Do legumes fix nitrogen from the air? Root nodules and crop rotation

About 78% of the air around you is nitrogen; you are breathing it in with every breath right now. Even so, nitrogen is one of the nutrients that plants in the field look for most. That is because the nitrogen in the air sits in a form plants cannot use. Legumes get past this obstacle with small partners that are hard to see with the naked eye. In this article we look at how chickpeas, lentils and beans manage it, the logic of crop rotation and how all this differs from fertiliser.

Why is nitrogen both plentiful and scarce?

Nitrogen is a building block of a plant’s proteins, its chlorophyll and its DNA. If there is too little nitrogen, the plant stays stunted and its leaves turn pale green.

Nitrogen in the air comes as N₂: two nitrogen atoms are joined by a triple bond, and this bond is very strong. With their roots, plants can take up ions such as ammonium (NH₄⁺) or nitrate (NO₃⁻) from the soil; but they cannot break N₂ apart. Turning the nitrogen of the air into a compound a plant can use is called nitrogen fixation. In nature only certain microbes can do it; lightning adds a very small share.

The partnership in the nodule

The roots of legumes (beans, chickpeas, lentils, peas, broad beans, clover, vetch) form a partnership with Rhizobium and related bacteria. The root gives off chemical signals, a suitable bacterium detects them, the bacteria enter through a root hair and the root grows small lumps to house them: root nodules.

Inside the nodule, the bacteria use an enzyme called nitrogenase to turn N₂ into ammonia (NH₃). The plant builds this nitrogen into its proteins; in return it gives the bacteria some of the sugars it makes by photosynthesis. Nitrogenase is sensitive to oxygen, so the nodule contains a substance called leghemoglobin that keeps the oxygen level low inside. The pink or reddish colour of an active nodule’s inside when it is cut comes from this substance.

It took time to understand this. In the mid-1880s Hellriegel and Wilfarth showed that legumes can take nitrogen from the air, and in 1888 Beijerinck isolated the bacteria from nodules. Ancient Roman farming writers had written long before that legumes “feed” the soil, without knowing why.

Crop rotation: sowing in turn

Crop rotation means changing the crops in a field in sequence instead of sowing the same crop every year. In a classic system a cereal follows a legume; for example, wheat can be sown after chickpeas or lentils. The legume leaves some nitrogen in the soil, the cereal benefits from it and may give a good harvest with less fertiliser. Also, different plants draw nutrients from different depths, and the cycle of diseases and pests tied to one crop is broken.

In many dry regions, a field used to be rested for a year (fallow). Sowing a legume instead of fallow is a way of getting a crop that also adds to the soil without leaving the year empty.

Comparison with fertiliser

Industrial nitrogen fixation became possible through Fritz Haber’s laboratory work in 1909 and Carl Bosch’s adaptation of it for industry in 1913. The Haber–Bosch process makes ammonia from the nitrogen of the air at high temperature and pressure and is the basis of synthetic nitrogen fertiliser. Today the feeding of a significant share of the world’s population depends on this fertiliser.

It is more accurate to see the two routes not as rivals but as different tools. The bacteria in a nodule fix nitrogen slowly and only beside a legume; a factory makes very large amounts on demand, but needs a lot of energy. Part of any nitrogen applied in excess can leach into groundwater and surface water. The right question is not “which is better” but “how much, and where”.

Do all legumes do the same job?

Legumes are used in different ways depending on the purpose of the sowing. Chickpeas, lentils and beans are grown as human food; clover and vetch are often grown as animal fodder. Some legumes, such as vetch, can be cut before flowering and mixed into the soil; this is called green manure. What they all have in common is that, when the bacteria in the root nodules work under suitable conditions, the plant gets part of its nitrogen from the air.

Common mistakes

• “Legumes take nitrogen from the soil and give it back.” No; they take nitrogen from the air, through their bacteria.
• “A farmer who sows legumes never needs fertiliser.” Part of the nitrogen leaves the field in the grain; also, legumes cannot fix phosphorus or potassium.
• “Every legume forms nodules in every soil.” If the right bacteria are not in the soil, no nodules form. That is why the seeds of some legumes are inoculated with bacteria before sowing.

Observe it yourself

In a garden, or at the edge of a field with an adult’s permission, carefully dig up a broad bean, bean or clover plant with its soil while it is flowering, and rinse the roots with water. If you see small lumps on the roots, those are nodules. Have an adult cut one open with a knife: if the inside is pink, the nodule is active. You may not find nodules on every plant or in every soil; that is normal. Do not pull up anyone else’s crop, and wash your hands with soap when you are done.

If you are curious about the living side of soil, read our article What lives in a handful of soil?, and for our lessons see the Nature and Farming topic.

Summary

• The nitrogen in the air (N₂) cannot be used directly by plants; legumes get around this with bacteria in their root nodules.
• The bacteria turn nitrogen into ammonia and the plant gives sugar in return.
• In crop rotation, sowing a cereal after a legume can reduce the need for fertiliser but does not always remove it.
• Haber–Bosch fertiliser and the legume partnership are different tools, and both have limits.