I generally do not try to define synthetic as I determine what to use for growing. Frankly, I have not seen a definition satisfactory to me that deals with organic growing. I will be very interested to see responses on this thread. Many of the NOP approved materials have a lot more complex processing and environmental impact than materials that some call synthetic. Are the NOP approved materials non-synthetic because they are approved? Fish emulsion, for instance, takes a lot more processing than urea, and yet urea is usually labeled synthetic, and fish emulsion is allowed as an organic addition. Calcium supplements are another example. Calcium nitrate is fairly readily available from other materials streams, and can be used easily in agricultural settings. Some of the approved calcium/magnesium solutions require significant processing which is readily reflected in their price. If I look at the definition of organic growing as having the least negative impact, then in both cases above (urea and calcium nitrate), for my situation, the so-called synthetic solution is actually more in tune with my spirit of organic gardening. I've seen the very amazing impact of fish processing plants as well as the small impact of a urea conversion facility. Sometimes you will see folks call materials natural if they require minimal processing to be available or usable. In this case, again, urea and calcium nitrate win out over the NOP materials. Another material folks often recommend to not use in organic growing is potassium nitrate. I have not done as extensive research on this material, but my determination was that it also had a lower environmental impact than available NOP organic solutions.
Much of this discussion really relates to philosophical differences in approach to organic gardening. I certainly know growers who would not use some of my choices for their organic gardens; they face different challenges than I do, and have made a philosophical determination that is different from mine. When dealing with other organic growers I always try to keep this viewpoint in mind and on the table: "We all are growing organically, but our approaches may differ significantly; those who decide to obtain certification need to pursue different solutions than those who inform the consumer about their growing methods, but do not seek government approbation."
Renais

Thanks Renais1.

How about others? I'd really like to know what you think defines a synthetic fertilizer in your organic philosophy. I certainly won't argue with you.

A material that is not natural in origin, is man made, has been made by an artificial chemical reaction, or has been treated in a manner beyond that what is necessary.
An example would be Urea which is made by capturing Nitrogen from the air and reacting that with Hydrogen to create Anhydrous Ammonia which is further processed to create Urea. While animal urine does contain some Urea it is not, and never has been, the primary source of Urea.
Another example is Super Phosphate where rock Phosphate is reacted with Sulfuric Acid to produce a product that has Phosphorus that is more readily available, unnecessary in a good healthy soil.

kimmsr, thank you for your reply. I noticed elsewhere that you consider Calcium Nitrate to be synthetic even though it does occur naturally. So I'm guessing you consider its extraction from by-products of the canning industry to be a synthetic process? If so, you won't get an argument from me.

Calcium Nitrate is made by reacting lime with Nitric Acid and is listed as the first synthetic fertilizer made. While some can form when manure is put on concrete and that might be natural what is sold is a synthetic product.

kimmsr, thank you.

For myself, I think of human-designed molecules as synthetic. An example is EDDHA amino acid that is used as a chelate in agriculture. So I guess my use of the word synthetic is more narrow. But I now understand your concern about manufactured compounds. In the past century calcium nitrate was mined for a limited time from dry lake beds in the western great basin, but you are correct that nearly all of it available to consumers today is a manufactured compound.

How about other people? What is your idea of a synthetic fertilizer?

Having just read several definitions of synthetic, it appears to boil down to something humans have a critical part in creating. No, I wouldn't include livestock manure in that even though a person is raising, tending and feeding the stock that would otherwise not exist.

There certainly is a critical delineation to the term organic that unfortunately gets missed on occasion, all organic molecules as science defines them have at least one carbon atom. If you look up the formula for urea you'll discover it has a carbon molecule though it is apparently considered by many to not qualify as an organic fertilizer. How urea is made and from what is a whole separate matter that folks with differing philosophies can debate.

To me, the most critical part of any plant growth aide (fertilizer) is getting only to the target crop what is needed, when it needs it and in the quantities it needs it. Irrigation in dry areas like mine is a crucial component for achieving that goal. I could recklessly manage my crop nutrients no matter where they come from and so can you, it's work to get it right as I expect many of you know.