Legumes: Nodulation, Molybdenum and Boron

Legumes such as soybean, pea, chickpea, faba bean and groundnut form a symbiosis with rhizobia bacteria, which live in root nodules and convert atmospheric nitrogen into a form the plant can use. The process is efficient, and a well-nodulated legume can supply much of its own nitrogen. It also leaves some nitrogen in the soil for the next crop, which is one reason why legumes are used in rotations.

The process depends on the plant's nutrient status, particularly molybdenum and boron, and on the presence of effective rhizobia in the soil. A legume that is short of either nutrient, or that lacks the right bacteria, will form few or ineffective nodules, and it will look as if it is short of nitrogen even when nitrogen is present in the soil. Diagnosing the cause is therefore essential before adding fertilizer.

Why molybdenum matters

Molybdenum is a component of the enzymes that convert nitrate to usable nitrogen and that support nitrogen fixation. Low molybdenum reduces nodule activity and can make the crop look nitrogen-short even when nitrogen is present. Molybdenum is most often short in acid soils, where it becomes less soluble, and liming can improve its availability.

The symptom of molybdenum shortage can resemble nitrogen deficiency, with pale, older leaves. A tissue test and a nodule inspection distinguish the two. A molybdenum seed treatment or a foliar spray can correct the shortage quickly in many legume crops, and it is usually a low-cost step.

Boron and flowering

Boron supports flower and pod set. A boron shortage can cause flower drop and poor pod filling, which reduces yield even when nodulation is good. Boron is also sensitive to excess, so the rate should follow a soil or tissue test and the label, rather than a routine rate.

Boron deficiency is more common in sandy and highly weathered soils, and it is worsened by dry weather, which reduces its movement to the reproductive organs. Irrigation or a foliar boron application at flowering may help in dry seasons, while a soil application is useful on soils that are known to be short.

Inoculation and rhizobia

Many legume fields need inoculation with the right rhizobia strain, especially where the legume is new to the field or where the soil has not held the crop for several years. Inoculants are sold as granular, peat or liquid products, and each has a shelf life and storage requirement. Use the inoculant recommended for the crop, check the date and the storage conditions, and apply it as the label directs.

Inoculation can fail for several reasons. Fungicide seed treatments can kill the bacteria, high soil nitrate reduces nodulation, and dry or very hot soil stops the bacteria from surviving. A seed treatment that is compatible with the inoculant, and a planting that is timed to avoid heat, improve the chance of success.

• Use a rhizobia inoculant that matches the legume species, and check its date and storage.
• Avoid fungicide seed treatments that are harmful to the bacteria, or apply the inoculant separately.
• Keep the seed and the inoculant cool and out of direct sunlight until planting.
• Check the nodules at flowering, to confirm that the inoculation worked.

A lighter nitrogen strategy

Legumes usually need little or no starter nitrogen when nodulation is working. Heavy nitrogen applications can suppress nodulation, so a small starter dose, applied only where the soil is low in nitrogen, is a common approach. The nitrogen that the legume does not need is better left out, because it reduces the value of the nitrogen fixed by the crop.

Monitoring and diagnosis

Dig up several plants at early flowering and inspect the roots for nodules. Count the nodules, check that they are pink inside, and note their position on the root. A healthy crop has many active nodules on the main roots, while a crop with few or pale nodules needs a closer look.

If nodulation is poor, check the soil pH, the molybdenum status, the boron status and the inoculant history before adding nitrogen. Correcting the cause usually gives a better and longer-lasting response than a nitrogen top-dress, and it supports the legume's own fixation for the rest of the season.

Crop choice and the value of nitrogen fixation

Choosing a legume for the rotation is partly a nitrogen decision. A grain legume grown in a rotation can reduce the nitrogen needed by the following cereal, and the value of that saving is part of the return on the legume. The value depends on the nitrogen price, the amount of nitrogen the legume fixes and the amount it leaves in the residue.

Pulse crops also break pest and disease cycles in cereal systems, which can reduce the cost of control. A grower who values the whole rotation, rather than the legume alone, is more likely to see the benefit of including it.

Nodulation success varies from field to field. A legume grown on a field with no history of the crop may need inoculation, while a field with a recent legume history may already have the right bacteria. Keep a record of the crop history for each field to decide when inoculation is needed.

Common questions

Do all legumes need the same inoculant? No. Rhizobia strains are specific to legume groups. Use the inoculant recommended for the crop, and check its date and storage conditions.

Can nitrogen fertilizer replace nodulation? Not in a good way. High nitrogen can suppress nodulation, and the crop then depends on fertilizer rather than its own nitrogen fixation, which costs more and reduces the benefit of the rotation.

Does a legume leave nitrogen for the next crop? Often yes. Nodulated legumes leave nitrogen in their roots and residues, which the following crop can use. The amount depends on the crop, the residue management and the season.

Recommendations depend on local soil, climate and crop conditions. Follow the product label and local regulations, and consult a qualified agronomist for field decisions.