus needing 16.7g of gypsum.

Am I overthinking this and does this soil indeed only needs a dust of gypsum?

View Image

Follow the recommendations that come with your test. Don't get confused by this thread.
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Old 02-20-2018, 04:54 PM #3970
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Originally Posted by paradoxlost View Post
So the way the CEC test works is this:

You flood the CEC sites with NH4+. Than you take a solution with a known concentration of K+ with a volume greater than the volume of the soil porosity. Than you flood the soil with a solvent and measure the concentration of K+ that were held on cation exchange sites. This concentration will tell you the amount of cations held per unit volume of soil.

This is the basic jist of the ammonia CEC determination. Obviously there is a little more too it, it's been a while since I have done this technique.

The limitations of this are with alkaline soils and saline soils, as salts will start to form ionic bonds which have the same effect as CEC, but are technically not part of CEC. Maybe the spectroscopy technique might have an accuracy issue. If you're running Atomic Adsorption Spectroscopy, versus something like an Induced Couple Plasma.

Idk what spectrum does btw as far as spectroscopy. But probably Atomic Adsorption. That is the most cost effective, and accurate enough for agriculture's purpose.
Thanks for reply, I do understand how the test works. My point was more about the quality of the people & processes along the way. How you take that soil sample, where, how many spots, how it's sifted, mixed and so on. The person in the lab, how detailed are they, how detailed is the labs process, how much pride do the people take in their jobs doing that work etc etc etc... Samples are also taken at a point in time - I was more trying to say it's has endless limitations but also offers invaluable detail.

My native around here in spots is > 12% OM, 40% is certainly a lot for OM but not unheard of.

reading this thread it just feels at times that some think their going to become a great farmer if only they get that perfect soil test result - I can't stress how wrong this is and IMO, leads many down the wrong rabbit hole...

Understanding is King, test results are simply one piece to that puzzle
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