# Varying recommendations for different soil tests



## jtmurphree (May 25, 2020)

I tried to look to see if this has been discussed much and I apologize if it has and is in another thread, but the variability in some of the fertilizer recommendations depending on who performed the soil tests is very drastic in what I have seen on the forum. I live in Alabama and in my opinion, we have one of the best Agricultural schools in the nation(Auburn University). I base what I am doing off of their study performed many years ago on Zoysia and Tif Bermuda. They compare different Alabama soils (which vary greatly based on what part of the state you live in) The conclusion is that K shows diffencies at levels less than 30 ppm and no significant improvements above 50 ppm. P shows diffencies less than 7 ppm and no real improvements above 15 ppm. I chose the Yard Mastery soil test because their recommendations were closer to these thresholds and will also be sending a soil test directly to Auburn next year for their evaluation and to compare with the YM soil test. 
From what I can tell Auburn's soil test recommendations are slightly higher than the thresholds mentioned above, which makes sense in that it would be expected all the fertilizer applied would not be "absorbed" or utilized by the grass.

My questions is, why is there so much variability in P and K recommendations? It seems like many of the soil tests are suggesting to put down way more fertilizer than what is really needed. I find it hard to believe there is that much variabiltly in soil/grass types for different labs to be so far apart in their guidance, even if they are in different states. I feel like a lot of people on here are being misled to apply way too much product to their lawn.

Most guidance on here for new members with issues/questions is to get a soil test, but if the soil tests have so much variability in their recommendations, is it really even helpful to get a test? I do think knowing the PH of the soil is important, but not as much as core aerating overcompacted soil is, which most people could do if they didn't blow so much money on fertilizer. I dunno, just my opinion only based on what little research I have done. Just wanted to see if anyone else bases their nutrient applications on actual studies performed or just go by what the lab says to do. Anyone else have any info that disputes the thresholds for P and K mentioned above?

Thanks,
JTM


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## Ridgerunner (May 16, 2017)

Could you link to that study?
Could you link to some soil test examples that you believe show this variability that you are seeing?


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## Virginiagal (Apr 24, 2017)

The short answer to your question about different ranges is that various labs use different extractants. So the same soil sample would show different ppm depending on the extractant used.

If you haven't read Ridgerunner's soil test thread, this will give you a lot of information on soil testing:
https://thelawnforum.com/viewtopic.php?t=1088

This thread will be of interest too:
https://thelawnforum.com/viewtopic.php?t=16135


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## jtmurphree (May 25, 2020)

https://www.google.com/url?sa=t&source=web&rct=j&url=https://aurora.auburn.edu/bitstream/handle/11200/2324/1565BULL.pdf%3Fsequence%3D1%26isAllowed%3Dy&ved=2ahUKEwjWhbzVxofrAhVChOAKHcZRDQ4QFjAAegQIAxAB&usg=AOvVaw1NQozDdkeNGuiG-l9jjtt3

Not sure if that link will work or not. It's a PDF doc.

If I would have found those threads or looked a little more for them, then I probably wouldn't have even asked the question. I have not had a chance to read more than a couple of paragraphs so far, but definitely will later tonight . Really assumed ppm was ppm no matter what the testing method is. If anything maybe this will send people to the guide provided by @Ridgerunner

Very cool. Thank you
JTM


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## Ridgerunner (May 16, 2017)

