What if Soil Microbes Mattered?: Our Health Depends on Them by Leo Horrigan was published in August 2025 by the Johns Hopkins Center for a Livable Future (CLF). In his latest book, Horrigan explores the roles that healthy soils play in agricultural systems by zooming into life belowground. Horrigan explains critical processes performed by soil microbes (fungi and bacteria) and the symbiotic relationships such microbes share with plants. Healthy crops, foods, and agricultural systems, Horrigan enumerates, all begin with healthy soils.
Horrigan is a food system correspondent for CLF, where he has worked since 1998 to study the food system in full – from agricultural production to food accessibility. He translates his findings into accessible educational materials for broad audiences, including three films, titled Growing Solutions (2020), Food Frontiers (2016), and Out to Pasture (2010), as well as a book titled What if CAFOs Were History: The Rise of Regenerative Farming (2023).
We sat down with Horrigan to learn more about his most recent book and his motivations for writing it:
Who is the intended audience of What if Soil Microbes Mattered?
“The title What if Soil Microbes Mattered? is a bit provocative because there are people who think they matter a lot,” says Horrigan. At the same time, in the world of mainstream and conventional agriculture, microbes and what they do for plants are less valued, he says.
Horrigan hopes that this book can present the implications of healthy soils and diverse microbial communities in a way that excites a general audience.
“I also have a special desire to reach people in [the field of] public health,” says Horrigan. “It is important for human health to understand how to nurture beneficial microbes and heighten their benefits as much as we can.”
What is the importance of writing about soil microbiology and agricultural practices in a more accessible way?
“For a scientist, sometimes it is hard to write about these topics in a way that can be amenable or digestible to a general audience,” says Horrigan.
Beyond that, Horrigan emphasizes the fact that issues related to soil health, crop health, and agricultural systems affect all humans.
“Farming, whether it is being done in a ‘good’ or ‘bad’ way, is going to have an impact on human health,” says Horrigan.
What motivated you to write this book?
Since before he was a teenager, Horrigan has been interested in gardening and farming. When the opportunity to work at CLF arose over 20 years ago, he jumped at the chance to combine three of his passions: agriculture, health, and the environment. For Horrigan, it is difficult to separate these three spheres; they are interconnected.
Over the years, Horrigan says, he has witnessed and covered disheartening statistics and stories about conventional agriculture. When he began researching regenerative agriculture, Horrigan grew hopeful.
“I was amazed by the possibilities for improving agriculture not just for the sake of agriculture, but for the environment and for our health,” he says.
“In terms of climate change, regenerative agriculture has tremendous opportunity to help mitigate or even reverse climate change,” Horrigan says. “It’s a controversial topic, but I think it’s uncontroversial to say that it can be a big piece of the solution to our climate crisis and that it has been undersold as [such].”
If a regenerative approach to agriculture took hold at a greater scale, what effects might there be?
“Conventional” agriculture, Horrigan says, is actually unconventional. For over 10,000 years, humans have relied upon a more traditional, chemical-free approach to agriculture to sustain themselves. In contrast, conventional agriculture, characterized by chemically intensive practices, has existed for less than a century.
“In a sense, conventional agriculture has been a long-running, uncontrolled experiment,” says Horrigan. “If it were an actual experiment, subjected to an ethical analysis, I think it would have been called off a long time ago because it subjects so many people to unnecessary health risks.”
The promise of regenerative agriculture, says Horrigan, lies in its ability to reduce these risks. “Much more than that, it would make farms more ecologically sound and sustainable,” he says.
Regenerative agriculture applies a more traditional philosophy and minimizes inputs and disturbances to the soil and surrounding ecosystem. In regenerative agriculture, Horrigan says, two things are important: what you do and what you don’t do. The “doing” includes things like applying compost and manure to the soil, rotating crops, or planting cover crops. The “don’t do” involves minimizing tillage and refraining from chemical inputs.
“The biology in the soil is what solves all of the problems that chemicals purport to solve. It suppresses weeds and brings fertility to the plant,” says Horrigan. While plants photosynthesize and provide sugars to microbes, microbes retrieve minerals from the soil and offer them to plants in exchange. This symbiotic relationship has been ongoing for hundreds of millions of years, says Horrigan.
An essential relationship therefore exists between humans and microbes, too.
“I think that humans have to be more humble in the way we look at nature and our place in it,” says Horrigan. “If you accept that the tiniest of creatures are essential to your very existence, it’s hard to think that humans are ‘all that.’ We are dependent on an awful lot of species just to exist.”
If regenerative agriculture is so promising, why has it not been more widely accepted?
According to Horrigan, there are a number of economic, political, and social reasons why regenerative agriculture has not taken greater hold among farmers.
“I think farmers get the message from so many angles that the only way to farm is with chemicals and monocultures,” says Horrigan. “They get that message not just from corporations and input industries, but from academia, government agencies, and neighbors.”
Beyond this, conventional agriculture has become so widely practiced and accepted in the past century that it has become entrenched in society – attracting government subsidies, increasing profits of input producers, accelerating speeds of production, and more. For a farmer, abandoning this status quo can seem daunting and costly.
“It’s not an easy decision for a farmer to change so drastically the way they do things,” says Horrigan. “Their whole livelihood is on the line and it’s not easy to just pivot.”
“Empathy is important. The last thing I want to do is demonize farmers because they are using chemicals – that’s not the way I look at it,” says Horrigan. “Tomorrow’s regenerative farmers are going to be yesterday’s chemical farmers. It’s a matter of changing mindsets and making a big leap to change.”