Healthy Soil Can Suppress Plant Pests, to a Point
Healthy soil + happy plants = healthy soil, unhappy pests. It’s a formula that seemingly works as a growing number of America’s farmers are using soil health management systems to improve the function of their growing medium.
“Managing for soil health could become an integral component of pest management, resulting in more resilient and productive cropping systems that provide multiple agronomic and environmental benefits,” according to a University of California publication titled, Managing for Soil Health Can Suppress Pests.
The same 2016 report noted that more work is needed, however. First, what exactly constitutes healthy soil?
“There is no universal definition of what constitutes ‘a healthy soil,” says Dr. Jim Walworth, Associate Head of the University of Arizona Department of Soil, Water, and Environmental Science.
“You almost have to define, or at least delineate, the term before you begin a discussion, and that’s difficult because soil scientists have been struggling with this for decades. And while there are some things most of us would agree on as indicators of healthy soils, there’s still no consensus. One thing the industry could benefit from is some sort of a rating protocol that any soil could be rated or evaluated against on the basis of its health.”
Beware One-Size-Fits-All Thinking
It is Walworth’s contention that there is no such thing as a universal soil and, therefore, no universal definition of how ‘good’ it might be.
“Soil in Arizona doesn’t resemble soil in the Midwest or Back East. It’s based on how much organic matter — carbon-based material — it includes, and that’s related to a couple of things, primarily climate. Go to Pennsylvania where it’s relatively cool with higher rainfall, and you’ll find a lot more organic matter. Out here in the desert, it would be considered good if our soil had 1% organic matter. But while we can’t make all soil look alike, we can manipulate their properties to some degree to try and come up with a soil combination that would be a deterrent to pests — insects, weeds, or diseases.”
And in that process, you may create one problem while trying to solve the other.
“Most people would say a ‘healthy’ plant is more able to resist disease, and that’s generally true, but you also help the pathogens, too,” he says. “Having good plant nutrition from good soil doesn’t necessarily ensure you’ll have fewer pest problems because, if you make conditions good for growing plants, sometimes you make the conditions equally good for some kinds of pests as well.”
Take weeds, for example. With more fertility, crops grow faster but so do the weeds.
“You can’t make conditions good for the crop and not for the weed growing next to it,” Walworth says. “To some degree, this is an unsolvable conundrum. We’ll never be completely free of pests, so we’re always finding compromises — what’s the acceptable amount of pest pressure we can live with? It’s an on-going compromise between how hard you fight those things and how much damage you can accept. We manage the best we can, but we’ll always have pests, just like we’ll always have hail and drought. These things are ever-present in the world of farming.”
Survival of the Fittest Applies to Plants, too
Mary Barbercheck is one of those aggie warriors looking for a better way.
An entomology professor at Penn State College of Agricultural Sciences and a researcher on the impact of agricultural practices on soil biodiversity and function, her work focuses on the biology and ecology of entomopathogenic nematodes and fungi, biological control agents that represent natural mortality factors for soil-dwelling insect pests.
“The healthier the soil, the greater preventive it is for pest attack because of its increased tolerance. Healthy plants are less attractive to insects than are unhealthy plants, those with a nutrient imbalance,” she says.
For example, she has seen plants emulate the wild where the weak animals in the herd were the ones that got attacked first through her experience growing field corn.
“In some parts of the field, there were thistle patches competing with the maize for nutrients … corn that was stunted and yellowed. The grasshopper population ate only the stunted and discolored corn and left the healthy corn alone,” she says.
Although her research is grant-funded from the USDA Organic Research Initiative and, therefore, focused on organic production systems, “the research we do applies to most production systems because they all have to do with best management practices and how those different practices affect pests. Although we target the organic, much of the information we’ve discovered can benefit all growers.”
One of the more common management practices to improve soil health involves reduction of inversion tillage (moldboard, disc, or chisel plow) — an action that disrupts the network of mycorrhizal fungi by reducing soil moisture and living space for soil-dwelling organisms. Increasing organic matter, reducing pesticide use, and rotating crops are other beneficial moves.
Barbercheck concurs that there won’t be a one-size-fits-all equation, because what works in the East for a certain kind of environment and soils probably won’t work out West.
“While there are many beneficial microbes in soil to help protect plants or even change plant physiology or chemistry, it differs by region, even by a specific site within an area, in the kinds of microbes that can survive and do their work in specific soils,” she says.
But there are some basic tenets she and fellow researchers have uncovered.
“There are good management practices that can help any kind of grower, and they include reducing soil disturbances in your system while building up its organic matter as well as having a lot of diversity in your system through cover crops and good crop rotation. These are basic, proven, ways to benefit your soils,” she says.
Working With Nature has Benefits
When pesticides are removed from the system, the insect population generally increases, both beneficial and pest insects. But that doesn’t necessarily mean you have to suffer more crop damage because pests are brought under control by beneficial organisms that live in the soil and by predatory insects.
For over two decades, Gabe and Paul Brown of Brown’s Ranch, a 5,000-acre operation in Bismark, ND, have followed a zero-till philosophy with cover cropping that has allowed them to eliminate use of synthetic fertilizers, fungicides, and pesticides to make their soils healthier.
Advising farmers to hang in there, help is on the way, Barbercheck says:
“We’re making progress and it should go more quickly from here on because of the high tech tools we now have that help us understand the microbial communities and what they can do — just the ability to characterize those communities and how they change with different environments and different production practices. We can now use technology to identify the soil’s DNA genetic material, then we can study the effects of whatever is in the soil and how plants and practices help determine who is living there. The soil microbial community is a whole new field of research that’s starting to explode.”