# Does phosphorous on a soil test really how what is available to plants?



## ktgrok (May 25, 2019)

Saw @thegrassfactor mention on his show that high phosphorous on a soil test might indicate a form that isn't really readily available, but he didn't go into detail. Research shows that it is tied up by either iron at low ph or calcium at high ph, but no real way to address that other than adjusting ph. Oh, and also increasing micorhyzae helps the plant break down the phosphorous. 
I'm Wondering if phosphorous can be absorbed via foliar spray the way potassium and iron can?
I noticed some significant improvement in a few patches of grass that were struggling and I'm wondering if it was partly due to phosphorous. Of course, I did a crap ton at once to it, so not sure what product, if anything, did the trick. 
Problem - thin, pale, weak grass, in a thinned out area that is where 1. seed washed out and 2. some spots have some shade part of the day
Applied-
molasses 
baby shampoo
miracle grow all purpose liquid(only fertilizer with phosphorous I've used since first few weeks)
humic
kelp
micorrhyzae

Now, I've applied the kelp and humic before, and plenty of nitrogen and recently several apps of potassium, so the new stuff was the baby shampoo, molasses, micorhyzae and the phosphorous in the miracle grow. Saw significantly healthier looking grass in 24 hours, and that improvement has lasted for 5 days now. 
Wondering if the grass needed some potassium and what is in my soil is too bound up. Ph is 6-6.5 depending on who tested it when, so ph shouldn't be a factor, but I have high iron and calcium in my soil, so maybe still binding it?

Wondering if my grass may actually need phosphorous, and if so, if applying it via a foliar spray would help. Going to shoot him an email and ask him to expand, but he's busy so not sure if/when he will get back to me and wanted some other people's thoughts. Lots of people in this forum with more knowledge of soil chemistry than I'll ever have


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## g-man (Jun 15, 2017)

Phosphorus deficiency is possible, but the ppm to be deficient is very low. Do you know your P ppm?


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## ktgrok (May 25, 2019)

My soil test (I know..soil savvy...won't use them again now that I know better) has it as high and Florida soil is typically high, so that seems right. I was just wondering if plants could still not get enough phosphorous despite it being high, because it isn't a form they can use. Hopefully that makes sense. Or maybe the phosphorous is fine and had nothing to do with anything going on in my lawn lol.


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## Virginiagal (Apr 24, 2017)

Don't know about foliar phosphorus but this article may be of interest:
https://www.lebanonturf.com/education-center/biological-plant-treatments/the-truth-about-phosphates-and-mycorrhizal-fungi
The mycorrhizae are fascinating. And such a great benefit. How did you go about doing your inoculation? If you want them to seek out phosphorus my understanding is too much easily available phosphorus inhibits them.


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## ktgrok (May 25, 2019)

Are used a product called root ruckus. It has the micorhyzae Plus kelpie plus humic acid plus compost. I will warn you, it does not smell good. In this particular instance I dumped half the dose into a watering can and out of water and then sprinkled it on the area. I was only treating a couple small areas.


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## Virginiagal (Apr 24, 2017)

I looked up the root ruckus and it recommends spraying foliage, which would make sense for the humic acid and kelp, but not for the mycorrhizae. They live in the soil, not in the plant. Ideally mycorrhizae would be worked into the soil to the root zone.

With your phosphorus testing so high and your pH ok at 6-6.5, I would not add any. Maybe your improvement came from the potassium you added recently.


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## ktgrok (May 25, 2019)

Are used enough that plenty got on the foliage and in the soil, LOL. It was a hefty dose.


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## Ridgerunner (May 16, 2017)

From a prior post:
Phosphorous is prone to bind with other elements in the soil, in high pH soils where Ca is in abundance, it mostly binds with Ca. In low pH soils where Iron and Aluminum are often in abundance, P binds with them. The purpose of soil testing is to attempt to measure how much of a nutrient will be available to the plant within a growing season period of time, That is, how much is in soil solution, the solution Pool (and immediately available to the plant-a very small amount) plus how much more is being held (bound "tied up" in chemical complexes or on exchange sites) within the soil where the bond "tie up" is weak enough to allow the nutrient to become available during that growth season time period, called the active pool. Some portion of nutrient is so tightly bound that it won't be available for years, called the fixed pool. The "trick" is to try and accurately measure the solution pool and the active pool and not the fixed pool. As @greendoc so well explained, the amount that is measured depends on the strength of the extract used, weak extracts report lower amounts, stronger extracts will report higher amounts. Debate rises whether weak extracts are measuring enough of the active pool and whether strong extracts measure too much of the fixed pool. Much depends on the type of soil. The usefulness of the results depend on how well they have been calibrated to plant response through multiple studies that have resulted in the development of "sufficiency ranges" specific to each test extract method. 
https://thelawnforum.com/viewtopic.php?f=24&t=7395&p=125862&hilit=phosphorous+available#p125862


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