Urea in Fertilizer


Question:  I’m a bit confused about the use of urea, and wonder if you can help me.  I was told sometime agao that we should not use any fertilizer with urea listed in the ingredients.  I’ve used fertilizers containing urea in my garden and it looked great!  Should I use my old fertilizer or toss it?

Answer:  There seems to be plenty of opinions on the subject, and it seems that the prevailing opinion of the violet hobby is the one you were given.  Still, urea has been an ingredient in fertilizers for years, and may successful growers us it.  For tha matter, we use it (4% or the 15% total nitrogen in our fertilizer), apparently without ill effect.  So what gives?

Let’s outline some important points.  To begin, don’t get distracted by “organic” and “synthetic”–though produced differently, chemically, they are identical.  Urea is just one particular form of nitrogen, an essential element needed for plant growth.  Nitrogen (N, the first number in you fertilizer formula) is the most essential element in any fertilizer.  Without it, your plant essentially starves.  it must have it.

Nitrogen can be supplied to plants in a number of forms, the most common being urea, ammonium, or nitrate (these should appear on your fertilizer label).  The crux of the matter is this: your plant can only use nitrogen in its nitrate form.  As ureas, it must first be broken down into ammonium (carbonate) then further converted into nitrate.  This is the basis for the “no urea” argument–since urea is not directly usable by the plant, why have it?  Wouldn’t a fertilizer containing only ammonium and/or nitrate forms of nitrogen be better?  If so, why is there any urea in fertilizer?  This is where the answer gets messy.

The simple answer is economics.  Urea is cheap, and fertilizers using it can be produced chapely and sold inexpensively.  But can it still be useful?  Yes, and it has been for many growers, for many years.  The secret is in making is possible for urea to be broken down into ammonium, then the usable nitrate form, of nitrogen.  This requires the work of microorganisms in the soil, which can oxidize the urea and ammonium forms.  In the “old” days, when growers commonly used compost, manure, or the like, in the soil mixes for their indoor plants, this was never an issue (though “sterilizing” these mixes undid some of this).  These mixes had plenty of bacteria (as well as other less desirable things, like nematodes and mealy bugs).  Modern “soilless” (peat based) mixes, though, contain no actual topsoil, compost, or manure.  Because of this, many growers believe they lack the microorganisms necessary to break down urea into usable nitrogen.  Unused, excess urea leads to an accumulation of “salts” in the soil mix, attracting water away from the plant’s roots and causing “fertilizer burn”, visible as brown leaf edges and plant centers.

Solutions?  First, good watering habits and occasional leaching (cleansing) of the soil with clear, unfertilized, water.  This will help prevent the build-up of excess fertilizer salts.  Use fertilizers at their recommended dilutions, keeping in mind your frequency of watering and the growth needs of your plants.  Can you still use urea-based fertilizers and soilles mixes?  Yes.  Even soilless mixes do contain some of the necessary bacteria to break down urea.  Soil conditioners ar available to add more, if you’d like.  Better, many of today’s soilless, peat-based, mixes do have these microorganisms–check the label (look for terms such as mycorrhize).  Good culture means more than just your choice of fertilizers.

2 comments

  • This was interesting. I have well-rotted manure that is a couple of years old. I’m guessing it’s good for adding humus but not for adding N. I recently discovered that this manure is relatively high pH (8.1). I was surprised as I just assumed it was acidic. So my question is: can manure and urea be applied at the same time? If not, when would be the best times to apply them.
    Thanks.

    • We’ve never used manure, so haven’t first hand experience with this. Not necessary, since any good fertilizer will provide all the nutrition the plant needs. That said, I see no reason you couldn’t do this, but with great caution, since it is possible to overdo it and burn roots (i.e. overfertilize)…everything in moderation.

Leave a Reply

Your email address will not be published. Required fields are marked *

 characters available