# Soil Test - Phosphorus Correction Question



## TurtleManRoshi (Feb 17, 2020)

Soil Test from Texas A&M shows I need 2.2 lbs P / 1000 sqft. Was pretty late to the soil test game this season before grabbing a small bag of fert. I used 24-0-6 Flagship 3lbs/1000 for my April app and got a pretty good growth spurt and color response. I am wanting to push monthly nitrogen apps this season at 0.75 N / 1000.

I have seen the Soil Remediation Guideline post and some phosphorus recommendations.

Does the 2.2 recommended P from my soil test need to be applied all in 1 season? Is there any risk of using a high P starter fert for my May, June, July monthly fert apps as South Texas temps get into the 90s? Since my lawn responded well to Flagship, I am looking to get something similar to Flagship's Nitrogen contents which is 25% SRN.

Thanks in advanced for any recommendations and discussion.

Bermuda Hybrid / 3.25 ksqft / HOC 2" / South East Texas

Soil test samples collected before my first Nitrogen Application.


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## MacLawn (Oct 26, 2021)

Same here , watching for details.


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## g-man (Jun 15, 2017)

> *General​*For most cases, don't try to remediate the soil during the winter if your soil freezes. Products applied to frozen soils might not travel deeper into the soil and could get washed away. *Do applications when the grass is actively growing*. Also avoid applications in the peak of summer under drought conditions since it might lead to more plant stress. This is a marathon and unless you are deficient (rare) most of the amendment we do is to get the soil to an ideal condition.
> 
> The soil test results from the lab might have some recommendations in the bottom. Those are normally for amount for the *entire year*. The standard practice is to limit the application of nutrients to be no more than 1lb/ksqft/month.
> 
> Lastly, the recommendations are for soil applications and not for foliar. Do not use these rates for foliar applications unless you *immediately* run the irrigation to wash the product off the leaves.


- Soil Remediation Guide


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## TurtleManRoshi (Feb 17, 2020)

g-man said:


> > *General​*For most cases, don't try to remediate the soil during the winter if your soil freezes. Products applied to frozen soils might not travel deeper into the soil and could get washed away. *Do applications when the grass is actively growing*. Also avoid applications in the peak of summer under drought conditions since it might lead to more plant stress. This is a marathon and unless you are deficient (rare) most of the amendment we do is to get the soil to an ideal condition.
> >
> > The soil test results from the lab might have some recommendations in the bottom. Those are normally for amount for the *entire year*. The standard practice is to limit the application of nutrients to be no more than 1lb/ksqft/month.
> >
> ...


Thanks.

I think I'll probably take the conservative approach this year and get a Nitrogen fertilizer that has a bit of phosphorus. Chip away at it monthly and get a soil test sometime early January and adjust accordingly. Maybe something in the 0.25 - 0.5 lbs P / 1000 sqft / month range.


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## Virginiagal (Apr 24, 2017)

Lesco has a starter fertilizer 18-24-12 with 25% slow release nitrogen which sounds like a similar product. 4 lb per 1000 sq ft would give you 0.72 lb of N and 0.96 lb of P and 0.36 of K. You could do two applications and it would get you fairly close to the P recommendation while giving you your preferred nitrogen rate. Other brands of starter are probably similar. Milorganite has N and P and is a slow release too. Bermuda grows well in hot weather. It will need water, either from rain or irrigation. Here is a guide to taking care of a Bermuda:
https://thelawnforum.com/viewtopic.php?t=1651

@MacLawn please note that OP is in Texas with Bermuda which is fertilized during the summer. Cool season grass should get the bulk of fertilizer in the fall with a smaller amount in the spring.


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## MacLawn (Oct 26, 2021)

Virginiagal said:


> Lesco has a starter fertilizer 18-24-12 with 25% slow release nitrogen which sounds like a similar product. 4 lb per 1000 sq ft would give you 0.72 lb of N and 0.96 lb of P and 0.36 of K. You could do two applications and it would get you fairly close to the P recommendation while giving you your preferred nitrogen rate. Other brands of starter are probably similar. Milorganite has N and P and is a slow release too. Bermuda grows well in hot weather. It will need water, either from rain or irrigation. Here is a guide to taking care of a Bermuda:
> https://thelawnforum.com/viewtopic.php?t=1651
> 
> @MacLawn please note that OP is in Texas with Bermuda which is fertilized during the summer. Cool season grass should get the bulk of fertilizer in the fall with a smaller amount in the spring.


