Know your soil's biological history

If the topic of fungi, bacteria or other microbes comes up regarding crop production, we tend to think of them as something that might harm a crop. We think of pathogens, challenges to plant health and lower grain yield. That’s a natural thought process. From fungicides to varietal resistance and cultural practices, disease management is a top priority. On the flip side, we seldom think about how to impact our beneficial bacteria—but it appears that awareness is growing. And that’s a good evolution.

In 2015, Missouri had over 1.5 million acres of prevented planting. If much thought was given to how 2015 fallow acres impacted the 2016 crop, it was generally centered on keeping weeds under control. Another consideration was how to adjust soil fertility. But there wasn’t a great amount of consideration on what might have happened to beneficial microbe populations. In 2016 we were reminded just how important the beneficial microbes in our soil are, and prevented planting acres from 2015 may still impact cropping decisions in 2017.

There is a common term used by plant pathologists to describe the necessary components for a disease to exist. It’s called the disease triangle. The three sides of the triangle are: 1) the infectious agent; 2) a host plant; and 3) favorable conditions. Without any of the three sides, the triangle is incomplete, and a disease does not advance. The same goes for beneficial microbes. They are infectious agents requiring a host plant and favorable environment. The only difference is they provide benefit to the host rather than harm. In 2015, on prevented planting acres, the host plant was removed. The result, in many cases, was that beneficial microbe populations plummeted. One possible result of reduced microbe populations is a condition called fallow corn syndrome.

Fallow corn syndrome received a lot of attention in 2016. While there is little doubt that fallow corn syndrome existed, to what extent is debatable. Fallow corn syndrome can easily be confused with nutrient deficiencies in the soil, herbicide injuries and common conditions associated with cool, wet weather. Fallow corn syndrome is a phosphorous and zinc deficiency in corn growing in soil that has adequate levels of those nutrients. The plant is deficient of these nutrients due to an inability to retrieve phosphorous and zinc. This failure results from a significantly reduced population of mycorrhizae.

Mycorrhizae microbes work as an extension of the corn’s root system helping to reach and solubilize nutrients that are highly immobile in the soil. Mycorrhizae have the ability to colonize many different plant species and do not necessarily need corn as a host to maintain populations. Planting cover crops in fields that would otherwise lay fallow is one prevention measure, and weeds are a host that can support populations. The most likely locations where fallow corn syndrome may have existed fit some specific parameters. They were non-planted fields where growers actually did a good job of controlling weeds, didn’t plant a cover crop, and planted corn in 2016. The actual impact of fallow corn syndrome depends on how quickly mycorrhizae populations recover. Biological products like Quickroots or Cue (designed to either inoculate seed with mycorrhizae or stimulate growth and reproduction of beneficial microbes) may speed mycorrhizae population recovery.

An issue that may be more common in 2017 than 2016 is reduced soybean nodulation in fields that were fallow in 2015. Like mycorrhizae, rhizobia bacteria, which fixate N for soybean plants, form a symbiotic relationship with the crop. Unlike mycorrhizae, different rhizobia species are require specific plants to survive. Rhizobia japonicum, the species associated with soybeans, will not maintain adequate populations to provide significant nodulation when soybean plants are not present. While inoculating soybean seed annually is an excellent practice, it becomes increasingly important during extended absences of soybeans. Because crop rotations were disrupted with non-planted acres, there are fields this year that may be planted to soybeans for the first time since 2014.

As time pushes on, we tend to forget about the impact some weather deliver and how they can continue to affect our soil. Past cropping practices don’t just have longterm effects on things like the weed seed bank, or soil compaction. They also affect the biological activity of soil. As always we need to remember the needs of this crop year. However, it pays to remember what fields have endured past years and what we expect of them into the future.