John P: Why Do "Desired Values" Change?

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John P
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John P: Why Do "Desired Values" Change?

Post by John P » April 1st, 2021, 4:10 pm

*** Split Posting because it hijacked another thread *** by Moderator

I noticed the lab keeps changing the "desired value" of the CATIONS with each test
And those changes appear (to me at least) significant
For example, from 2019 to 2020, they raised his "desired value" for Mg ~65%
But then from 2020 to 2021, they lowered his "desired value" for Mg ~20%
Same story for Potassium
From 2019 to 2020, they raised his "desired value" for K ~65%
And then from 2020 to 2021, they lowered his "desired value" for K by the same 20%
Over the same 3 years, his pH stayed in a fairly tight range, 8.1 in 2019 to 8.4 to 8.3.

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andy10917
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Re: John P: Why Do "Desired Values" Change?

Post by andy10917 » April 1st, 2021, 6:34 pm

I moved your posting to a thread of its own, because a posting that is not related to the topic that the original posting or thread is considered "hijacking the thread. In the future, please don't attach a response/question to someone else's thread or posting...

Your question is interesting, and it leads to "why we use a lab that provides the best soil information".

The "desired value" field is not a single fixed number - it is a calculation that is based on the ability of the soil to hold nutrients. This number is shown as your TEC or "Total Exchange Capacity". If your soil could hold more cations, but doesn't have them at the proper Base Saturation amount, then there is a gap in that nutrient. For instance, if it could hold 100 lbs/acre and is expected to be 70% of the total amount of cations, then the desired value is what it could/should have. Explained another way, as the TEC rises, the desired number will rise, too. If it falls, so does the desired value. Where NONE of the cations are filling the available exchange sites, then Hydrogen will do the job and the result is an acidic soil.

Many soil test labs do not calculate TEC, and therefore they are not giving us a critical number needed to explain YOUR soil. Instead they use a theoretical "typical soil". Less than 10% of the soils that we see are "typical". Therefore, we don't use labs that do not explain the properties of YOUR soil, including TEC.

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Re: John P: Why Do "Desired Values" Change?

Post by John P » April 1st, 2021, 8:14 pm

Thanks for the reply. It was just a question that occurred to me while reading that specific thread. Attempted hijacking does sounds pretty awful,

It seems that pH is a part of the calculation as well as it moved in the same direction as the other numbers, but by a much smaller percentage.

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andy10917
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Re: John P: Why Do "Desired Values" Change?

Post by andy10917 » April 1st, 2021, 8:37 pm

It seems that pH is a part of the calculation as well as it moved in the same direction as the other numbers, but by a much smaller percentage.
Nope! pH is not an input to the calculation. pH is a measurement of how much Hydrogen is held in the exchange sites. It is an abbreviation for "power of Hydrogen". If all of the exchange sites have cations, the pH will be 7 or higher, and the Exchangeable Hydrogen will be zero. As more and more exchange sites are filled by Hydrogen, the Exchangeable Hydrogen will go up, and the pH will go down.

pH is a measurement of the end result of Hydrogen using empty exchange sites. It tells you nothing about whether the amounts of Calcium, Magnesium, Potassium, Sodium and a few "other" cations are at the right levels. Base Saturations tell us that info.

Paying too much attention to pH is like reading the last page of a book - you may know "whodunnit" but you have no idea what the book was about. Seriously!!

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MorpheusPA
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Re: John P: Why Do "Desired Values" Change?

Post by MorpheusPA » April 1st, 2021, 9:59 pm

Sometimes you don't even really know whodunnit, you just know where the gun ended up being. I usually describe pH as "unimportant in and of itself, but something people expect to see."

I angered somebody over on another site with that description but...well, that's another story. pH, as noted, just measures how much hydrogen hangs around.

It says nothing about where it comes from...and, more importantly, if it's off, how to fix it. And if it's right...is it coming from the right things? I could design a picture-perfect pH soil that's completely sterile and unable to grow anything very, very easily, just by starting with a flawless bark and peat mix (commonly used) and balancing it with lye to a pH of 6.8. Dandelions or crabgrass might be able to survive...for a while.

pH doesn't move very much as some cations rise and fall as pH is a factor of ten for each number; 6 is ten times as basic as 5. And so on. And cation influence can be kind of subtle, depending. Plus when we add the material to the soil to change the pH, we can do things...a little differently. There are times when adding calcium can cause a pH to drop.

Back to the base question, in addition to the calculated changes Andy described, simple sampling variance comes into play here as well. No soil is completely identical everywhere. And soils change year to year as well depending on the amount and type of organic matter included in the sample, so the total capacity actually will change over time as well.

I fully expect a normal variation in sampling year to year and don't worry about it too much, even from my own soil where I take twenty samples, mix them, dry them, mix again, and take a subsample to send. Closer to perfect it's not possible to get, yet there's always going to be variation.


John P
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Re: John P: Why Do "Desired Values" Change?

Post by John P » April 2nd, 2021, 8:29 am

Thank you gentlemen. When I was first contemplating the question, I was thinking in terms of PPM. But the CATIONS are stated by this lab in terms of lbs per acre. So even though a "desired value" of 491 lbs per acre of Mg (2020) over 298 ppa (2019) does not seem as much a variance even if it is a 60% year-over-year increase.

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Re: John P: Why Do "Desired Values" Change?

Post by Spacklerstyle » April 2nd, 2021, 10:31 am

John, I also suspect (as Morph eluded to) that my early sampling was a bit inconsistent. In the last couple years I have done a better job pulling soil from a 3-4” depth (in the early days, I probably only had 4” before I hit the limestone bedrock below, so it was a more shallow sample back then... I have been building the soil up slowly over time with topdressing / sand for leveling / composting, etc.)

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