'Scotts winter guard' is pretty quick.......anything quicker- wouldn't be a good idea this late in the season.

Milorganite?

It's my understanding that the last fertilization should be applied while active growth has slowed, but before the ground freezes, and be fast release urea type. 10-10-10 would be fine too.

And only apply it after growth has completely absolutely stopped.

I wonder if Ammonium Sulfate may be a better choice as a winterizer in cold weather than urea.

@j4c11: I haven't thought much about urea and cold temperatures, other than it is all nitrogen and that's what we want for over wintering. I did a little googling and I'm not sure there is a benefit to using ammonium sulfate over urea in cold locations, which of course is the only place you are going to be "winterizing." Either way you go, biological activity is required to convert the nitrogen into a nitrate for uptake by the grass. The only difference between the two is that urea requires an additional step of converting the urea into ammonia first. Since the ground is not yet frozen and the urea is watered in, it seems these reactions happen at an acceptable pace. But like I say, you need bio activity either way you go. Urea is probably cheaper and easier to find. Other than that, I haven't found any info on the activity of the bacteria of interest at different temperatures. Maybe it isn't supposed to be all converted into nitrate too quickly, so that it slowly releases over the winter even though it it technically fast release urea. That's just conjecture, though. I'm sure the people at ATY would have more answers.

Plants, turf grasses, use nitrogen primarily in two forms: ammonium and nitrate. Although studies have shown that grasses can absorb some small amounts of urea and use it, the vast amount of urea gets transformed by the enzyme urease which converts it to ammonia and in the presence of water, the ammonia is quickly hydrolyzed into ammonium (a form that the plant can use) and if temperatures are warm enough, microbiology will convert the ammonium into nitrate (another form that plants can use). Unfortunately urease is also temperature dependent, so other than the small amount of urea that the plant can directly take up and use, urea fertilizer will just sit in the soil if temperatures are too cool. Using an ammonium fertilizer like ammonium sulfate will supply an immediate temperature independent plant useful form of N: ammonium. Another temperature independent fast release fertilizer would be ammonium nitrate or any other nitrate fertilizer like calcium nitrate, potassium nitrate etc.

I'm sure the people at ATY would have more answers.

All the cool kids hang out at TLF now :-)

Went to Site One and got a bag of Lesco 30-0-5

I did a little research into urea conversion to ammonia based on j4 and ridge's comments above. It's interesting and more complicated than you'd think. It seems there is a lot of variation in studies that look at how temperature affects the conversion by urease in soil.

Of several studies I found, they maddeningly reserve the actual numbers to the full study that you have to pay for. However, I did find a thesis paper that I skimmed closely enough to find interesting information. The question is how cold does the soil have to get in order to effectively stop the reaction? The short answer is that the reaction continues unless the soil is frozen solid.

The summary paragraph says that in warm soil (72F) the reaction is about 3 times faster than in soil near the freezing point (37F). So this puts a number on the speculation about the conversion rate from urea to ammonia. Based on this information, I'm going to say it is a non issue. Urea conversion in warm soil is very fast, so a third the speed is just kind of fast, and this assumes the coldest possible conditions. Let's ballpark that urea conversion around Thanksgiving will be about half that compared to July 4th.

This is really just a cursory view, and maybe I'm missing other important information, but it does seem to me that the urea to ammonium step is not a limiting factor in cold weather. This kind of jives with my assumption that if urea really wasn't optimal, all these lawn experts online wouldn't be recommending it. (I know, that isn't usually smart thinking). Also, there are issues with ammonium sulfate being a salt, lowering pH and so on.

What do you think? Oh, here's the link to that one study:

https://archive.org/stream/effectsofsoiltem00moyo/effectsofsoiltem00moyo_djvu.txt

Nice job of research danielj. My understanding was that urease basically stopped functioning somewhere above freezing (about the same time as soil microbes @<45F?). I stand corrected. Maybe urea is a temperature independent source of N. However, much of what I have read over the years says not; that a fast release (AS or a nitrate) should be used for winterizing. (btw, I do use urea as a winterizer.)

The study you cited only measured the loss of urea in the soil not the amount of ammonium created. The issue I see remaining is whether or not ammonium is being produced in useful quantities.

My understanding of the reaction (pardon my attempt at writing the formula):

2(NH2)CO (urea) +H2O (water) +urease (catalyst) = CO2 (carbon dioxide) + 2(NH3) (ammonia).

NH3 + H3O (Hydronium, a proton donator?) = NH4 (ammonium)

Correct? or have I oversimplified?

Somewhere, I recall that cold temperatures reduce atom activity. If so, is there a low (soil) temperature at which hydronium (or another donator) would be less likely to give up the extra hydrogen atom needed to create ammonium from the ammonia?

