Are a third of the proteins we consume dependent on synthetic fertilizer?

Question

In July 1997, Vaclav Smil wrote an article in Scientific American on humankind's use of the Nitrogen Cycle (emphasis added):

Synthetic fertilizers provide about 40 percent of all the nitrogen taken up by [cultivated plants]. Because they furnish—directly as plants and indirectly as animal foods—about 75 percent of all nitrogen in consumed proteins (the rest comes from fish and from meat and dairy foodstuffs produced by grazing), about one third of the protein in humanity’s diet depends on synthetic nitrogen fertilizer.

While the article includes references to four books/articles as "further reading," this is an interesting and uncited assertion. I assume "depends on" to mean that the nitrogen in a third of the protein in humanity's diet is fixated through synthetic, industrial processes. In other words, "derived from" synthetic processes.

^{At first glance, 33 percent is such a large number! How can humanity be so dependent on an "artificial" chemical processes? Then with more thinking, the number is not obviously wrong. And then with even more thinking, it's a small number! Only 33 percent? I always thought synthetic fertilizers were more significant to mankind. They are everywhere and reporting is consistent on its negative effects (environmental damage from runoff, etc.).}

A separate journal article states:

Now, over 80% of the nitrogen in the average human body originates from the Haber–Bosch process (Howarth 2008).

As the Haber-Bosch process is used to (synthetically) produce ammonia for plant fertilizers, I would assume that the above statement is equivalent to

80% of the nitrogen in humanity's diet is derived from synthetic nitrogen fertilizer. (my own inference)

And, according to the Royal Society of Chemistry's General Briefing Sheet (emphasis added):

Nitrogen is an important part of your DNA, which defines what you are like in many ways. We cannot survive without nitrogen in our diet – we get [nitrogen] in the form of protein.

So, I thus refined my earlier statement

80% of the protein in humanity's diet is derived from synthetic nitrogen fertilizer. (my own inference)

This statement does not agree with Smil's statement, yet it seems to reflect cited science.

Do "one third (33%) of the protein in humanity’s diet depend on synthetic nitrogen fertilizer?" 80%? Some other number?

Of course, this number may have changed between July 1997 (Smil article) and December 2008 (Howarth article). I am interested in Smil's statement.

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Note (based on comments):

@sumelic correctly points out:

Although the quotation uses the word "depends", it might be easier to avoid opinion if you used a different phrasing such as "Is the nitrogen in a third/80% of the proteins we consume derived from synthetic fertilizer?"

Hence, the question is more simply stated as "Is 33% of the protein in humanity's diet derived from synthetic nitrogen fertilizer?"

Examples:

1. If the nitrogen is fixated through rhizobia, it is "natural."

2. If the nitrogen is fixated through the Haber-Bosch process, it is "synthetic."

3. If some corn is grown using synthetic fertilizer, a cow eats the corn, and a human eats the beef, the nitrogen in the beef proteins is "synthetic." (credit @Jan)

@Jan also notes: "Currently vegetal sources of protein dominate protein supply globally (57%), with meat (18%), dairy (10%), fish and shellfish (6%) and other animal products (9%) making up the remainder." (source)

Answer 1

I think the premise is a bit backwards here. The most direct and shortest answer is: No, it does not depend on so much artificial fertilizer. "A uses B" is not the same as "A depends on B". The plants are not going to die if they have no synthetic fertilizer applied.

1) Non-synthetic fertilizers could be used. There is nothing to cite here, as that would be proving a negative. Instead, if anyone were to believe non-synthetic fertilizers were necessary, the onus of burden would be on them to prove why.

2) Another option is to use no fertilizers at all. There are options other than fertilizers that can be used. Letting a resource rest and replenish itself is one tactic. Rotation is another. Farmers can plant different crops in a field each year. The different plants have different needs, so by planting crop A this year they consume more of chemical B from the ground, but next year plant C needs chemical D, so we're all good - further, some plants actually put certain nutrients into the ground which could be good for next year's crop. You can also let a field lie fallow. You can also just completely ignore a field for 1 or more years so it has plenty of rest. All of these options can be done using zero fertilizer.

3) You can also just ignore good farming practices and just do whatever you want, just keep planting the same thing in the same place and accept that your plants will eventually perform poorly.

So many non-synthetic fertilizer options.

Many of these options will result in at least slightly reduced crop yield, so you need to utilize more land area to get the same amount of product output. But again, that does not mean that there is a dependency on the synthetic fertilizer, just that the synthetic fertilizer is chosen instead of using more land area.

So you see, there are other resources that can be used instead of synthetic resources, such as: non-synthetic fertilizers, land, different choices in field planning. Even if you do use these other options, however, once you have used all your non-synthetic fertilizer, all the land available, and if you are using proper field planning techniques, then that leaves you with X amount of produce. If you want greater than X, then you need to find some other way to increase production, and if you're already doing everything else then the answer is either use synthetic fertilizer or accept the amount of produce you have.

So no, there is no dependency. It is a choice, a choice which not everyone in the world makes. Synthetic fertilizer is just another tool in the toolbox, and each tool user decides what tools they use. Because it's easiest and provides the maximum produce per unit area, many decide to just order up a tank of fertilizer, whether it is synthetic or not, and drop it on the field.