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Im a bit confused on how crucial Nitrogen fixation with the Haber-Bosch process really was. On the Wikipedia page of the Haber-Bosch process it says:

In combination with pesticides, these fertilizers have quadrupled the productivity of agricultural land.

With average crop yields remaining at the 1900 level the crop harvest in the year 2000 would have required nearly four times more land and the cultivated area would have claimed nearly half of all ice-free continents, rather than under 15% of the total land area that is required today.

Due to its dramatic impact on the human ability to grow food, the Haber process served as the "detonator of the population explosion", enabling the global population to increase from 1.6 billion in 1900 to today's 7 billion.

However I have read that the products labelled "Bio" in the stores in Switzerland that don't use nitrogen fertilizers have only about 20% lower yields, which is a dramatically different number than what the Wikipedia page on the Haber-Bosch process claims (quadrupling of productivity).

What am I missing? Did the nitrogen cycle change so much so that we don't need nitrogen fertilizer nowadays? Did new technology enable us to get similar yields without nitrogen fertilization? What explains these numbers? Was crop rotation an unknown concept in the 1900’s?

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    $\begingroup$ The first sentences states "In combination with pesticides, ...". Therefore, I would assume that a combination of pesticides, fertilizers (also phosphorus!), and new plant species yielded this 4x yield. $\endgroup$ – daniel.heydebreck Aug 11 '17 at 8:55
  • $\begingroup$ In addition (I am not an expert on agriculture -- so this can be wrong!): (a) I would assume that you can reduce the need for fertilizers considerably by crop rotation and planting intermediate nitrogen-fixating crops. (b) For example in northwestern Germany, we have a lot of industrial-like animal husbandry. A lot of manure is available and put onto fields. This leads to a considerable excess of nitrogen in the soils and to threshold-exceeding nitrate loads in some ground water bodies. What I mean: in some regions there is not need for industrial N-fertilizers in the moment. $\endgroup$ – daniel.heydebreck Aug 11 '17 at 8:59
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    $\begingroup$ The aim of crop rotation is also to reduce the vulnerability to insect pests and to have plants with different root depths and nutrient needs. Nitrogen fixating plants can also be planted in the monoculture setup: spring+summer grows the default crop (soy etc.) and in autumn the nitrogen fixers. But yes: If you neither have manure nor nitrogen fixers (e.g. due to too less water), you probably need nitrogen fertilizers. $\endgroup$ – daniel.heydebreck Aug 11 '17 at 11:18
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    $\begingroup$ @daniel.neumann Just pointing out that soy is a nitrogen fixer. $\endgroup$ – Spencer Aug 12 '17 at 14:39
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    $\begingroup$ @RodrigodeAzevedo Which point do you mean? Item (b) of my second comment? I agree with you. However, the feed often is not grown where the livestock ist fed and the manure is produced. That was the background for my comment. I should have clarified it. But, right: without application synthetic fertilizer at another location, we couldn't "produce" that much manure here. $\endgroup$ – daniel.heydebreck May 9 at 7:16
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There are a number of factors that went/are going into the population explosion, food is only one vaccination and antibiotics are a big factor as well. within food fertilizer is again only one of many factors. Better crop strains, mechanized and better farming techniques, soil science, pesticides, refrigeration, and a dozen other factors contributed. Keep in mind artificial fertilizers get less important as you go back in time as well so calling it the detonator is incorrect and misleading.

Now when considering crop rotation, crop rotation has pros and cons, there are several nutrients plants need and rotation only helps with some of them, and it relies on having compatible crops to rotate which not every crop or environment has.

Keep in mind the piece wikipedia is quoting is not a peer reviewed paper but an essay in nature, which explains it is referring to several modern countries that fertilize almost exclusively with artificial fertilizers. It is not claiming they cannot use other fertilizers.

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    $\begingroup$ Add to the last paragraph that time scales for the Wikipedia article and the Bio-Swiss reference are different. A 20% difference using today's technology versus a x4 increase from standards used over a century ago are two different metrics entirely. $\endgroup$ – Jeffrey J Weimer Nov 14 '18 at 21:23
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There are several aspects to be considered in this apparent difference.

First, your quote from Wikipedia describes the change in productivity but does not account for other factors that have lead to productivity increase such as changed farming practices (larger, more sophisticated farm equipment, improved use of weather and soil data, etc.) changes to crop species and cultivars - especially the increase in use of hybrids, and increased irrigation. Thus, the contribution of nitrogen fertilizers is only one factor, although an important one.

Secondly, you can compensate for the lack of chemical nitrogen fertilizer through a number of methods, but that doesn't negate the changes due to the widespread adoption of nitrogen fertilizers. You can use organic fertilizers to increase production but there may be limits on the ability to do so on the scale of conventional agriculture. You may be able to use more labor-intensive techniques on a small scale that could not be adopted on the scale of modern agriculture.

Additionally there are cost considerations. It may not always be financially feasible to increase productivity through organic farming methods. Or at least society may not be willing to pay the price

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