OrchidGuide Digest Thursday, March 29 2001 Volume 03 : Number 076
Date: Wed, 28 Mar 2001 18:35:11 +0200
From: "S.M. Wellinga"
Subject: [OGD] [OGD V3 #74] Re: Urea
Dear all,
The claim that urea 'releases' its nitrogen too slowly to be useful as a
fertiliser for orchids, is indeed, as Marty Epstein has pointed out, a
myth, which however is widely spread and even used by manufacturers of
orchid fertilisers ("formulation 100% urea free!"). A bit of literature
study will reveal that many plant propagating media rely on urea as a
(cheap!) source of nitrogen. As most of you will know, flasking media
basically are nutrient ('dilute fertiliser') solutions which normally are
solidified by a gelling agent. If it indeed were true that nitrogen cannot
but become available after degradation of urea by soil-bound bacteria, I'd
be curious to know how orchid seedlings are able to grow (and grow well!)
on sterile media in which urea is their sole source of nitrogen...
Sincerely,
Simon M. Wellinga / SymPhyto - Laboratory for in vitro plant propagation
Heerenveen, The Netherlands
Worldwide delivery of orchid species flasks for hobbyists and professionals
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Date: Wed, 28 Mar 2001 14:35:40 -0500
From: Martin Epstein
Subject: [OGD] Urea vs Ammoniacal nitrogen
In OGD # 75, Brad writes:
>I see several people have said that the use of urea is fine for orchids.
>But from my understanding, this statement is poor advice. In order for
>urea to supply nitrogen to a plant, it must be broken down by a chemical
>process which involves bacteria in the soil. Since I grow all my orchids
>mounted, and have some bromeliads, the use of urea would be a bad choice
>for me, and a lot of others here. And I would ask the question, does the
>bacteria thrive in orchid mixes. Instead, use ammoniacal nitrogen, which
>is a ready source of nitrogen that can even be absorbed through the
>leaves.
>If someone would like to argue against this, I can offer you many
>reliable sources of this information. Do you suppose every book I've
>read is wrong? ...Brad
On this subject, possibly yes. Myths have a way of being repeated and
perpetuated until they are taken as truths.
There have been carfefully controlled experiments on the hydrolysis of urea
in a broad range of potting media. The results show that urea is rapidly
hydrolyzed in all of them. The rate of hydrolysis does vary with the
nature of the mix, and is initially faster in mix that had been previously
cropped than in fresh mix. It even occurs in rockwool. The nitrogenous
products are ammonia (or ammonium ion), nitrate, and a trace of nitrite.
Here are some literature references of actual studies:
Elliott, George C., 'Urea Hydrolysis in Potting Media', J. Am. Soc. Hort.
Sci. 111(6):862-866 (1986).
Wright, Robert D., 'Nitrogen Availability from Urea in a pine Bark Medium',
HortScience 22(1):70-72 (1987).
Klougart, A., 'Urea as a Nitrogen Source for Pot Plants", Acta Hort.
64:103-109 (1976).
Those who believe urea is not a good source of nitrogen should read these.
I don't know of any studies of hydrolysis rates of urea on mounted plants
(there may be some) but I don't see why it should differ materially from
plants potted in bark or any soiless medium. Urease bacteria (or their
spores) are ubiquitous.
There are many excellent growers who use urea based fertilizers, and many
who don't. Actually, I believe that in most cases urea based fertilizers
may have a slight advantage in that they provide a continuous supply of
ammoniacal and nitrate nitrogen over a period of a day or two, and the
ionic strength and therefor the osmotic potential is somewhat lower. The
only argument against the use of urea is that its hydrolysis rate may be
slower at low temperatures and that it may not be as effective in a cool
(45F) greenhouse. I don't know of any actual studies in this regard but
again there may be some.
Marty Epstein
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Date: Wed, 28 Mar 2001 18:49:41 EST
From: FJ Bergman
Subject: [OGD] Re: OrchidGuide Digest V3 #75 Urea vers ammonia nitrogen
Brad, no not every book your have read is wrong, just those that say urea is
no good. Fred Bergman.
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Date: Wed, 28 Mar 2001 22:31:55 -0800 (PST)
From: Roel Mendoza
Subject: [OGD] Urea
I don't think urea needs to be broken down by bacteria
before its nitrogen is made available to the plant. I
have read that plants have the enzyme urease which
does the breaking down. Also urea is said to be quite
readily absorbed by the leaves (foliar fertilization)
and also facilitates the foliar absorption of certain
trace minerals. Urea may have certain drawbacks but
probably, nitrogen unavailability is not one of them.
