Inoculum to be mixed with seed before planting is available on a variety of carriers; the most common carriers are peat. Peat has proven to be better than most other carriers in preserving live bacteria under unfavorable conditions (high temperature, late planting).
Inoculating seed
When inoculating seed, two conditions must be satisfied to get good nodulation:
1. The roots must be in contact with the Rhizobia bacteria, and
2. The Rhizobia must be alive and able to infect the plant root.
For the bacteria to be in contact with the roots of every plant, inoculum should cover each seed. To achieve the best distribution, the inoculum should be mixed with seed in a large space rather than in a planter seed box—on a tarp-covered floor, in a tub, in a cement mixer (paddles removed), or in the bed of a pickup.
Using an adhesive (also known as a sticker) helps the inoculant to adhere to each seed. This is especially important with small-seeded forage legumes, which need more inoculant per unit of seed-surface area. Both commercial and homemade stickers are effective. A homemade sticker can be prepared as a 1-in-10 dilution of syrup or molasses, diluted cola or milk also can be used.
Mix seed with enough stickers to just moisten all seeds. Too much liquid may cause premature germination of the seed. To the moistened seed add inoculant and mix to coat the seeds. Air dry by spreading the coated seed in the shade. Drying may be speeded by adding additional peat-based inoculant or finely ground limestone. The seed must be dry to flow properly through the planter. Calibrate the seeder with inoculated seed when setting desired seeding rate. Seed should be planted as soon as possible after inoculation because bacteria begin to die in the drying process. If not planted within 24 hours, inoculate.
The rate of inoculant to use depends on the amount of time elapsed since the legume was last grown in that field and on the conditions for bacteria survival at the time of planting. Start with the manufacturer's recommendations. If the soil is dry and germination of the seed is expected to be delayed, then a higher rate of inoculant is required to make up for loss of some Rhizobia. For soybeans being planted into a new field, three times the normal rate of inoculant is recommended. A good way to achieve this is to moisten the seed with liquid inoculant applied at the normal rate, then mix seed with twice the normal rate of peat-based inoculant.