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?
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 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 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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