A farmer’s choice of inoculant largely depends on personal experience and product availability, relative cost and perceived efficacy. But there are situations where one inoculant may be a better fit than others. It all comes down to numbers and, like real estate, location, location, location.
If the goal of inoculating legume seed to is to introduce a large number of viable host-specific rhizobia in order to increase infection rates, which ultimately leads to higher yields, then we need to get the live rhizobium as close to the roots as possible so that infection can occur. Why? Because rhizobia are fragile and their movement within the soil is limited.
Inoculants are available in several formulations — seed inoculants (liquid or peat-based powder) and soil inoculants (granular) which can either placed with seed or banded separately in close proximity to the seed. Since movement of rhizobia is very limited in the soil, nodulation requires the roots come into contact with the rhizobium bacteria in the soil.
One of the most common inoculation methods involves treating the seed with a peat-based powder or liquid inoculant before planting. Studies show, however, that some rhizobia applied to seed can die before seeding or shortly after seeding if exposed to seed treatment chemicals, seed coat toxins, dehydration and/or excessive heat. Another choice are the granular inoculants which have been shown to be more tolerant to stressful conditions unfavorable to rhizobium survival or plant growth such as low nutrient status soils, dry or unfavourable moisture conditions or when germination is delayed.
EARLY SEASON CHECK UP
In order to evaluate the performance of an inoculant type or formulation, careful evaluation of nodulation should be done beginning three to four weeks after emergence. Generally, seed-applied inoculant results in nodule formation on the primary root system in the crown area. Soil applied granular inoculants typically produce nodules on both the primary and secondary root systems within the treated layer of soil. Nodules found on secondary roots away from the primary root and crown area are more likely from residual soil rhizobia.
There has been some discussion that nodulation in both the crown and lateral root area is superior to only one location. I think this relates more to the fact that nodulation through the entire root system is likely better than nodules located only in the crown or only on lateral roots. This is likely especially true in fields that have unfavourable surface pH underlain by more favourable subsoil pH. Nodule number, position and function are the main factors to consider. Active and properly functioning nodules should be pink to beef red inside. Brown, green or tan coloration means the nodules are not actively fixing nitrogen for the plant.
INOCULANT FORMULATION BENEFITS
Major advantages of granular inoculants are ease of storage, handling and application. Soil inoculation using granules separates the rhizobia from seed-applied chemicals and seed-coat compounds that can be toxic. Disadvantages are the bulk of the granulars with the high rates of application (five to 10 kg/ha versus 0.25 kg/ha for peat inoculants), the increased transport costs and problems if the granules are not free-flowing.
Each inoculant manufacturer has specific and unique recommendations (compatibility charts) regarding tolerance to a variety of seed treatments and seeding windows following inoculation.
SO WHICH IS BEST?
Good, better, best? It depends. Using peat or liquid based inoculant can increase capacity at seeding by reducing the number of fills needed in a day. Farms with high pH soil may benefit from the close contact of peat-based products. Granular can be tricky as it can bridge, so keep an eye on your tanks. The form of inoculant used seems to impact where nodules form (and the total amount) so perhaps a combination of both seed and soil applied might ensure the best response
The bottom line is the best inoculant product or formulation is specific to your seeding system or field/environment conditions. At the end of the day, proper application is the most important consideration. Choose an inoculant that allows you to do this on your farm with your equipment in a timely manner.
TABLE 1. LIQUID
POLY MER COATED
Least forgiving — not ideal for first time field or hot, dry soils
Good for optimal conditions — warm, moist soils
Limited time between inoculation and seeding
Works well as sticker for peat inoculants
Low cost Some protection for rhizobia
Rhizobia nutrients in carrier
Effective in first time fields
Effective in harsh conditions
Consistent seed coverage can be difficult Resilient formulation — good in harsh conditions
Application rates may be high
May need third tank on seeder
More costly Convenient, no extra tank or handling needed
Cost similar to granular