Does Inoculation Pay?

Most farmers have stopped inoculating soybeans but with todays prices, is it worth considering?

In most cases, if soybeans have been grown in the last 3-5 years, the soil contains rhizobia

(Bradyrhizobium japonicum) bacteria which fix atmospheric nitrogen in their root nodules to

produce soybean seed. Although these soils have rhizobia, do you have enough and does it pay

to inoculate every year.

Dr. Jim Beuerline’s Ohio research (2004) showed farmers gain 1.94 soybean bushels per acre

when they inoculated their crops every year. Since inoculant costs $5 to $7 per acre, at $14 per

soybean bushel, that’s a return of 4X-6X your investments. For some farms the yield increase is

2-6 bushel ($28-$84)/acre or a return of 4X-16X. In most cases, inoculating soybeans is an

investment that is at least worth considering! Here are some cases where it really pays.

First, if your soil has been flooded or have standing water for more than 1 week, inoculation can

pay off. On the other hand, if you inoculate, plant your field, and it stays flooded for a week, it

may not pay. In almost all cases, if your soil problems or issues persist, it may not pay.

Soybeans need good soil structure (no compaction) down at least 4 inches. Soils that are

compacted tend to flood and have poor air movement. Rhizobia bacteria need access to

atmospheric N to make nodules.

Soils that are bare and get really hot can have reduced bacteria levels. Bare soils during a hot dry

summer can reach 1400F which kills all soil bacteria. Think back to last year. The air

temperature in June reached above 900F for many days and soil temps could have reached 110-

1300F on bare fields when soybeans were still emerging or had not reached full canopy. Soils

with low soil organic matter and sandy soils tend to benefit from inoculation.

Crop rotation can also affect rhizobia bacteria populations. If soybeans have not been planted for

at least 3-5 years, the rhizobia soil counts decline. If brassica cover crops (radish, kale, rape,

turnips) or buckwheat cover crops have been planted by themselves, often the rhizobia bacteria

population declines. Soils that have eroded have less SOM and may also be low in rhizobia.

If you decide to inoculate your soybeans, check your seed treatments for insecticides and

fungicides to see if they are compatible with rhizobia. Most times it is not an issue, but it is

worth checking. Also check your soil pH. At a pH below 5.5, soybeans struggle because certain

micro-nutrients like molybdenum are not available. On these fields it pays to lime to correct the

soil pH. Rhizobia also need cobalt to make nodules that fix atmospheric N. During the growing

season, pull up a few soybean roots to look for soybean nodules which should be pink inside if

they are healthy. High pink soybean nodules generally translate to higher yields if the soybean

plants are healthy.

Sometimes, rhizobia bacteria can lose their N fixing capacity or simply become less efficient at

the process over time. According to a Manitoba soybean study, bacteria genes exist in large

genetic packages called symbiosis islands that can be detached from the bacteria. The gene pool

can change over time, and the rhizobia may not be quite effective as fixing N for the soybean.

General recommendation for inoculating: Store your inoculant in a cool place (LT 50

0F) and keep out of sunlight. Apply inoculate to seed about 4 hours before planting with a slurry or get it pre-inoculated by a commercial company. Buy fresh inoculate each year. Watch both

insecticide and fungicide soybean seed pre-treatments. Never mix a fungicide or insecticide

together with a rhizobia pre-treatment.

When loading a drill or planter, fill the seed box 3 inches deep and apply inoculate over the seed,

mixing it thoroughly. Continue to add seed in 6-inch layers, treating each layer until the seed box

is filled. Moisten seed slightly to increase the adherence of dry inoculate. Individual seeds need

no more than 3-5% coating of dry material. Liquid materials will usually cover most seed. Liquid

inoculants are generally good for 120 days, sterile peat products 30 days, and planter box

treatments 2-4 days as long as it is cool and out of the sun.

Each plant species needs its own special type of rhizobia inoculant. For cover crops, alfalfa clay

inoculants can last 2 years, red clover clay inoculants 1 year, but peat-based products only 48

hours. For Crimson Clover pre-inoculants (48 hours maximum), Hairy Vetch and Winter Pea

planter box treatments (12-24 hours maximum). Rhizobia inoculation is a good farming and soil health practice and it pays.