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Into the Weeds with Soil Balancing – The Ohio State University Soil Balancing Team

By Matthew Kleinhenz

Maybe you remember learning in school about the scientific method – the way that we humans study the world around us (question, hypothesis, test, data analysis, conclusions, retest, etc.).

According to the scientific method, research begins with curiosity and observation, and progresses to a hypothesis (or educated guess) about how and why the world functions. Farmers, researchers, and other professionals who need real answers, continue the process. They look for evidence to support or refute these educated guesses and they create ways to test their ideas. Depending on the situation, this could be a farmer talking to his neighbors for advice, or taking close notes on a new practice; or it could be a more formally designed experiment or survey that involves a team of researchers. The same basic steps apply.

In this article, we would like to take you on a behind-the-scenes journey to see how one university researcher followed the scientific method to reach a new opinion about an old idea.

Asking Questions about Soil Balancing

Doug Doohan is a weed scientist and state extension specialist at The Ohio State University. In the 1980s, he read a book called Weeds – Control Without Poisons. The book’s author, Charles Walters, theorized that weeds flourished on a piece of land in response to conditions of the soil. In this theory, weeds are indicators or even stewards of the soil, flourishing to fill a gap in the farm ecosystem. For example, grasses are more common on compacted soils, and certain weeds with deep taproots grow on soils that need nutrients pulled up from deeper soil zones. At the time, Doohan admits he didn’t give the theory much credence. He figured anything that helped or harmed the crop would affect the weeds similarly.

But in 2007, during a study of organic farmers in Ohio and Indiana, he spoke to a group of 30 farmers in depth. He found about half of the group believed that weeds responded intensely to soil management and particularly to managing the relative levels of calcium (Ca), magnesium (Mg), and potassium (K) in the soil, a practice otherwise known as Base Cation Saturation Ratio or soil balancing.

Another observation Doohan took away from these interviews, and others since, was a perceived connection between standing water in the field and weeds.

“Flooding was mentioned specifically as – not just a contributor – but as a major, if not, the major contributor to weed problems and to the spread of weeds,” he recalls. “At first I thought they were talking about flooding along river banks or stream banks.”

But later he came to suspect that they were talking about ponding of water in the fields. Standing water in fields creates anoxic conditions in the soil that impede plants’ ability to exchange air and water with the soil. It almost always leads to poor crop performance and often death.

These two ideas stuck with Doohan – that soil balancing reduces weeds, and that standing water increases weeds. As he began to learn about soil balancing, the two concepts slowly merged into a possible hypothesis.

Forming a Hypothesis on Soil Balancing

Doohan says he had never heard or read an explanation of how soil balancing would affect weed populations. In fact, past research by the scientific community had mostly dismissed soil balancing as an ineffective practice. But it was mentioned often enough by the farmers in his study that he began to wonder if previous research had missed the mark. Why would farmers repeat a practice that didn’t help? And why would the balance of calcium (Ca) and magnesium (Mg) affect weed populations?

From previous research, Doohan learned there was a theoretical basis and at least some evidence that high levels of Mg in the soil, relative to Ca, result in slower drainage or “tighter” soil. Tipping that ratio toward higher Ca, as soil balancing prescribes, may then result in a “looser” soil with better drainage and water infiltration. Ca also helps build organic matter, which aids in water absorption and retention in the soil.

“As we reduce flooding in the fields, we end up with a healthier crop in many cases – a crop that survives, versus a crop that dies out under flooding conditions – and as a result, fewer weeds,” Doohan explains. “When the crop dies or when the crop is not vigorous, what grows in those areas of the field are weeds, primarily. And we’ve all seen those dead areas in the field that come up in foxtail and other weeds.”

Field comparison - flooding
A low spot in a field early and late in the season. Notice the area by the tree line which is flooded and then weedy.

Once weeds get a foothold in the low spots, Doohan says, they can go to seed and become a problem in the healthier areas of the field as well. He found some observational data suggesting grassy weeds may grow better than broadleaf plants in compacted soils. The fibrous root structure of most grasses can grow without penetrating deep into the soil, lending an advantage over many broadleaf plants with deeper-growing taproots.

“What I think we might eventually learn is that soil balancing doesn’t affect weeds directly, but it affects weed occurrence in the field indirectly, because it reduces the likelihood that we would have these anoxic conditions in some fields. I may well be proven wrong, but it’s the best theory I’ve got,” says Doohan.

