Eli5 why plants which need nitrogen from the ground do not take it from the air which is 70% Nitrogen
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As people have mentioned: nitrogen in the air is N2, which is extremely difficult to break apart into single nitrogen atoms – and no plants can currently do this.
Instead, plants rely on soil bacteria to do this for them. This is a slow and difficult process; legumes help by providing cosy root nodules for the bacteria to live in, and the plants reap the rewards.
Farmers used to rotate their crops, planting legumes in a field every few years, then leaving it to lie fallow the next season for the leftover root nodules to rot down and release the nitrogen compounds into the soil.
The most enormous advance in agriculture of all time was the Haber process – a system for combining atmospheric nitrogen with methane, and producing ammonia to use as fertiliser. Something like 80% of the nitrogen in animal life these days is as a result of artificial fertiliser produced this way.
However, the Haber process is not very environmentally friendly – it takes a whole lot of energy, and produces greenhouse gases. And on top of that, runoff from the actual application of fertiliser (since the water in the soil leaks into the water table, rivers, etc), causes a whole bunch of other problems down the line.
As such, there’s some really interesting work going on to genetically engineer the nitrogen-fixing enzymes from the soil bacteria directly into plants, which would then indeed be able to get their nitrogen directly from the air.
It’s a major challenge, though – bacterial DNA is not like plant DNA, so you can’t just insert the relevant chunk into a plant nucleus and expect it to work. Instead, they’re looking at inserting it into the DNA of the mitochondria within the cell.
Mitochondria used to be independently-living bacteria, but they formed a symbiotic relationship with other cells about a billion and a half years ago, trading some neat metabolic tricks for an ongoing supply of nutrients. The relationship is complex and the lines between the two have become blurred – but mitochondria are a hell of a lot more similar to the nitrogen-fixing bacteria than plants are, and a lot of the research is based around translating the necessary genes into ones that will run within the mitochondria.
If they get it right, it will be *huge*.
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