Nitrogen in fertilizers and the truth about Urea
Sorry for being late on the discussion about fertilizer. It is all a matter of finding time. the one thing I want to dispel is that urea is not good for plants or orchids contrary to often what is written about it. I posted a similar post to this one years ago on this forum, but information gets lost especially when there is a lot of misinformation.
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.
Most orchids can absorb nitrogen directly in the forms of cationic ammonium ion (NH4+), the anion nitrate (NO3-), and urea. The nitrogen in the ammonium ion (NH4+) 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. Ureas 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 and can be used 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. Contrary to older orchid book statements urea does not take a year to break down, and can be used by orchids directly.
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. 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 a 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.
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 take up urea directly through their leaves and roots. The initial study for plant uptake of urea is Hartel, H (1977) Wirkung einer Harnstoffernahrung auf Harnstoffumsatz und N-Stoffwechsel von Mais und Sojabohnen. PHD Thesis, Technische Universitat, Munchen.
Other books used in this post are Mineral Nutrition of Higher Plants, Horst Marschner, 1995, Academic Press and
Hydroponic Food Production, Howard M. Resh, PhD, 1995, Woodbridge Press Publishing Company