Thank you, still trying to wrap my head around application rates based on test results and percent on the bag. Getting easier every day thanks to the help here


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## TurtleManRoshi (Feb 17, 2020)

Virginiagal said:


> Lesco has a starter fertilizer 18-24-12 with 25% slow release nitrogen which sounds like a similar product. 4 lb per 1000 sq ft would give you 0.72 lb of N and 0.96 lb of P and 0.36 of K. You could do two applications and it would get you fairly close to the P recommendation while giving you your preferred nitrogen rate. Other brands of starter are probably similar. Milorganite has N and P and is a slow release too. Bermuda grows well in hot weather. It will need water, either from rain or irrigation. Here is a guide to taking care of a Bermuda:
> https://thelawnforum.com/viewtopic.php?t=1651
> 
> @MacLawn please note that OP is in Texas with Bermuda which is fertilized during the summer. Cool season grass should get the bulk of fertilizer in the fall with a smaller amount in the spring.


Thanks for this, it is really helpful!


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## ABC123 (Jul 14, 2017)

Anything above 10ppm for P is a sustainable level for turf. I would try to raise it a little though, but you dont have to go crazy.


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## Ridgerunner (May 16, 2017)

TAMU uses M3 for testing which can overestimate available P levels in higher pH soils. The higher the pH, the more the reported levels will be inflated. @TurtleManRoshi with a pH of 7.9, P availability can be improved by adding smaller amounts of P more often in a 1N:0.23P ratio.
@MacLawn Mass. soil pH is usually acidic. If you pH is between 5.5 and 7, you can adjust your P applications at greater increments at up to 1lb of P per month until desired levels are achieved.


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## Virginiagal (Apr 24, 2017)

My goodness, @Ridgerunner, you're complicating things. You're saying he may actually need more phosphorus than the report recommends, right? But only in small amounts at a time, because in a high pH soil, phosphorus is quickly going to get bound up and unavailable to the grass.

If he uses starter fertilizer, at 18%N and 24%P, one pound of it per 1000 sq ft will give him 0.18 lb N and 0.23P. He could get the other N he wants (he wants 0.75 lb a month) by using a nitrogen only fertilizer. Let's say he does one lb of starter every two weeks. That would be 0.36 lb N and 0.46 lb P in a month, leaving 0.39 lb N to come from a nitrogen only source. He could do half of that extra nitrogen in each application or all of it in one (he's using slow release products). . So one lb of starter every two weeks plus some other nitrogen. In four months (May-August) he would get 1.84 lb of P. In five months (May-September) he would get 2.3 lb of P.

Question: how hard is it to spread 1 lb of a product over 1000 sq ft? Do you have some suggestions on doing spoon feeding?


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## Ridgerunner (May 16, 2017)

https://www.siteone.com/en/agronomic-maintenance-fertility-nutrition/c/sh1315110?q=%3Arelevance%3Asocategory%3ASH1315110101%3AMaintenance_npkratio%3A13-3-13&searchtype=product&viewtype=All&NPK+Analysis=on#


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## Virginiagal (Apr 24, 2017)

Another idea: 12.5 lb per 1000 sq ft of Milorganite (6-4-0) will provide 0.75 lb of N and and 0.5 lb of P per 1000 sq ft. You could do that once a month May-September and get a slowly available supply of N and P with a total of 3.75 lb of N and 2.5 lb of P per 1000 sq ft. Slowly available is similar to spoonfeeding.

Ridgerunner is right that phosphorus quickly gets tied up and unavailable in high pH soil. I began wondering what farmers do. They can't go around spoon feeding all the time. I found this article from Texas A&M which is worth reading:
https://agfax.com/2021/02/01/texas-key-considerations-about-phosphate-applications/

Also, read the second paragraph under Application Methods here:
https://passel2.unl.edu/view/lesson/0718261a1c9d/7
Leaving clippings on the lawn could be helpful in improving phosphorus delivery to the roots.


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## Virginiagal (Apr 24, 2017)

@Ridgerunner, did you mean overestimate or underestimate? You say overestimate. The 13-3-13 you're suggesting would indicate you think he needs less phosphorus than is being recommended on the test, meaning the test underestimated the P level and he needs less than is being recommended. So I'm looking around for whether Mehlich 3 overestimates or underestimates in high pH soil. I found this:
"Iowa research shows that the M3 P results are similar to the Bray in acid and neutral soils, but are much better in many high-pH soils. Similar conclusions apply to many soils of nearby states, although the M3 does underestimate P in many soils of western states that have higher (and perhaps a different type of) calcium carbonate."
https://www.agronext.iastate.edu/soilfertility/info/mnconf11_22_99.pdf

I'm thinking about broadcasted P and how it gets tied up in high pH soil. It gets tied up easily when it's incorporated. But lying on top of the ground, there is less contact with the soil. So maybe it stays available a little longer. The roots close to the surface would get it, and roots further down wouldn't. If it's on top of the ground, would it matter it about timing? Maybe spoonfeeding isn't all that important?


