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Comment by universa1 | Hacker News Reader

But in the end you have to replace at least some of the nitrogen (and other nutrients) you are taking from the field in some way. And without a major shift in consumption patterns (less meat) this will mean fertilizer, as the alternatives usually lead to a much lower yield.

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Meat farming replaces artificial fertilizer, by providing a source of manure, so doesn't that relationship run the other way?

But having 1/4 of the land under soy beans or other locally appropriate nitrogen-fixing plants doesn't seem like it would hurt yields too much.

Nitrogen can also be replenished by crop rotation. Legumes will naturally add nitrogen back.

At the same rate as artificial nitrogen fertilizers? I doubt it. And, is the profit per hectare the same when planting legumes? Again, I doubt it.

I was very disappointed on my last trip to Iowa to see that most farmers weren't even bothering with cover crops.

I wonder whether genetic engineering can give nitrogen fixing abilities to plants that currently lack this ability?

It's not the plants themselves that fix nitrogen but endosymbiotic microorganisms in the soil.

I know. For simplicity, I was talking about the plants in the same generic sense that your gut microbiome is a part of you, and the dead tissues that form your hair and skin are also a part of you.

You still need to hack up the eg cereal plants so they can actually engage in that symbiotic relationship (or perhaps actually directly fix nitrogen all by themselves, without any outside help at all).

N-fixation does happen in the soil, but legumes have root nodules that host rhizobium bacteria and can assimilate the N much more easily

You could do that, the problem is the same as in making nitrogen fertilizer in a chemical plant: energy cost. It just takes so much energy to break the nitrogen tripple bond.

Even if you made a plant that fixes nitrogen extremely efficiently, every joule of sunlight it pumps into the ground is not available as calories you harvest. And fixing nitrogen will take an amount of energy per acre on the order of what you harvested from that acre in a year.

Well, only being as efficient as existing nitrogen fixing plants (or rather their microbes) would already be quite interesting.

Btw, I don't think plants are close to optimal efficiency in terms of using sunlight. See eg C3 vs C4 plants.

> Well, only being as efficient as existing nitrogen fixing plants (or rather their microbes) would already be quite interesting.

My point is that you can't have corn that is as nitrogen-fixing as a legume and still produce nearly as much corn - the plant (or its microbes) will need the majority of the available photosynthesis products to fix nitrogen. This directly makes the cobs smaller.

> Btw, I don't think plants are close to optimal efficiency in terms of using sunlight. See eg C3 vs C4 plants.

That's true, even photovoltaic panels (which are still far away from their theoretical maximal efficiency) are an order of magnitude more efficient at pulling energy from the sun than plants are. But significantly improving photosynthesis in crop plants is far beyond our current genetic engineering ability.

And I'm not aware of any way to organically fix nitrogen that uses energy outside what is provided by photosynthesis - or gets its energy from digesting dead organic matter, which also doesn't beat the limits of photosynthetic efficiency on a per-acre basis.

> My point is that you can't have corn that is as nitrogen-fixing as a legume and still produce nearly as much corn - the plant (or its microbes) will need the majority of the available photosynthesis products to fix nitrogen. This directly makes the cobs smaller.

I can believe that. However for people who don't want to use nitrogen fertiliser, this might still be useful.

You can see it as an alternative to clover (or manure), that happens to produce eg a bit of grain.

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