Soil Inoculants


As planting season starts, some farmers are applying soil bio-inoculants to promote improved plant growth. Dr. Jay Johnson (retired), former OSU fertility specialist, touted inoculating soybeans with Rhizobium bacteria yearly to increase soybeans yields 1-2 bushels. The Rhizobium bacteria increased nitrogen in soybean nodules which improved crop yields. Today, many farmers are experimenting with soil bio-inoculants with variable results. Evaluating and using soil inoculants requires some careful management to be successful.

Underneath a single footprint exists more soil microbes than humans in the world! Soil microbes and plant roots evolved together, feeding each other, and require certain environmental conditions to flourish. Most beneficial soil microbes and plants require well aerated soils with high levels of soil organic matter (SOM). Farmers converting from conventional tillage systems to no-till generally get the most benefit from soil bio-inoculants. Conventional tilled soils may be too wet, lack enough oxygen or be low in SOM to support the soil microbes long-term. Longterm no-till soils may benefit from inoculation but if cover covers have been used regularly, it may not be needed. Once the soil heals itself, most beneficial microbes will come back. Generally, once farmers get the right soil conditions, soil bio-inoculants are only needed for 1-3 years because they reproduce quite rapidly.

There are several types of soil inoculants including Rhizobium bacteria, Arbuscular Mycorrhizae Fungi (AMF) and beneficial bacteria. AMF increase nutrient availability and these products use spores or root fragments to inoculant seed. Unfortunately, with 150-250 AMF species, farmers need to know several things: What AMF species are present, which species do I need (what does each AMF species do), and what AMF species can I buy? In most cases, these are hard questions to answer.

Many companies do not list the AMF species on their label. The easiest AMF species (Rhizophagus) to reproduce are common and does not require soil inoculation. Every gram of soil has 8-90 AMF spores or up to 40,000 AMF spores per pound of healthy soil. A test of AMF inoculant products found that only a few had live AMF spores and those had mainly Rhizophagus species, which are not needed. The Rhizopagus species are numerous, they thrive in tilled soils, are mostly beneficial, but if they become too dominate (which they often do) may reduce crop yields. Regular crop rotation and multi-specie cover crops use reduces Rhizophagus domination and crop yield loss by diversifying AMF species that improve crop yield.

There are several AMF products on the market, so it pays to read the label. Look for AMF species and spore concentration? Sunlight (UV light) and high temperatures reduces spore viability (store bio-inoculants below 550F out of direct sunlight). One product (Valient’s MycoApply EndoPrime) lists 4 AMF species (one species has been proven to increase soybean yields 10%), has a full label, and a money back guarantee IF a test strip is used to verify yields. In a test strip, always plant the control (no inoculant) first, followed by the inoculated seed to avoid microbial contamination. When applying bio-inoculant products as a spray, use a large nozzle size with at least a 50-mesh screen (or no screen) and apply at 55 psi or less. Adding a humic substance or molasses as a food source helps to improve bio-inoculant survivability.

Some farmers are now experimenting with static compost as a seed treatment to improve beneficial aerobic bacteria, actinomycetes, and good soil nematodes. Static compost uses common carbon sources (leaves, sawdust, straw) composted under aerobic (high oxygen) conditions without regular turning. The process takes 60 weeks to complete and is a combination of composting and vermiculture using red wigglers (worms). The red wigglers inoculate the carbon residue with thousands of beneficial species (no AMF species, that is a different process) and slowly decompose the organic material. This bio-inoculant can be applied as a seed treatment (1-2 pounds per acre) and it’s a cheap easy way to reinvigorate a “dead” or unhealthy soil.

Scientist are just now starting to understand some of the benefits of these beneficial soil microbes. Pseudomonas is a gram-negative bacterium known to produce plant hormones that increase plant growth. Pseudomonas also produces antibodies that have anti-fungal characteristics, reducing soil-borne fungal disease organisms like Pythium and Fusarium. Bioinoculants can be used to improve plant and soil health, but the microbes will not thrive unless your management improves the environmental conditions necessary to create a healthy soil.