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Row Crop Fertility Update – Harris

Glen Harris here with some requested comments on how getting a “pile” of rain (I have another word for it) can affect fertilizer plans for 2021 Georgia cotton…

The good news… I don’t think there is reason to panic…for a number of reasons:

  1. Even if you already put out your preplant N-P-K (which is probably most growers)…phosphorous is basically immobile and even though K is mobile it is not as mobile as N.
  2. We have time to correct this issue without suffering significant yield reductions.
  3. So the concern is nitrogen…but not all of the N was in the leachable “nitrate form”…(ammonium and urea N is not susceptible to leaching)
  4. And what we got was not what I would call a “leaching rain” i.e.  lots of rain over a long period with a chance to soak in and move down through the soil profile .  We got what one of my old colleagues would call a “frog strangler” or what I call a “runoff” rain, i.e. where a lot left the field going sideways instead of down.
  5. And the official recommendation is to only put out ¼ to 1/3 of your total N at planting – this is designed to get you to sidedressing time and one of the reasons we don’t like to put a larger portion of N out at planting (too much vegetative growth early is another). I already got a call where a grower already put out 2/3 of his total N. News flash – some of that will need to be replaced at some point.
  6. But what about potassium? It’s half way to the gulf of Mexico too right? I don’t think so. Like I said, it’s not as mobile as N and may have gotten pushed down into the soil profile. But once the cotton tap root gets down deeper (and that probably happens a lot faster than you think) it should still be there.

So…what should you do?

Let’s agree we can take phosphorous off the table and potassium to some extent. I am actually more worried that the “packing rain” we got this weekend will do more to compact soil and limit root growth — and therefore potassium uptake –than I am worried about what K we lost.  I wouldn’t look to replace potassium now, but if you already planned to put some more K out I would do it no later than sidedress time (first square to first bloom). Don’t forget we can also foliar feed K once we start blooming if need be.

I also would not be in a huge hurry to replace any nitrogen you think you may have lost,  but would simply evaluate what the cotton looks like at first square and be prepared to go with an early N sidedress. Tissue sampling would be amazingly helpful in this regard also. Take a tissue sample sometime right before squaring or at squaring (should be around 35-45 days after planting) to check on N levels. The beauty of this is that we can also check on P and K and everything else – including sulfur !

Sulfur is also highly mobile in soil, like N. so I would recommend including sulfur with your N sidedress. This is pretty easy to do with liquids (like 28-0-0-5(S) and 18-0-0-3(S) and by adding to ammonium sulfate to urea or ammonium sulfate solids. There is also Amidas, urea and ammonium sulfate homogenized into a 40-0-0-5.5(S) granular.

Back to N, if you haven’t planted yet and have the capabilities of putting starter fertilizer in a “2 x 2” (2 inches to the side and 2 inches below the seed), you might consider putting 10 gallons of 10-34-0 in a 2 x 2. Or likely a more economical treatment would be 3-5 gallons of 28-0-0-5(S) in a 2 x 2.

Bottom line…it may not be economical to replace 30 lb N/a right now, especially on large acreage, so be prepared to sidedress N at on the early side of the window (first square rather than first bloom) and maybe bump up the rate 10-25 % then.

And tissue sample right before sidedressing if possible.

Hope this helps.

As always, contact your local county agent if you have additional questions.

They know how to get a hold of Bob and I !

Oh, and what about corn ?

If you haven’t finished putting N and S out yet then I would consider replacing about 25 % of your total N rate (include some S too , like with cotton).

If you already had all your N and S out on corn, maybe wait a week and take a tissue sample and see where you stand and if you need to figure out a way to put some more out.

Glen Harris – UGA-Tifton, Extension Agronomist for Soils & Fertilizer