Roel Mendoza
Metro Manila, Philippines
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Date: Thu, 29 Mar 2001 22:03:04 EST
From: Mark Sullivan
Subject: [OGD] Myths, PPM, and the good the bad, and the ugly of Nitrogen Sources
In the definition of myth both the words fiction and half-truth are used.
Martin Epstein's list of myths is generating useful discussion, but one of
the problems with list is they can over simplify. Truth or debunked myths
without content can make for dubious information. Something Marty decries.
Example from Marty's list: * A total dissolved solids level of 1000 ppm
(substitute your own number) is too high for orchids. This is both true and
untrue. It depends on what the totally dissolved solid(s) is or are. Table
salt or a herbicide at 1000 ppm would be too high for orchids. As Sharon in
South Carolina already pointed out salt is not good for orchids. Flour or
sugar (I am in a baking mood) at 1000 ppm would not matter to orchids. I
choose the above ingredients to make the half-truth point simple. Using
solids that are used in fertilizer to illustrate the point would require
discussing at what various points fertilizer solids deterred verse benefited
an orchids growth or just the ability to just stay alive which would have
more variables to juggle.
Marty's myth list also includes: * Urea is bad for orchids and should not
be used as a fertilizer. * Urea releases its nitrogen too slowly to be a
useful fertilizer. Without content these are half-truths and it depends on
how you look at them. I don't think most books or fertilizer makers say that
urea is bad for orchids, or that it releases its nitrogen to slowly, or that
the nitrogen can't be used by orchids, but they say there are better sources
of nitrogen then urea for the purpose of growing orchids. In those
fertilizers that tout they are urea free they use nitrogen from one or more
of these sources: ammonium nitrate, ammonium phosphates (both make up
ammoniacal nitrogen in the guaranteed analysis though are not necessarily
found together in the same fertilizer); and nitrates like potassium nitrate
(common in fertilizer), and some other less common nitrates in fertilizers
like calcium nitrate, and nickel nitrate. Nitrates make up the nitrate
nitrogen in the guaranteed analysis. So why do they say these sources are
better then urea? I think the following but I can't speak for fertilizer
makers or the authors of books: Urea is CO(NH2)2. It is a more complex then
ammonium nitrate NH4NO2, the nitrogen sources I have chosen to compare it
too. Ammonium nitrate is also very common in urea free fertilizers. Urea as
already acknowledged takes some breaking down for the nitrogen to be absorbed
by plants while nitrogen from ammonium nitrate is readily available to
plants. Ammonium nitrate breaks down easily to N2O. Urea is a weak base, and
form salts with strong acids. Besides the difference in availability of
nitrogen to plants of these two chemicals, it is what is left over that may
matter to plants and growers - salt. Urea is a cheap source of nitrogen, so
to a farmer raising vegetables, cheaper is good. To hydroponic grower of
vegetables the cheapness of urea verses other sources of nitrogen is only
part of the consideration of using it, a lot depends on the type of
hydroponic system used and the sensitivity of the plants to salts. Many
orchids are salt sensitive. In using a urea fertilizer verses a urea-free
fertilizer on orchids may depend on whether the orchids are mounted, or in a
pot, terrestrial or not, how much fertilizer you go through (cost of
fertilizer), or how often you flush salts. Bottom line is that it depends on
many variables as to whether use urea. This does not make urea either good or
bad.
From what I know, bacteria are not the only way urea can be broken down.
Heat, ultraviolet light, and chemical reaction can break down urea. When you
use urea in a growing medium for a flask, you sterilize (heat). Urea as a
foliar feeder is exposed to ultraviolet, heat, and bacteria.
I am sure if the above is wrong or partly wrong someone will correct me. I
would love to hear it and the reason why, and any added information.
I would also love to hear anyone talk both pro and con about the "myth": *
Certain fertilizers containing methanol give remarkable results.
Orchid growing is not black and white, but a tapestry of colors.
Mark Sullivan
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Nurserymen and horticulturists are fully aware
that urea, like ammonium and nitrate, is a short term fertilizer, that is
it it has a rapid but only short term affect. To remedy this an insoluble
polymer was devised from the reaction of urea and formaldehyde which
hydrolyzes very slowly and is ideal as a lawn and garden fertilizer. This
material is called a ureaform resin. I speculate that someone did not
understand the difference and attributed the properties of ureaform resin
to urea. To my knowledge ureaform resins are not used in soluble orchid
fertilizers but they could possibly be a component of a time-release
fertilizer.
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From: "Alex"
Subject: Re: [OGD] Urea, Urea, Urea
I would think you are right Marty - I have just looked at my special Kew
Gardens orchid fertiliser label and it says the N is urea. BTW, I presume
that most nitrogen used by epiphytic orchids in the wild is rain water
diluted bird droppings. Does anyone know what kind of nitrogen this would
be?
