Urea as a source of nitrogen?
 

I know the general consensus is that urea is an inefficient form of nitrogen for orchids. But there have been times in my research I have seen urea championed and seen statements that it breaks down quicker then the common belief. Could someone with experience with this lend me their expertise.

Thanks,
Hylian

Urea is a great form of nitrogen. It is especially effective for foliar application but it works well in the root zone also. Urea had gotten a bad reputation many years ago possibly because of urea that was heavily contaminated with another chemical called biuret which does have some toxicity to plants. Now that old bad reputation has passed into folklore with people not really knowing why it is bad, just that that is what they heard, and as a consequence there are various erroneous stories floating around as to why it is supposed to be bad.

If you use urea, you want to be certain it is sourced from a large and reputable vendor who guarentees that it is low in biuret.
http://www.ipni.net/publication/bettercrops.nsf/0/A49BB197243BA024852579800081B53D/$FILE/Better%20Crops%202007-3%20p6.pdf

Thanks David

Some studies have shown that some orchids grow better when given nitrogen derived from nitrate rather than urea or ammonia. I've read that some experts believe that nitrogen from urea is unavailable to plants until it's broken down by bacteria. That process could probably happen pretty quickly in some organic mediums but it would be a problem for mounted orchids or orchids grown in inorganic mixes. On the other hand they grow beautiful orchids in Malaysia using organic fertilizers that are mostly urea. Personally, I believe I see better growth when I use fertilizers that get a significant amount of their nitrogen from nitrate.

For each one of these 3 nitrogen sources you can find multiple papers that determine that particular nitrogen source to be the best for orchids.

Here is one that finds urea to be the best:
http://www.crh.ulaval.ca/uploads/tx_..._ca_absorb.pdf

Here is one that finds ammonium to be the best:
http://elmu.umm.ac.id/file.php/1/jur...v2000/1236.pdf

If I look a bit more I will find one that determines nitrate to be the best.

Here is a fertilizer company that extolls the virtues of their nitrate only fertilizer which is carefully formulated to exclude any of that nasty old urea.
Growth Technology - Orchid Focus instructions

Sounds to me like the more we know, the more we we know that we really don't know!!...Jean

The problem in this respect is that different orchids will have different tolerances.

If you're talking about Phals, it is known that nitrogen derived from urea will work just fine.

Talk about something else, and it may be a whole different ballgame.

If you check some of the plant nutrition textbooks, you will find that foliar absorption is through the "plasmodesmata" regions surrounding stomata. The density of stomata varies pretty significantly among orchid genera.

Plasmodemata are pretty good for transfer if less-polar molecules like that of urea, but pretty poor with ionic species like nitrates and ammonium compounds.

It seems that the roots have just the opposite capability, favoring ionic species. Velamen, apparently, has sites that specifically trap the nutrient ions immedately upon exposure - seemingly an evolutionary adaptation allowing them to harvest the nutrients from the rain cascading through the forest canopy - it's going to be most concentrated at first, more dilute as the rain continues, so it makes sense to grab stuff as soon as it can.

I have to analyze a lot of peer reviewed research papers for my job; so I thought I would throw my two cents at this.

DavidCampen wrote,
Everyone should remember that while mulitple studies may be present that report conflicting conclusions, it is best to delve into exactly what the study was testing and from where the study was sourced. Any study that is paid for by a vendor or supplier of fertilizer should immediately receive high scrutiny. Also, some studies are highly targeted in that they are really attempting to answer one very specific question under a very specific set of circumstances, and a lot of times the conclusions are stated absent of those experimental parameters.

I found this study to be very convincing. The study did a lot to test and ensure that nitrogen was being absorbed by the roots and was actually testing for the nitrogen rather than measuring growth amounts. For instance, it protected the leaves from coming into contact with the substrates and fertilizers (because it is well known that urea can be absorbed by the leaves.) And it tested the substrate after the plants were extracted for evidence that the urea had not hydrolyzed (hydrolyzed urea forms ammonium, which is already known to be absorbed by roots). They found that the sterilization was effective and that the urea was unchanged in the substrate.

What I liked about this study was that it tested three forms of nitrogen uptake: nitrate, urea, and ammonium against each other. To test if the nitrogen got into the plant, they used special isotopes of nitrogen, which they later tested for from the leaves.

While more nitrogen was found in the leaves from a urea source, it was not statistically more than ammonium. However, the amount of nitrogen uptake from ammonium and urea was 3 to 4 times higher than from nitrate.

The orchids used were phalaenopsis and the substate was sterilized agar. The one caveat was that very small traces of nickel might need to be present so that the plant can incorporate nitrogen from urea into its cells. The amount necessary is likely present naturally in most fertilizers.

This study looked at Catasetum fumbriatum and found that both ammonium and glutamine were the best absorbed, but it did not even look at urea, it was only comparing it to nitrate. Finally, the study notes that C. fumbriatum grows best natuarlly in rotting tree nodes. Clearly this study is very different than the phalaenopsis study above and it is looking to answer a different question: can C. fumbriatum absorb nitrogen from organic glutamine? It did not look at the absorption mechanism. But the study does show that different orchids may have evolved for different habits.

This web page can no longer be found.

Troy, this is exactly the way any question should be researched. You did your own due diligence, looking at multiple viewpoints.

I personally believe nitrogen can be utilized from many sources. From an evolutionary standpoint, it would make little sense for orchids (which live in a nutrient-challenged environment in the trees) to turn down nitrate in favor of urea, or to put it another way, to not use urea directly, and instead preferring only ammonia, or only nitrate.

I have personally used urea-based fertilizers with good results.