Testing & Results in Weed Populations

If Doohan’s hypothesis is correct, field experiments should show a shift in weed populations as a soil comes into the Ca:Mg “balance” prescribed by soil balancers. (Traditional soil balancing recommends a base saturation of 65-70% Ca, 10% Mg, and 5% K.)

Over the past four years, the research team has conducted an experiment at two locations – one on a silt loam soil near Doohan’s research facility in Wooster, Ohio, and a second site in Bowling Green, Ohio, with a higher clay-content soil.

Researchers measure weeds in soil balancing test
Ohio State researchers measuring weeds in a test field.

At the Wooster site, researchers compared soils treated with various common soil amendments to change pH and increase Ca or Mg. Treatments included high-calcium lime (CaCO3), gypsum (CaSO4), epsom (MgSO4), dolomitic lime (CaMg(CO3)2), and a control (no treatment).

At the Bowling Green site, the Ca:Mg ratio indicated that the soil was close to “balanced,” so treatments involved adding Ca to maintain balance or adding Mg to force the soil out of balance. (Researchers used gypsum or Epsom.)

From 2015 to 2018, the team tracked many data points, including soil health and fertility, crop yield, and nutrient content, weed emergence, and weed seedbank counts. The team compared results between the treated and non-treated areas. Although they were eventually able to change the balance of the soils with most of the treatments, as of 2017, there were few significant differences to note in soil or crop health between the different treatments.

The lack of early results was no surprise. The research team had helpful input from an advisory board of farmers and consultants. Committee members and others surveyed or interviewed during the project have repeatedly said the process takes time. Since most previous studies on the effects of soil balancing have lasted two years or less, patience was a key element of Ohio State’s study.

Preliminary results from 2018 suggest that soil balancing treatments are beginning to affect the growth of two weeds, giant and yellow foxtail, which both occurred in higher number where Epsom (high magnesium) had been used.

Foxtail weed counts from soil balance test
Average counts of giant foxtail in 2018 weed seedbank samples from an Ohio State test site.
Yellow foxtail counts in soil balancing test
Average counts of yellow foxtail in 2018 weed seedbank samples from an Ohio State test site.

Similar results have not yet been found on the Bowling Green site. However, this summer, for the first time, project researchers found improved water infiltration in Ca-amended versus Mg-amended soils on the heavier Hoytville clay soil at the Bowling Green site.

Conclusions (so far) Concerning Soil Balancing and Weeds

These are exciting numbers for the team after three years of nothing significant to report, but so far the results are inconclusive and do not yet fully support Doohan’s idea that changes in soil structure lead indirectly to changes in weed populations. And even in time, the results may not support his proposed model. Keeping an open mind is a critical aspect of the scientific method. Doohan freely admits there may be a better hypothesis out there or a completely different set of mechanisms at work that would explain his observations. But it’s been exciting to him to reopen the question of soil balancing and to learn from the experience and on-farm observations of growers.

If the team can find additional evidence that soil balancing improves soil structure and water infiltration, it may add an important and accessible management tool to Extension’s list of recommendations — one many organic farmers in the Midwest already appear to be using, according to the team’s survey research.

Doohan cautions that further study will be needed to gain scientific credibility for the long-dismissed practice of soil balancing. For a hypothesis to graduate to a theory and eventually to recommended practice, the steps of the scientific method will have to be repeated by other researchers. And even then, it may not be appropriate for all situations and soils.

“I’m not saying that balance is not important, but what I am suggesting is that, like farmers everywhere, like human beings everywhere, people are looking for magic bullets; and probably soil balancing is not going to be a magic bullet,” says Doohan.

Regardless of what science has to say about soil balancing, he encourages all farmers to review their management practices for ways to build organic matter, for example, the use of cover crops, manures, and compost. Everyone can agree on the benefits of organic matter: increased water holding capacity, a reserve of nutrients, and increased biological activity.

“As an organic farmer, you’re pretty much restricted to the use of physical methods of weed control. And physical methods of weed control – tillage, and cultivation in particular – tend to disrupt soil structure and lead to reduction in high-quality organic matter in the soil.”

Doohan further recommends some common sense approaches to weed control that can be practiced by all farmers: late season mowing to prevent weeds from seeding, cleaning equipment to prevent spreading weed seeds between fields, use of crop rotations to disrupt weed cycles, and just good general crop establishment.

“Anything we can do to optimize the soil for crop growth will help keep down weed populations.”


Learn more about Ohio State’s Soil Balancing team at go.osu.edu/SB or in the previous EcoFarming Daily article, “Soil Balancing: Worth Another Look.”