Every young farmer hereabouts is taught, that over the year,worldwide, the equivalent of many millions (billions?)of tons of nitrogen fertiliser is put into the soil by lightning strikes.
How? How does a massive jolt of electricity 'put' nitrogen into it; what is the chemical reaction brought on to release nitrogen in the soil? Where is the nitrogen before it is made available to the plants?

It's all about getting it into a chemical form that the plants can use. Nitrogen (N₂ ) is plentiful in the atmosphere (about 80%), but isn't usable. The high energy of lightning causes the nitrogen to react with oxygen and form NO and N0₂, which, in turn, combines with water to seep into the soil and form HNO₃, a form that the plants can use.

Nitrogen-fixing bacteria do a similar job. Fertilizers use a combination of animal matter (manure & decaying animals - so really just recycled plant) and synthetically fixed nitrogen (via the Haber process which reacts nitrogen gas with hydrogen).

Thanks, methos. ( Why did I think that you would answer this? ). There are plants we grow that fix nitrogen and so enrich the soil too, of course; they all seem to be of the pea and bean family. Strangers sometimes wonder why we are suddenly so keen on growing a field of beans which are only ever grown as animal feed; that's really the reason (though a bonus is that come harvesting you can get enough rabbits from the middle to feed the village ).

It's the above-mentioned bacteria that live by the roots of the legumes that fix the nitrogen. The beans take advantage of this and the excess ends up in the soil. In fact, they sell the bacteria separately so that you can add more efficient strains.

Crop rotation, which you mentioned, has been known to increase yeilds since at least Roman times. We now know that it works largely due to alternating plants that add nitrogen to the soil with those that require nitrogen from the soil. It also tends to cut down on diseases and bugs because there is no carry-over from year to year and because some plants produce effective pesticides.

Thinking of how interdependant nature is, it isn't surprising that a system using multiple species works better than trying to isolate a single species.