Regards
Alex Scott
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Date: Mon, 2 Apr 2001 18:50:12 EDT
From: FJ Bergman
Subject: [OGD] Re: OrchidGuide Digest V3 #79 More on urea
I want to thank all of you who answered my inquiry about Bob Gordon's
statements on urea. My original purpose was a rebuttal to the gentlemen who
said he had never read a book that said urea was no good. It was an
interesting to see which OGD participants had his book, and who believed it.
I studies urea vers ammonium nitrate, using Phals, several years ago and
presented the findings in the Phalaenopsis Fancier. This was a well designed
experiment which lasted two years and relied on tissue and media analysis for
the comparison. I found that the experimental results really didn't change
anyone's mind, just stirred up a reaffirmation of everyone's previous
opinions. I stopped trying. For those of you who are interested, I'm with
Marty. Remember, those who write books don't have time to grow orchids, and
those who grow orchids don't have time to write books. Fred Bergman
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Date: Mon, 2 Apr 2001 23:42:47 EDT
From: Mark sullivan
Subject: [OGD] Urea, ammonium, and nitrate uptake by orchids and plants
Like Marty, I am not championing any specific source of nitrogen or have it
out for urea. I only opened my mouth because many of the postings did not
seem balanced, and I was beginning to wonder if urea was the long lost
miracle nitrogen source we had forgotten about. I also am hoping to learn
more of what goes happens on the molecular level as to the fertilizing of
orchids then what I have read so far. I am interested in expanding my
knowledge as with any other aspect of orchid growing, and don't want to give
the impression of an over importance to fertilizer or its ingredients. No
fertilizer will overcome poor culture. Certainly, orchid growers use many
different types of fertilizers and amendments, and are satisfied with their
results.
The questions on urea got me digging up information and reading (not orchid
books) to get at the truth. Unfortunately now that I understand more, I am
not sure I have it down enough to easily explain the process that urea and
other sources of nitrogen have in orchid growth without raising questions that
are harder to answer. Almost all published studies done on fertilizers and
plant nutrients have been done on crop plants. I do think though that much of
this is transferable to orchids. In my last posting I made some mistakes at
the end which I would like to correct. I also butchered my comparison of
urea and ammonium nitrate, mainly do to concerns as to the time and length the
posting was taking.
A question: Marty Epstein writes in OGD V3 #78: "Water and fertilizer are
applied but the roots stay wet for some time. There is also a reservoir of
water and nutrients in the root vellum. The plant absorbs water and nutrients
over a period of time. During this time some of the water evaporates and the
residual solution becomes more concentrated. The osmotic potential rises,
and salts, especially calcium carbonate may precipitate from solution. At
the next watering (with or without fertilizer) residual salts are flushed
away and are replaced with whatever is present in the new solution. This
means that the nutrients, including nitrogen, are absorbed over a period of
time. Urea makes nitrogen available to the plant over a period of time."
Skip a bunch of stuff. "The breakdown of urea into ammonium and its
subsequent conversion into nitrate is really very fast and, depending on the
total system, may be complete in a day or two (or it may take longer,
depending on the total system). It is, in a sense, a time-release
fertilizer, but over a relatively short time horizon."
I don't understand why there would be any advantage to the "time released"
urea source of nitrogen then to any other source of nitrogen? Either the
orchid absorbs the nitrogen or it is washed away by water flush. To spell
this out: If orchids can only take up a certain amount of nitrogen at a time,
or you believe that orchids only take what they need (the thought of those
who use a balanced fertilizer year around), then some nitrogen will be left
around in the surrounding wetness to be absorbed later whether it is from
urea or another nitrogen source. If 200-ppm nitrogen is used to water two
separate orchids with all variables controlled except that one orchid
receives its nitrogen from urea and the other from ammonium nitrate, both
orchids can only absorb a max of 200-ppm nitrogen for as long as moist
condition allow ion exchange to take place.
Now, I am going to go really against the grain of many previous postings,
though some of this actually supports Marty's arguments though probably not
in the way he would have thought. Bob Gordon is not correct in his statements
about urea, except in the case of a few orchids that do not have the urease enzyme and lack chlorophyll.