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## TurtleManRoshi (Feb 17, 2020)

This all great stuff.

I think I'm going to go with a starter fert in two weeks. Something like 18-24-12, then sub in a fertilizer with less P for the remainder of my season apps to be more on the conservative side.

I'll re-test soil in January before my first prodiamine app.

I've used Milo before and didn't have great results color/growth wise. Most likely because my lawn was on top of New Construction Soil which is probably when I should have applied a starter fert to jump start things.

I do mulch my grass on every mow except if I am scalping.


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## Ridgerunner (May 16, 2017)

@Virginiagal Overestimate. The stronger acid P extractants (e.g. Mehlich and Bray P2) will release P that is non-labile (tied up with calcium in unions that would not be released under normal soil conditions during the growing season)in high pH soils. the same non labile release of P bonded with Fe and Al occurs with using Olsen in low pH soils. There is debate over the significance of this trait with observations of M3 P extraction in high pH soils appearing to be linear (although research I've seen on this infers that M3 has been buffered to neutral) and with Olsen testing used for lower pH soils in Australia and NZ.
There is also the discrepancy in the amount of measurement dependent on the instrumentation being used. ICP spectrometers (popularly used by many labs as it can measure multiple nutrient elements at once) report significantly higher values than is measured by colormetric. 
The issue is calibration. Is the P measurement calibrated to plant response for the pH range? That a reported range of values is sufficient or optimal for whatever crop (tissue testing) is being grown.
IMO probably with the exception of FeAlO strip testing, none of the current testing extractants are without some margin of error but Bray and M3 have been extensively calibrated for P in acidic soils and Olsen in alkaline soils with reasonably reliable recommendations and results. Until there is consensus, I think it's best to dance with the one that brung you for now. 
You make some good observations regarding fertilization (your one suggestion might be similar to banding of P in results without spoon feeding). My suggestion of the 13-3-13 every 2 weeks supplies .25 lbs of continuous P application for every pound of N (approximately the N rate of plant usage) and by applying N every two weeks does the same thing as slow release. 6 of one, half a dozen of the other.
There is a lot I don't know. I do my best.


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## Virginiagal (Apr 24, 2017)

@Ridgerunner I see what you're saying, that Mehlich is reporting more P than an Olsen would report. So the Olsen number would be even lower. So there would be an increased need for P, not a decreased. That's what I found confusing. Spoonfeeding 13-3-13 would give a continuous supply of P and would work if one doesn't mind spoonfeeding. It's also giving unnecessary K. Did you read the Texas article? It's like they were saying, yeah, yeah, we know Mehlich isn't ideal but that's what we're using, like it or not. I am wondering if they have worked that Mehlich issue (too much non available P being extracted) into their recommendations, doing some sort of conversion from their many years of research using Olsen. Phosphorus is such a problem. Use a little or use a lot, it's going to get bound up in high pH soil.


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## Ridgerunner (May 16, 2017)

Correlation and calibration. Always.
@Virginiagal Yes I read the article. I have no idea what TAMU is doing. They have a really unique perspective that just doesn't make a lot of sense to me. Example: the amount of N they recommend (always less than a pound) actually IS their annual N recommendation.


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## Virginiagal (Apr 24, 2017)

@Ridgerunner I think maybe they put the very lowest annual nitrogen up at the top. That applies to probably low maintenance centipede. You'll notice in the finer print below the chart that they they say to make additional applications of 1 lb per 1000 sq ft at various intervals if you have this kind of grass or that. I found this on their website about nitrogen for various turf grasses:
https://agrilifeextension.tamu.edu/library/landscaping/lawn-fertilization-for-texas-warm-season-grasses/
@TurtleManRoshi This article has a lot of useful information in it.


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## Ridgerunner (May 16, 2017)

That's not how I understood TAMU correspondence that I read from a couple of years ago. Maybe something was lost in translation. I still wonder why TAMU uses M3.


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## TurtleManRoshi (Feb 17, 2020)

@Ridgerunner , thanks for the link!

The Nitrogen on the top right is the initial application recommendation, then follow up with additional applications as needed. P, K, and the other nutrients are season total recommendations. See snapshot from the e-mail they send with results.

They also have a pretty nifty calendar as a guide too. https://cdn-ext.agnet.tamu.edu/wp-content/uploads/2018/10/bermuda-Image2.jpg


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