Secondly, as much of the free H+ atoms in soil are produced during the decay of OM by microbes, is it possible when soil temperatures in the Fall get too low for the microbes that there may not be enough H+ in the soil for effective ammonium creation?

Maybe the recommendation to use ammonium or nitrate fertilizers as a winterizer is just based on the slowed availability of urea and the short time between the winterizer application and ground freeze. Just a mater of efficiency. 2/3 less production is significant.

EDIT: You need to be a chemist to appreciate the complexity. I'm not a chemist and I'm overwhelmed. It appears it isn't so simple. There are a lot of other reactions that occur:

2(NH2)CO (urea) +H2O (water) +urease (catalyst) = CO2 (carbon dioxide) + 2(NH3) (ammonia).

BUT THEN:

2(NH3) + CO2 = NH4(H2NCO2) (ammonium carbamate)

THEN apparently:

NH4(H2NCO2) +(spontaneous hydrolysis) 2(H2O) = NH4 + NH3 + H2CO3 (carbonic acid) +OH

I think. (one description I read stated that bi-caronate somehow fits in there too)

Bottom line: If you use ammonium sulfate or a nitrate fertilizer, you'll get more usable N to the plant faster than if you use urea. Seeing as studies show that plants use increasing less amounts of N as temperatures drop, it probably wont make a difference either way as urea even as a temperature dependent source of usable N, will likely still supply more than what the plant can use in late Fall (Novemberish).

@ ridge: I just now had time to reply... busy week. Let me say a few things to think about. First, with the temperatures we are talking about, like 70 or 80F vs 35 or 40F, there is little difference in the "activity" of the various species in solution (ammonia, hydrogen ion, sulfate, hydroxide, etc..) The only time atoms are at zero activity is at absolute zero, or -460F! ;o) So strictly from the view where biological organisms are not involved in the chemistry, temps are pretty irrelevant, meaning if ammonia is going to pick up a hydrogen ion it doesn't matter what the temperature is for our purposes. It is a spontaneous reaction.

A little on ammonia chemistry. It is not necessary for there to be proton (H+) donors such as decaying matter to allow ammonia NH3 to turn into NH4+. The H+ can come from pure water in a hydrolysis reaction. There is a small amount of H+ and OH- in water all the time, and ammonia soaks up some of the H+. This leaves an excess of OH- ions, making the solution basic with a high pH. I think it is around 11 in pure water.

Ammonia in the NH3 form is a weak base, meaning that it will become NH4+ and generate those extra OH's in solution, but it doesn't really want to do that. It would prefer to remain as NH3. People might be surprised to know that household ammonia is mostly just that, NH3 in solution. Only about 1% of the ammonia is in the NH4+ and OH- (ammonium hydroxide) state.

So what does this say? If we take a salt like ammonium sulfate and dissolve it in water, it will drop the pH down and become mildly acidic. The sulfate has nothing to do with it. It becomes acidic because, again, it would rather exist as NH3 when in solution. So when the NH4+ gets wet it can then shed most of those H+ and get back to NH3. The extent to which this happens depends on the pH of the solution to begin with. The more alkaline (pH above 7), the more the ammonium ion converts to ammonia, NH3. Conversely, the more acidic, the more NH4+ will remain. IN GENERAL we can say the equilibrium heavily favors NH3 in solution. This means that most of that ammonium you've added from the ammonium sulfate will be converted back to ammonia!

So let's consider the two situations and just assume neutral pH in the soil:

First is urea. Once we have NH3 in solution from the urease reaction, about 1% of it will want to turn into NH4+ and it will do this in pure water without the need for an H+ source other than the water itself. As the ammonium is consumed either by the plant or is converted to nitrites and nitrates by bacteria, more ammonium ions will be created immediately to replace those that were "lost."

Second is ammonium sulfate. When you add it to the soil, it will quickly want to shed most of the H+ ions to become NH3. As above, any remaining NH4+ will be soaked up one way or another, and they will be replaced when NH3 takes back a proton. In reality, the extent to which this all happens depends on the pH of the soil. Again, if it is alkaline, the reaction will go to completion and virtually all the NH4+ will now be NH3.

Of course a lot of other reactions are going on, as you say. I guess the take away from this post is that things change as soon as you drop that ammonium sulfate on the ground. So you can't make an assumption that with urea you only have NH3 and with the salt (ammonium sulfate) you only have NH4+, so you are ahead of the game if you use the salt.

Having said all that, I agree that in general you'll have more ammonium available right away with adding the salt vs adding urea, but I don't really know if it is that important. There are pluses and minuses as well, such as soil acidification from adding the salt.

As an aside, I always thought NH4+ was bad for plants and living things, and the natural process of converting it to nitrates for plant use was preferred. I guess in small amounts the NH4+ can be used directly by the grass, hence inorganic fertilizers.