Most orchids can absorb nitrogen directly in the forms of cationic ammonium
ion (NH4+), the anion nitrate (NO3-), and urea. The ammonium ion (NH4+) upon
absorption, the nitrogen is available immediately to the plant for the
production of amino acids and other compounds. Nitrate nitrogen (NO3-) on the
other hand has to be reduced in order to be used by plants. Nitrate nitrogen
is more "time released" then urea. Urea's nitrogen availability is more the
middle ground between nitrate and ammonium. Urea hydrolyses into ammonium
(NH4+) and C02. This breakdown of urea can take place in the surrounding
soil, bark or coir and in the roots and leaves of orchids. The urease enzyme
breaks down the urea in leaves and roots. Urea is readily absorbed by roots
and leaves, as has been noted by a few OGD posters who use it in foliar
feeding. While urea is broken down in a growing medium and the resulting
ammonium nitrogen is absorbed by the orchid, it is not necessary as orchid
can directly absorb urea and break it down within the orchid. I would suspect
that the hydrolysis of urea in the medium more furthers the breakdown of the
medium then supplies nitrogen to the orchid. Contrary to Bob Gordon
statements urea does not take a year to break down, and can be used by
orchids directly
While heat initially may play a role in releasing urea's nitrogen during
orchid seed flasking, I am sure that urease provides the breakdown of urea as
the seedling continues to mature. The use of urea in flasking medium is
probably advantageous as you get (NH4) and CO2 in a closed system then using
a nitrogen source such as ammonium nitrate..
Ammonium Nitrate NH4N03 is like a quick fix nitrogen drug for plants, as the
plants readily take up both ammonium NH4 and nitrate N03. Because ammonium is
directly assimilated into plant metabolism and growth, it benefits only work
in very good light conditions or in correcting nitrogen deficiencies. Many
orchids grow in a constant swing of moist to dry to moist, and I imagine this
is also the case with orchids use of nutrients as they become available then
unavailable. The use of ammonium in this situation is probably advantageous.
Excesses of ammonium in plants can cause many problems in plants reducing
their ability to absorb and utilize other nutrients. Urea only provides
nitrogen in the form of ammonium NH4. This is why I think Mary Lou saw a
difference when switching from a urea-based fertilizer to a urea free
fertilizer. Nitrate nitrogen can be stored by plant. Orchids or plant
preferences for ammonium or nitrate nitrogen is directly related to their ph
preference for growing (acid medium growing plants have a preference for
ammonium, plants with a preference for an alkaline growing medium prefer
nitrate nitrogen), though both nitrogen sources are used by plants throughout the ph
range. For these reason and others, the highest growth rate is a mixture of
nitrogen supplied by both ammonium and nitrate.
I can see how Bill Bergstrom blew holes through his orchid leaves with his
urea fertilizer. I would not be surprised if that fertilizer contained little
in the way of nitrate. Bill are you sure the stuff was fertilizer and not tree stump remover?
Lastly, why would urea free orchid fertilizer makers' use other sources of
nitrogen that are more expensive then urea, and require more government
scrutiny? They don't do it for economic reasons. I would hazard to guess that
they use other sources of nitrogen for better control of nutrient uptake to
maximize growth by people who use their fertilizers. Urea only provides
ammonium, while ammonium nitrate and ammonium phosphate provide both ammonium
and nitrate. My other guess is that urea based fertilizer help in the quicker
decomposition of popular growing mediums like bark.
This is a very condensed explanation, and it is still a very long post. I
just hope it is coherent enough. I realize that the most controversial
statement made is probably the ability of plants and I am sure orchids to
uptake urea directly through their leaves and roots. For a reference cite for
plant uptake of urea: Hartel, H (1977) Wirkung einer Harnstoffernahrung auf
Harnstoffumsatz und N-Stoffwechsel von Mais und Sojabohnen. PHD Thesis,
Technische Universitat, Munchen.
Helpful books: Mineral Nutrition of Higher Plants, Horst Marschner, 1995,
Academic Press
Hydroponic Food Production, Howard M. Resh, PHD, 1995, Woodbridge Press
Publishing Company
Now if somebody says there are no urease enzymes in orchids but I
doubtthat is the case.
Mark Sullivan
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Date: Tue, 3 Apr 2001 02:42:48 -0400
From: "Laurence T. May Jr."
Subject: [OGD] FertilizerUrea
An interesting article on fertilizer urea may be found at:
http://www.extension.umn.edu/distribution/cropsystems/DC0636.html
Larry May
Cambridge, MA
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I offer a few things from Dr. Yin-Tung Wank, Ph.D. Professor of
Floriculture/Nursery Crops at Texas Ag Experimental Station ( Texas A&M)
with whom I just have hung up the phone....
