Vetch

It fixes atmospheric nitrogen by nodulating with this bacterium. A useful detail for the gardener: inoculation has often proved ineffective, the soil's native strains outcompeting the ones introduced. A large part of its nitrogen does come from the atmosphere, but note, that's the nitrogen CONTAINED IN THE VETCH, not the nitrogen delivered to the following crop (see below). North American figures, not transposable as-is.

This is where the popular story parts from reality. Two independent isotope-tracer trials converge: the following maize recovers only about 12 to 15% of the nitrogen from the vetch residues in the first year, most stays in the soil. Put plainly: the vetch BUILDS a nitrogen stock over the long term, it does not pay an immediate advance to the next crop.

The best-supported association on this sheet, by a meta-analysis of 55 site-years. The cereal acts as a support for the climbing vetch, captures residual nitrogen (which pushes the vetch to fix more), and its C/N balances the mixture's decomposition. But the nuance matters: against the better of the two sole crops, the mixture mostly BREAKS EVEN on nitrogen, its real gain is 'as much nitrogen + the rye's services' (cover, weed control, anti-leaching), provided the vetch makes up at least half the seed. A real effect on fixation, but inconstant from one year to the next.

The vetch exudes molecules that reshape the neighbourhood, cyanamide and canavanine identified. But the effect in the garden must absolutely not be overstated: these compounds are only detected around the roots, transiently (a few weeks after germination), at doses far below those that act in the laboratory. An important caveat: cyanamide also inhibits lettuce and tomato, it's not a selective weedkiller, and a direct sowing behind a vetch may suffer from it.

The most counter-intuitive lesson of this sheet. The vetch bears real nectaries outside the flowers that attract ants, a real resource, not a myth. But the reference study shows that these ants, while removing certain pests, also protect the pod-boring caterpillars from their parasitoid wasps: where the ants are abundant, seed damage INCREASES. 'The vetch attracts beneficials that protect the garden' is therefore not demonstrated, the study rather documents the opposite for the vetch itself.