This was a little rambling but I was just trying to throw out some chemistry facts that would help you make comparisons between urea and ammonium sulfate. It is complicated.

That is some pretty informative rambling.

First some house keeping. Some of this I can't give a citation and I and you must rely on my memory. Make of it what you may.

Regarding plant preference/use of nitrate vs ammonium.

1. Plants can use both and do. I've read studies where plants prefer ammonium early in the year and then nitrate. As I recall, these studies where based on crops which are planted annually, So the supposition is that what is really happening is that young plants prefer ammonium and as plants mature, they prefer nitrate.

2. There has been some research with established turf grasses where turf will perform poorly as total N soil levels exceed 20-25 ppm, or where soil ammonium levels exceed 7 ppm or when nitrate: ammonium levels fall below 3:1. https://www.paceturf.org/member/Documents/0109.pdf

3. As to why plants prefer ammonium: Sugars are produced in the leaf. Due to the availability of oxygen, when a plant uses ammonium, N reactions take place in the root and sugar must be transported down from the leaf to the root. When nitrate is used, it is transported up to the leaf for the N reaction. Maybe moving sugar expends more energy than moving nitrate. but temperature seems to dictates where the N reactions most efficiently take place. http://www.smart-fertilizer.com/articles/ammonium-nitrate-ratio

Your ramble has really got me wondering, If NH3 is the preferred state (and ammonia is toxic to plants) there has to be something else intervening to create NH4 that we haven't addressed (otherwise how does Bermuda survive the ammonia and get N in an pH 8 soil?). Something other than spontaneous hydrolysis must be creating plant available N (ammonium or nitrate) from ammonia.

I may (or not) have found a possibility: enzyme mono-oxygenase. If the urea enzyme can convert over a pound of urea/M daily, maybe mono-oxygenase can convert enough ammonia into ammonium to counter-act ammonium's natural tendency to return to NH3? I have a feeling that it can convert ammonia to ammonium is also a bit over-simplified. The name oxygenase bothers me, but a number of sites make conclusory statements that it converts ammonia to ammonium. Not much explanation though. Maybe it converts NH3 into some other stable molecule eventually useful as N to the plant and not ammonium?

Edit: More research says NOPE, oxygenase appears to create nitrite, another plant toxic form of N. Maybe NH4's ability to be held on cation sites helps create a less toxic soil solution NH3/NH4 balance and NH4 availability than science would infer.

Maybe it's that your explanation is true in saturated soil solutions, but NH4 is more prevalent in soil at a "normal" moisture content? If that makes sense.

Well as you've demonstrated it is really complicated. I just assumed plants only used up nitrates but your link clearly shows how important ammonium can be as well. All I really know about it is from water chemistry for fish tanks. Fish create urea which is converted to ammonia by urease and some of this turns into ammonium. Another bacteria converts the ammonium to a nitrite, and this is quickly further oxidized by yet another bacteria into a nitrate, which the plants then remove from the water. The ammonia is eventually depleted because the reaction is being driven forward by the bacteria.

To your other point, I think as long as the ammonium fertilizer is actually dissolved and is in solution, it will follow the equilibrium balance. I suppose the ammonium could be shielded from conversion to ammonia if it is attached to a cation site.

I'm sure there are people with some of the answers to this stuff, but I just don't have the time to research it all. Interesting though.

I guess Joe has moved on with his life while we struggle with the minutiae. :o)

Lol. I'm kinda tiring of it too. :)

Went to Site One and got a bag of Lesco 30-0-5

Joe, I thought you did that already. :o)

You're a hoot danielj.

I think Joe was subtly reminding us that the discussion has been moot for quite a while as he already made his decision.

So I guess my hoot is moot. Back to the topic, this is the first year I'm using urea for winterizing. I have been using it this fall per Andy's aggressive weekly program. I think there is still an issue of the urea volatilizing if not watered in, so I'm thinking to see if I can manage to apply it just before a rain. I'm going mow the lawn Sunday, but I think it is still growing a bit. Probably apply urea over Thanksgiving weekend depending on temps.

I read the bag today (applying tomorrow) and there's mostly Urea in it.....FWIW

I read the bag today (applying tomorrow) and there's mostly Urea in it.....FWIW

Then I'd say your set.

danielJ, If you're going to get hit by this big rain front coming through I'd time it with that. Even in NJ, I'd say your grass has slowed down enough even if it hasn't stopped. I (Ohio about even w/Pitt) put mine down earlier this week before a day of rain.

Thanks for the heads up ridge. I'll take a look at the weather report.

Fertilizer applied.

Rain coming today.

See you all in the spring.

Happy holidays!

Likewise. I wasn't so lucky. Working today so didn't have time for urea application. Hopefully next weekend.

Same to you, Joe.