Soil microbes fuel nutrient cycling

I vividly remember sitting in soil science and plant nutrition classes at my alma mater land grant university, wondering if the conventional agricultural paradigm was really just one long sales pitch. 

Despite the so-called “sustainable agriculture” title in my degree, I spent entire semesters listening to professors talk in circles about soil chemistry panels and endless inputs that supposedly “need” to be added to the soil for crop success. 

Our homework involved meticulous calculations of how many pounds of fertilizer to add to a figurative field-crop scenario. Soil microbiology was only ever mentioned in passing. We covered the nitrogen, phosphorous, and potassium cycles extensively. Yet, the professor never once mentioned the microbiology that fuels these cycles, along with every other natural cycle on Earth.

Fast-forward to five years of working on organic vegetable farms around the country, and I was astonished to realize that even we organic growers have been sold the idea that inputs are absolutely necessary for proper plant nutrition. We can’t farm without manure, fish, fertilizer meals, lime, gypsum, or other fertility inputs, right? 

Ironically, all the latest science tells us that soil microorganisms are in fact the key drivers of soil nutrient cycling on farms and beyond. How can we harness this microbial power for healthier soil and healthier food? Let’s start by digging into the fundamentals of the soil food web.

 

What is the soil food web?

By now, most folks have heard the term “microbiome” circulating in food and health conversations. The trillions of microorganisms in our guts and in our soil are intricately tied to the health of humans, farms, and whole ecosystems. At the very base of our food supply are the microscopic workhorses that make plant life (and all life on Earth) possible: bacteria, fungi, and the lesser known protozoa, nematodes, viruses, and microarthropods. 

Microbiologist Dr. Elaine Ingham coined the term “soil food web” over four decades ago, but her discoveries are only recently coming to the mainstream of ecological agriculture. We always knew microbes were doing something down there in the soil, but most farmers only paid attention when it was something bad. While plant pathogens were receiving all the attention, incomprehensible quantities of soil microbiota were hard at work fueling the nutrient cycles of our crops, just like they have been doing for millennia. 

As I dug into the research on the soil microbiome, I became more and more confused by the conventional soil principles we were taught in school. How did ancient forests and early farmers grow plants without any of these inputs? Is it necessary to mine nutrients from far away or haul industrial agriculture’s byproducts to our farms to give plants the nutrients they need? 

 

Beyond rhizobium

When it comes to soil microbes, perhaps the biggest celebrity amongst farmers is rhizobium. These nitrogen-fixing bacteria form close associations with legumes (crops from the Fabaceae, or bean family). The bacteria feed off of root exudates from leguminous plants and use it to transform atmospheric nitrogen into ammonium. Studies have shown that biological fixation of nitrogen is the primary source of nitrogen in soils across the world. 

But the nutrient cycling power of microbes is not just found in rhizobium. In fact, all natural phosphorus, potassium, and micronutrient cycling is fueled by microorganisms. Mineralization is this process of transforming minerals into plant-available nutrients. Soil scientists and farmers have only touched the tip of the iceberg when it comes to our understanding of soil microbes’ role in plant nutrition. 

 

Microbial middlemen and nutrient cycling

Soil microbes are essentially the middlemen between soil or atmospheric nutrients and plants themselves. Recent research has uncovered how protozoa and nematodes act as the biological grazers of the soil world. As protozoa and nematodes (two less-talked-about soil microbes) eat fungi and bacteria, they transform nitrogen and phosphorus into plant-available forms. Basically, they are eating other microbes, absorbing their nutrients, and excreting the excess into the soil solution of our crop’s root zone. 

This process is comparable to the above-ground food chain between pasture grasses, livestock, and humans. Just like we cannot readily digest the nutrients from grass, plants cannot directly absorb rocks, minerals, and organic matter. They need a middleman. 

A cow eats grass, digests and processes those nutrients (with the help of billions of microbes in their rumen, of course), and then makes that nutrition from the grass available to other organisms when they consume the cow’s milk or meat.

