Rhizobium inoculant

A rhizobium inoculant is a carrier material — often finely milled peat, a liquid, or a granule — carrying a high population of live rhizobia selected for effective nitrogen fixation with a particular legume. It is not a chemical fertilizer and carries no guaranteed NPK analysis; its value lies in the number and viability of the bacteria it delivers.

Active component

Living rhizobia (e.g. Rhizobium, Bradyrhizobium species)

Carrier

Peat, liquid, or granular material that keeps bacteria viable

Specificity

Each legume requires a compatible rhizobial strain

Analysis

No NPK grade; effectiveness depends on live bacterial count

Rather than adding nitrogen, the inoculant enables the legume to obtain its own. Compatible rhizobia infect root hairs and form nodules where the enzyme nitrogenase reduces atmospheric nitrogen to ammonia, which the plant assimilates. A well-nodulated legume can meet much of its nitrogen need this way and can leave residual nitrogen for following crops.

• Fixes atmospheric nitrogen into plant-available forms within root nodules
• Reduces or removes the need for applied nitrogen fertilizer on legumes
• Can leave residual soil nitrogen benefiting a subsequent crop

Inoculant is used when establishing legumes, particularly where the crop is new to a field or the compatible rhizobia are not already present, or where a high, reliable fixation is wanted. It is applied to seed just before sowing or placed with the seed, choosing the specific product matched to the legume being grown.

• Most valuable when the compatible rhizobia are absent or scarce in the field
• The inoculant strain must match the legume species being grown
• Applied to seed or in-furrow at sowing, following the label carefully

Because the bacteria are alive, inoculant viability is fragile: heat, direct sunlight, drying, and expiry all reduce the live count and effectiveness, and some seed-applied fungicides or high seed-zone fertilizer salt can harm the bacteria. Fixation is also reduced by high soil nitrogen, extreme pH, drought, and other stresses, so an inoculant is an enabler of fixation, not a guarantee of it.