Interesting Read for Members Interested in Soil Test Interpretation

Learn how improving your soil can lead to a better looking lawn
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andy10917
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Interesting Read for Members Interested in Soil Test Interpretation

Post by andy10917 » July 9th, 2019, 8:59 am

The member SCHREIBDAVE asked an interesting question about his soil test interpretation, and to answer it I had to try to explain the processes we do when interpreting soil test results and building a plan for remediation. If you're one of those people that actually want to understand what we go through when thinking out a plan, this could be an interesting read for you.

If you read it and want to discuss it (with questions), please use this thread and let's avoid hijacking his thread.

Here's a direct link to his question and my response: Dave's Soil Test Question. The soil test results being discussed are up higher in the thread.

Here's his question:
If I put 1 lb of P and K down monthly does anyone have an opinion as to how many apps I will need before those numbers get into the acceptable range? Is this a multi year effort?
and here's my response:
This is going to be confusing for you, because the numbers on your test show amounts at the 6" test depth, and most of the time we do calculations at the 4" depth for grasses. The numbers we use are therefore (4/6th for the conversion), or 67% of our normal numbers for anything calculated in lbs/acre. Things calculated in PPM do not need conversion. Sorry, that's what happens when you have the tests listed at the 6" test depth, which you did.

Soil chemistry is far more complex than "if I need to get from 58 lbs/acre of Phosphorus to 170 lbs/acre, and I apply 1 lb/K monthly (43 lbs/acre), then it will take X number of months to reach my target".

You've got a boatload of Calcium in your soil. In a "perfect scenario", we'd want to see the Calcium around 68% base saturation. You're at almost 84%, and there is no way to remove Calcium. Now, Calcium/Magnesium/Potassium are all vying for exchange sites, and Calcium has an advantage in a head-to-head competition. This means that Potassium is going to struggle to bind to your soil and find a permanent home. If it doesn't, it can flush out ("leach") over time. It's sort of like pouring water into a glass with a hole drilled in the bottom - it is leaking out at some rate as you pour it in. What is that rate? We don't know, but can take educated guesses once we have a year-to-year trend.

So, if we lose Potassium to leaching, is this a hopeless cause? No - because we're adding it monthly, so there's always some present. Optimal? No, but it's better than not putting it down.

Now, the raw number for Potassium in your soil is actually not terrible - it's 144 lbs/acre (215 * 0.67). We'd like to see that at around 175 lbs/K. But (another complication) - the high TEC means that the raw numbers and the "desired numbers" are not in the same ballpark. Soils can act funny when ratios of one nutrient is not in balance (by ratios) to another. The short-term goal is to at least get the raw numbers working, and move slowly toward the balanced ratios in the "desired values" over time.

So, the Potassium situation in your soil is not horrible. But the Phosphorus numbers are. Converted to the 4" depth, you have about 39 lbs/K - far below the 170 lbs/K I'd like to see even for raw numbers. That will take a long time to correct, but at least Phosphorus doesn't vy for exchange sites.

I could go on and on, but I feel I'm just going to confuse you more. The bottom line is that you'll be applying Phosphorus and Potassium for quite a while, but we'll watch how things go with the lawn's quality as we get at least the raw numbers addressed.

I'm sorry that this is probably way more information than you were expecting, but I wanted to get you understanding that your question isn't straightforward. Ask more questions if you'd like to.

Now you've got a preliminary feel for what we (and Logan Labs) consider when we interpret your soil test and why we don't just publish stupid target numbers via a single calculation - it doesn't work well to do that.

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HoosierLawnGnome
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Re: Interesting Read for Members Interested in Soil Test Interpretation

Post by HoosierLawnGnome » July 9th, 2019, 12:49 pm

Great point. It isn't as simple as a mathematical calculation. You need real world observation. Micro environments are too variable and complex. They aren't sterile, controlled labs.

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ken-n-nancy
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Re: Interesting Read for Members Interested in Soil Test Interpretation

Post by ken-n-nancy » July 9th, 2019, 3:18 pm

I'm so glad you started this thread with the response you provided to SchreibDave -- I had bookmarked that posting so that I could refer back to it later. Now I can bookmark this one instead, and hopefully also learn further from future postings in this thread, too!

Dargin
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Re: Interesting Read for Members Interested in Soil Test Interpretation

Post by Dargin » July 13th, 2019, 12:56 pm

I'm thankful for this topic being started as well. I've been dipping into reading on all of this, but I'm still pretty scattered; trying to piece concepts together. I've got questions coming for sure, just trying to work past the ore obvious/lazy ones.

Dargin
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Re: Interesting Read for Members Interested in Soil Test Interpretation

Post by Dargin » July 14th, 2019, 7:58 am

I hope this isn't to broad, but reading this brought this question to mind. I'll start a new thread if preferred.

If phosphorus is an anion, not vying for exchange sites,
and exchange sites like clay and OM particles are negatively charged, what keeps phosphorus hanging around longer than an anion like, potassium?
I'm pretty sure I've read analyses here where it's been stated that phosphorus is at luxury levels and won't need amending for a long time, if ever, but I dont recall ever hearing that in regards to potassium. It seems like phosphorus should leach quicker. What relationship keeps it around?


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andy10917
Posts: 29739
Joined: February 23rd, 2009, 10:48 pm
Location: NY (Lower Hudson Valley)
Grass Type: Emblem KBG (Front); Blueberry KBG Monostand (Back)
Lawn Size: 1 acre-2 acre
Level: Advanced

Re: Interesting Read for Members Interested in Soil Test Interpretation

Post by andy10917 » July 14th, 2019, 9:08 am

Phosphorus is complex - it reacts pretty quickly with other nutrients to form less-available forms. These forms slowly either slowly convert back and forth to available forms in solution, or can leach out of the soil. But run-off is the biggest issue ecologically.

There are organic forms of Phosphorus and mineral forms, some of which are stable and some of which go into solution and are available to plants. There is far more Phosphorus in soil than is usable, but it is tied up and can't be counted on for plant growth. They are extremely slow to weather and be bioavailable. Search for "Phosphorus Cycle" using Google to get more details about the forms, mineralization and leaching/run-off for more details.

Dargin
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Re: Interesting Read for Members Interested in Soil Test Interpretation

Post by Dargin » July 14th, 2019, 11:12 am

Thanks, Andy. More reading! :yahoo:

By the way.
Dargin wrote:
July 14th, 2019, 7:58 am
...what keeps phosphorus hanging around longer than an anion like, potassium?
Meant to say "a cation like potassium." Just so nobody is confused.

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