The cow is the mediator between grass and other organisms, just like protozoa and nematodes are the mediators between minerals and plants. Microbes are the mediators of all soil fertility. They make clever swaps with plants that are mutually beneficial. 

Microbes get “cakes and cookies,” called root exudates, from their plant hosts. It’s estimated that plants give away up to 60 percent of their hard-earned sugars from photosynthesis. They send those carbohydrates, proteins and sugars out through the root zone. Amazingly, research shows that plants are continually modifying their exudates to entice the exact microbes they need at any given moment.

They send off their microbial allies to collect and mineralize the nutrients they need, in the quantities they need. This is the complex process of mineralization that happens every second in our soil if we nurture a healthy soil food web.

 

Hardest working farmhands

Needless to say, microbes are powerful. They are the hardest working farmhands you’ll ever find. Understanding and nurturing the soil food web is essential for any grower interested in cutting down on fertilizer costs, improving plant health, and growing more nutrient-dense, microbiome-friendly food. 

Soil microbiologists like Ingham have demonstrated that fertilizer costs can be cut by 100 percent — completely eliminated — simply by nurturing the complex below ground soil food web. This is how Mother Nature has been growing plants without fertilizers for millions of years.

With all the focus on the potential of soil fungi to sequester carbon and regenerate soil, we have to remember that the benefits of a healthy soil microbiome extend far beyond carbon credits. Even on the smallest scale, restoring the soil food web can profoundly impact our agroecosystems and the humans that eat from them. Soil scientist Nicole Masters is adamant that restoring soil health at any scale — from market garden to industrial — is crucial for our food supply and farm viability into the future. The simplest way to improve soil health is to continuously work to revitalize the soil food web and bring it into balance.

 

Four simple methods to support the soil food web

First, adopting no-till or low-till methods is the first and most vital step toward a healthier soil food web. Tillage destroys soil structure, the habitat for soil microbes. Tillage profoundly disrupts the balance of soil ecology and thus thwarts microbial nutrient cycling. It releases hard-earned carbon back into the atmosphere and creates bacterial blooms of a handful of species, eliminating essential diversity.

Second, the continuous addition of high-quality organic matter is the easiest way to feed soil microbes. Good compost, mulch, cover crops, and leaving roots in soil are excellent ways to add organic matter.

Third, keep the soil covered with living, growing plants as much as possible to maximize root exudates and maintain constant photosynthesis.

Fourth, get biological soil test panels in addition to standard soil tests (which only measure soil chemistry at a snapshot in time). Biological analysis under a microscope will give insights into what is really going on. You can find a directory of labs that do biological soil tests at soilfoodweb.com. Solvita soil tests are another option that can give a fuller picture of the biological health of the soil, not just soil chemistry. 

 

Digestive link

As you can see, microorganisms are the vital mediators of nutrition in our world. In our guts, they digest our food into compounds that our bodies can absorb. On our farms, soil microbes transform minerals into plant-available forms that our crops can use to grow.

In other words, the soil food web is like an external digestive system of the plant. So goes the saying, “feed the soil, not the crop.” Dan Kittredge of Bionutrient Food Association says synthetic and water-soluble fertilizers are like an IV drip, bypassing the plant digestive system entirely. 

This is the perfect metaphor for how our current food system treats our human digestive system as well. In a world reliant on an endless array of vitamins, pills, and fortified processed foods, one must stop to ask: Why is our food losing nutrient density? Why aren’t the endless agricultural inputs (organic or synthetic) truly fixing the nutrient issues in our crops and in our bodies? Could restoration of the soil microbiome be the secret to restoring our gut microbiome function? 

Only time will tell. A major shift in the agricultural and health paradigms is in the works, if only we stop trying to substitute nature’s microbial magicians with endless inputs.

 

Logan Hailey is a farmer, writer, and co-owner of Ramblin Farmers, LLC, a mobile freelance farmhand service. She travels to farms and forests around the country in her off-grid school bus tiny home with her partner and three dogs. As they ramble and farm, they’re searching for a place to put down roots and start a farm of their own.