@jtmurphree Thanks for posting the link, it's an interesting read. 
As to your observation and question regarding "The conclusion is that K shows diffencies at levels less than 30 ppm and no significant improvements above 50 ppm. P shows diffencies less than 7 ppm and no real improvements above 15 ppm.": First keep in mind that although the study states they used the testing methods identified by N. R. Page (1965), we don't know what those were. We'll assume M1 testing was used and secondly, there is very little documentation as to how they came to that conclusion which would have helped us understand exactly what those numbers represent. However, I'm reasonably confident that they are referring to "snap shot" measurement of soil P and K levels. This is similar to results reported by PACE turf in their MLSN (Minimum Levels for Sustainable Nutrients) recommendations. For reference PACE turf developed their MLSN using sand based soils with CEC under 6 using M3 (not M1) testing processes which will give similar values for K, but will result in higher values for P. MLSN minimums for K are 37 ppm via M3 (Auburn: 30 ppm K via M1) and 21 ppm for P (interestingly PACE has re-evalulated and found min. levels t much lower--@g-man can you help with PACES newest min P levels?) via M3 (Auburn: 7 ppm P via M1). In both cases, that means if a soil test today is at or above those min. levels, turf shouldn't suffer in quality, but as the turf uses nutrients and those levels drop below those thresholds, the turf can/will suffer tomorrow. On the maximum end (Auburn: 50 ppm for K and 15 ppm for P--PACE hasn't done a study to my knowledge), that makes sense. Common sense tells us that there must be a maximum level of any nutrient at which point the the plant can't utilize the nutrient (not to mention issues of detrimental interactions and toxicity). Again, keeping in mind that that 50 ppm and 15 ppm is most likely a "snap shot" measurement. Those might be the values found today, but as the plant takes up and uses nutrients, those values are going to drop day after day. Most of the people on TLF strive for maximum potential (not to be judgemental, to each his own)-that extra 1%- based on appearance and quality of growth. That means maintaining levels that don't fall below maximum potential (e.g. per Auburn 15 P and 50 K), but don't reach detrimental. So, per the Auburn study, what's detrimental? I don't see a determination where they report any detrimental effects of K at high levels for Bermuda or Zoysia, nor do they report any detrimental affects for high levels of P for Bermuda, but they do report yellowing of Zoysia at the max 88lb/a levels. I'm not surprised, 88lbs/ac is 44ppm. That's a LOT, year after year when the estimated amount of P used per pound of N/ac is only approximately 2.9 ppm. I'd classify yellowing as detrimental. Consequently, we can conclude that for Zoysia grass grown on those soils, there is a detrimental level somewhere between 44 and 88 lbs/a/yr.
If my assessment of the the Auburn study is valid, could you clarify how the Auburn study would influence your approach to fertilizing vs the methods you see commonly suggested? Is your position that P and K levels should be maintained at no more than 15 and 60 ppm, and no less than 7 and 30 ppm respectively? How would you insure those levels? How often would you want to test to make sure those levels are are not exceeded?
MySoil/YardMastery and Soil Savvy testing have not documented that their tests are correlated or calibrated nor how accurate and repeatable their methods are. I'm not a "trust me, the check is in the mail" person. Stick with the established testing methods performed by reputable labs.


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## Ridgerunner (May 16, 2017)

> It seems like many of the soil tests are suggesting to put down way more fertilizer than what is really needed.


I assume that "way more fertilizer than what is really needed" meant more than the Auburn study of 15 ppm of P and 50 ppm of K.
As mentioned in my post above, we are assuming those levels will result in maximum turf growth potential (MGP), at least for Zoysia and Bermuda and to maintain MGP, soil P and K content need to be maintained at the 15 and 50 ppm levels per the Auburn study and not fall below. As asked above, how is that done, do you test daily and add the P and K lost to plant uptake and lost by other processes to insure levels stay at 15 and 50ppm?
Rather than try to maintain a specific, in time, defined target (e.g. 15 and 50) the practice of making fertilizer recommendations is to base it on an annual target. The annual targets have been found to supply a crop with all the nutrients needed for a whole year in order to result in approximately 90% of MPG. Due to the recommended levels being for an entire growing season, they will be large. If you were to start out with a soil test level at 50 ppm of K for MGP, you tested daily and added K daily to maintain 50 ppm, I'd venture at the end of the season the total amount of K you added for the year would be very close to the amount of K fertilizer recommended using the traditional annual methods for determining K rates on the annual basis.


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