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Nature and farming · Soil and fields

Crop rotation and legumes

The air is 78% nitrogen, yet wheat can run short of it. Discover how bacteria in the root nodules of chickpeas and lentils fix nitrogen from the air, how this differs from the Haber–Bosch process, and the logic of crop rotation from Anatolia to Rome.

  • 17 min
  • 2 simulations
  • Mini quiz

Updated:

Preview

The question

Picture a wheat field on the Konya Plain in central Türkiye. One year the farmer sows wheat; the next year, chickpeas or lentils in the same field; and in some years the field is left empty, with nothing sown at all. Why not sow wheat, the best-known crop, every single year? What is more, nitrogen, one of the nutrients wheat needs most, makes up about 78% of the air we breathe. How can something so plentiful ever be in short supply?

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Soil layers

Roles in the field

Choose a role from the list for each plant; a role can be given to more than one plant. Read the short notes on the cards too. When you have matched them all, press Check.

Prediction

Predict first

Which of these plants do you think can get its own nitrogen from the air?

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Ordering

How does a nodule form?

Order the cards by dragging them or moving them with the keyboard, then press Check.

Prediction

Predict first

What do you think the small nodules on chickpea roots, pink inside, are?

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Frequently asked

5 questions

With so much nitrogen in the air, why do plants run short of it?

The nitrogen in air is the N₂ molecule, two atoms locked together by a triple bond, and no plant can break that bond on its own. Plants have to take nitrogen from the soil as ammonium or nitrate dissolved in water.

Do legumes take nitrogen from the soil and give it back?

No. Rhizobium bacteria in their root nodules turn nitrogen from the air into ammonia with the enzyme nitrogenase, and in return the plant gives them sugar. That is why legumes can supply their own nitrogen even when the soil is low in it.

Does a farmer who grows legumes never need fertiliser?

Not always. Part of the fixed nitrogen leaves the field in the chickpea and lentil grains, and legumes cannot fix nutrients such as phosphorus and potassium. Crop rotation reduces the need for fertiliser, but does not always remove it.

What does crop rotation do besides adding nitrogen?

Some wheat diseases and pests decline when they find no host in the field the next year. Crops sown at different times make it harder for the same weeds to set seed every year. Shallow- and deep-rooted crops also use different layers of the soil.

What does it mean if a nodule is white inside?

Active nodules are pink or reddish inside because of leghaemoglobin, which keeps oxygen away from the enzyme. If a nodule is white inside, it is not yet mature or not active, and it is most likely not fixing nitrogen.