A root nodule is a small bump on the roots of legumes and a few other plants, formed when nitrogen-fixing bacteria take up residence inside plant tissue. The nodule is a living factory: bacteria convert atmospheric nitrogen into ammonia, and the plant supplies them with sugars and a protected environment.

Nodules form only when the plant needs nitrogen and the right bacteria are present in the soil. The plant releases chemical signals that attract compatible strains, which enter through root hairs and trigger cell division. The whole process takes a couple of weeks. If soil nitrogen is already high, the plant may skip nodulation entirely, since the partnership costs it energy.

Reading a nodule

• Pink or red interior: actively fixing nitrogen
• White or green interior: inactive or ineffective
• Large, clustered nodules: healthy symbiosis
• Absent nodules: wrong bacteria or sufficient soil nitrogen

Farmers sometimes dig up a legume mid-season to check nodulation. If nodules are missing or pale, inoculating the next legume crop may be worth the cost.