Can you see the P?

The MLSN guideline for phosphorus (P) is 21 ppm. I usually recommend enough P fertilizer to keep soil P, as measured by the Mehlich 3 extractant, from dropping below 21 ppm.

But there’s another way to decide when to apply P. That’s by looking at the grass. Can you see the P in this image?

P caused by P fertilizer at OJ Noer Facility in Madison, Wisconsin The letter “P” on this creeping bentgrass green at the O.J. Noer Turfgrass Research and Education Facility in Madison, Wisconsin, was caused by an application of P; the darker turf around the letter “P” did not receive P.

One can wait to see a deficiency, which is evident as turf that gets a purple tint, or grows a bit less, or has a darker color. When those deficiency symptoms are observed, one can correct the deficiency by applying P.

For example, I’ve been growing some variegated Axonopus compressus at #ATC南店 and when I returned from a long trip on 5 December, I saw this grass had symptoms that looked like a P deficiency.

probable P deficiency on variegated tropical carpetgrass Probable P deficiency symptoms on variegated Axonopus compressus on 5 December 2019.

On 6 December, I added a bit of diammonium phosphate and urea to this pot of grass. Then I left on another trip. When I came back on 19 December, the purple color was gone, and the grass was back to normal—green leaves with light-colored stripes. That purpling went away after adding N and P. I’d guess this removal of purple color was caused by the P I added and not by the N.

variegated tropical carpetgrass growing normally Variegated Axonopus compressus on 19 December 2019 thirteen days after application of diammonium phosphate.

I wrote above that I usually recommend enough P to stay above 21 ppm. That’s pretty much guaranteed to prevent any visual symptoms of P deficiency. And I think that is the way most professional turfgrass managers would like to keep their turf—without having to worry about a visual deficiency showing up. There is a particular case when I do suggest waiting to see a P deficiency before applying P. That is when one is trying to prevent Poa annua invasion. More about that in a future, and probably longer, post. But I thought these photos were interesting and wanted to share them today.

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Micah Woods
Micah Woods

Scientist, author, consultant, and founder of the Asian Turfgrass Center