"...(paraphrase quote) ...Urea use can depend on your climate and media
types. Cold climates and the urea may not be released soon enough for
uptake by your orchids. Also, it tends to be better utilized when used
in conjunction with peat or peat based media mixes, which is NOT to say
that bark media should not have balanced urea nitrogen fertilizer. The
bacteria can and do break down the urea in bark mixes. Growth from too
much urea nitrogen can be softer and weaker due to incorrect
environmental conditions that do not support the added growth of the
plant...."
This data is supported by studies done by Dr. Wang and presented to the
IPA annual meeting a few years ago. But to quote the Bard.."Argue with
no true believer."
Jim Roberts
Roberts Orchids
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Date: Mon, 02 Apr 2001 15:57:37 +0100
From: Oliver Sparrow
Subject: [OGD] Re: On taking the urea
I have to say that I am with Paul Johnson. Two obvious physiological
points. First, methanol at anything like 25% would pickle a plant nicely,
and kill its owner. Tiny amounts of methanol dissolved in large amounts of
water would be quickly metabolised through the C1 photorespiration pathway,
much as ethanol > acetaldehyde > acetyl CoA > krebbs cycle.
Second, urea and nitrates. Plants do not use nitrogen as nitrate, and have
the 'nitrate reductase' family of enzymes in the leaves specifically to
take NO3- >> NH4+. Urea, delivering reduced nitrogen in soluble form, ought
to be rather helpful in this regard, and many plants are stuffed with
urease. The jack bean is about 40% by weight urease. Certainly,
conventional plants relish it over Nitrates. It is of limited use only when
the soil is very alkaline and nitrate is lost as gaseous ammonia. Stuff has
been said of soil bacteria, but I am not sure to what end. There is a
continual flux of carbon and nitrogen in the soil which leads to the
production of surplus nitrogenous compounds (such as lignin) as the carbon
is lost as CO2. This affords a bank of N on which plants can draw, but is
not an essential conversion process (save N-nodules fixing atmospheric
molecular Nitrogen in e.g. legumes.) This said, epiphytes are immersed in
bacteria when grown in an organic mix. Loamy soil contains around 10^9
micro-organisms per gram, and orchid mix in greenhouse conditions cannot be
far behind this! Bare weathered cork has remarkable numbers of
microorganisms on it.
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Date: Wed, 4 Apr 2001 17:52:22 -0400
From: Martin Epstein
Subject: [OGD] Still More on Urea and such
I posted only the abstract of the paper as given. Perhaps I should have
added the conclusion:
"These studies present evidence that urea is a suitable source of N for the
nursery industry with respect to the availability of N supplied by urea for
plant absorption. Even though urea is not absorbed by the bark medium and
is subjected to leaching as is NO3- N, urea is rapidly hydrolyzed to NH4-N.
The NH4-N fraction is then available for adsorption to the negative charges
of the medium prventing leaching, or can be readily absorbed by the plant
as NH4 or NO3 after conversion by nitrification"
Please note that when the medium dries out the fertilizer doesn't go away.
It is still there, but dry. Also, fertilizer solution absorbed by the
vellum, though dry to the touch, is certainly present as well.
Lest you think is is an isolated example Elliot, George c. "Urea Hydrolysis
in Potting Media", J. Amer. Hort. Soc. 111(6):862-866, 1986 reported
results for uncropped (fresh) commercial media for
Fafrad Peat-Lite,
Fafard Mix No. 2,
Fafard Mix No. 3,
Fafard Mix No. 4,
Master Mix,
Metro-Mix 200,
Metro-Mix 300,
Metro-Mix 350,
Metro-Mix 500,
New Plant Life,
RePeat Grower Mix,
Sunshine Mix,
VSP Peat Lite Mix, and
VSP II.
The results varied between mixes but support the thesis that urea
decomposes relatively rapidly.
He also reported thta hydrolysis was more rapid in cropped mix than
uncropped mix, that is, the older the mix (two weeks in this case) the
faster the hyrdolysis.
With regard to Ray's post on osmotic potential, one must differentiate
between osmotic potential and diffusion rates of ions and molecules.
Diffusion rate through a semi permeable membrane does, as you say, depend
on the relative concentration of the same ion on both sides of a membrane.
It also depends on the size of the pores and the size of the ions or
molecules. By the way most biological membranes have diffusion pumping
mechanisms to counter this where necessary for the well-being of the plant.
Osmotic potential (simplified) refers to the difference in concentration of
water molecules on both sides of the membrane. To a first approximation
the size of the ions and molecules are less important than their numbers.
We tend to use TDS as an inexact measure of this.
I don't know what the equivalent osmotic concentration is in plant tissue.
that is, how much dissolved solids in water would just counterbalance that
of the sap (or whatever liquid is present) in the roots of orchid plants.
Marty Epstein
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