Legume

What counts as a legume

Legumes are the pea and bean family. In a UK veg patch that means peas, runner beans, French beans and broad beans, plus things like mangetout and field beans grown as a cover crop. They all share one useful trick: they pull nitrogen out of the air and bank it in the soil.

How they feed the soil

Plants can't use nitrogen straight from the air, even though it makes up most of what we breathe. Legumes get around this with a partnership. Soil bacteria called rhizobia move into the plant's roots and form small swellings, or nodules — break open a healthy pea root and you'll see them, often pinkish inside when they're actively working. The bacteria convert airborne nitrogen into a form the plant can use, and in return the plant feeds the bacteria with sugars. This is what we mean by "nitrogen fixation".

The upshot is that legumes largely feed themselves, and leave a little nitrogen behind in the soil for whatever you plant next.

Don't over-feed them

Because they fix their own nitrogen, legumes don't want a nitrogen-rich diet on top. Pile on high-nitrogen feed or fresh manure and you get lush leafy growth but disappointing pods — the plant gets lazy and the bacteria switch off, since there's no need to do the work. A free-draining soil with a bit of compost worked in is plenty. Save the rich feeding for the hungry crops that come later.

Cut the tops, leave the roots

Most of that fixed nitrogen is stored in the roots, not the leaves. So at the end of the season, resist the urge to yank the whole plant out. Cut the tops off at ground level for the compost heap and leave the root system in the soil to break down. As the nodules rot, the nitrogen is released gently for the next crop — a small free fertiliser you'd otherwise throw away.

Their place in crop rotation

This is exactly why legumes earn a permanent spot in a rotation plan. The classic move is to follow them with brassicas — cabbages, kale, broccoli and sprouts — which are greedy for nitrogen and lap up what the beans left behind. A simple beginner's order is roots, then legumes, then brassicas, working round the plot each year so no bed grows the same family twice in a row. Rotating like this keeps the soil balanced and helps dodge the pests and diseases that build up when one crop sits in the same spot.

If you're just starting out, broad beans are the gentlest way in: hardy, forgiving, and quietly improving your soil while they crop. See our guide to growing beans for sowing dates and spacing, or browse all the vegetable growing guides to plan your rotation.