do synthetic fertilizers kill soil microbes

Gabe Brown is a rancher and farmer from North Dakota who spent decades transitioning his land away from synthetic inputs. He documents what he found in Dirt to Soil, and the picture is striking.

When you add synthetic fertilizers to the ground, he explains, you damage the microbial and fungal communities that exist there. The mechanism is this: when you supply nitrogen in a readily available, soluble form, which is what synthetic fertilizers do, you take away the reason for plants to maintain their relationships with mycorrhizal fungi.

Plants normally trade carbon sugars down through their roots to feed soil fungi, and in exchange those fungi go out into the soil and mine minerals and nutrients that the plant can't access on its own. It's a transaction. It's been running for hundreds of millions of years. When you flood the root zone with soluble fertilizer, the plant essentially says "I don't need the fungi anymore." And the fungi, no longer getting fed, pull back.

Brown puts it this way: you've taken the most productive biological machinery out of your soil and replaced it with a bag from a factory. You get a short-term result, the crop grows, but you've interrupted the living system that makes sustained fertility possible.

Short term, synthetic nitrogen fertilizers can actually increase microbial biomass. Bacteria eat nitrate. It's a nutrient. You add more of it, you get a temporary population boom. This is the data point that conventional agriculture likes to point to.

But long-term studies tell a different story.

A 12-year study tracking the effects of continuous chemical nitrogen fertilizer application found a significant decrease in bacterial diversity, primarily driven by nitrogen-induced soil acidification. The mechanism is straightforward: ammonium-based synthetic nitrogen releases H+ ions as it breaks down in soil, lowering pH year over year. As the soil acidifies, acid-sensitive bacterial taxa decline, community composition shifts, and functional diversity narrows (Zhu et al., Frontiers in Microbiology, 2022). That's not a temporary fluctuation. That's a 12-year trend with measurable, documented consequences for the microbial community you depend on.

A landmark meta-analysis synthesizing 1,235 experimental observations across eight biomes confirmed that synthetic fertilization is one of the global change factors that significantly alters soil microbial community structure and functional capacity — alongside drought and land use change (Ren et al., Nature Communications, 2020). The scale of evidence here is not a single study with a limited sample. It's over a thousand observations across ecosystems worldwide all pointing the same direction.

Microbial diversity matters more than most people realize. Research demonstrates that with hundreds of thousands of taxa per gram of soil, microbial diversity dominates soil biodiversity and is directly and significantly linked to organic matter decomposition — a major process underpinning virtually all ecosystem services the soil provides. Reduced microbial diversity impairs carbon cycling, particularly under elevated nutrient conditions (Wagg et al., Applied and Environmental Microbiology, 2018). In other words, when you narrow the microbial community, you narrow the soil's ability to function.

Fungal diversity matters even more specifically. Bacteria are flexible — they can multiply fast and adapt. Fungi grow slowly, build complex networks over years, and perform functions bacteria simply can't replicate. Mycorrhizal networks are the infrastructure for phosphorus and micronutrient delivery to plants. When synthetic nitrogen eliminates the economic reason for the plant to maintain that partnership, the fungal networks decline. You lose infrastructure that took years to build, and it doesn't come back overnight when you stop using the fertilizer.

It's worth understanding exactly how synthetic nitrogen degrades soil biology, because the mechanism is specific and it explains why the damage accumulates over time even when you're doing everything else right.

The most common synthetic nitrogen fertilizers — urea, ammonium nitrate, ammonium sulfate — all release ammonium ions into the soil solution. Soil bacteria (specifically nitrifying bacteria like Nitrosomonas) convert ammonium to nitrate through nitrification. This process releases hydrogen ions: each molecule of ammonium that gets nitrified generates two H+ ions. Those protons drive down soil pH.

Soil pH is not a cosmetic number. It controls which microbial species can survive, which nutrients are soluble and available to plants, and whether aluminum and manganese reach toxic concentrations in the soil solution. As pH drops from 6.5 toward 5.5 and below, acid-sensitive species begin to disappear. Many beneficial bacteria cannot maintain enzyme function below pH 6. Earthworms abandon acidic soils. The structural diversity of the microbial community collapses toward a narrower set of acid-tolerant organisms.

The Zhu et al. 12-year study documented exactly this process playing out over a decade of fertilizer application. It's not theoretical. It's measured. And it happens even when the farmer sees good yields in the near term, because the yield is being supported by the synthetic input while the underlying soil capital is degrading.

Synthetic fertilizers almost always get applied without a corresponding addition of organic carbon, the carbon-rich organic matter that soil biology needs to function.

Soil biology runs on carbon. Bacteria and fungi and nematodes and earthworms are all processing organic matter as their food source. When you add nitrogen without carbon, which is what synthetic fertilizers do, you create a biological imbalance. The carbon-to-nitrogen ratio in healthy soil should be somewhere around 15 to 25 to one. Pile on synthetic nitrogen without adding organic matter, and you burn through the carbon reserves in the soil faster than they're being replenished.

This is the mechanism behind what I've seen over and over on ground that's been conventionally farmed for years. The soil is gray. It's tight. It doesn't hold water. There are no earthworms. The carbon is gone. And without carbon, the biology has nothing to work with. The fertilizer is keeping crops alive in the short term, but the underlying system is degrading every season.

Albert Howard saw this a century ago. In An Agricultural Testament and in The Soil and Health, he argued that the industrial model of plant nutrition, treating soil like a chemistry problem to be solved with synthetic inputs, was fundamentally misunderstanding what soil is. Soil is a biological system. Its fertility comes from the decay cycle. Feed organic matter in, get minerals out in forms plants can use. Interrupt that cycle, and you get declining fertility that you then have to paper over with more synthetic input. It's a trap.

The Organic Farming Research Foundation points out that the issue is not just fertilizers in isolation, it's the combination of synthetic fertilizers, pesticides, and tillage together. These practices create systematic interruptions in the natural processes that make soil work.

This matters because the research picture gets even clearer when you look at total management systems rather than individual inputs. A soil under no-till management with cover crops and compost shows dramatically different microbial community structure compared to a soil under conventional tillage with synthetic fertilizers — and the difference compounds over years. A meta-analysis of 43 peer-reviewed studies found that no-tillage significantly increases soil bacterial diversity by maintaining soil organic carbon and nitrogen content that tillage destroys (Li et al., Soil and Tillage Research, 2020).

Each disruption — tillage, synthetic nitrogen, pesticide application — pushes the biological community in the same direction: toward lower diversity, lower fungal dominance, and higher dependence on external inputs to maintain production. The combination of all three, which describes most conventional agriculture, creates a system where the soil has progressively less capacity to function without them.

One of the most telling data points is what happens when growers transition off synthetic fertilizers. In the short term, often the first one to three years, yields typically drop. Sometimes significantly. The soil biology has been suppressed for so long that it can't provide the nutrients the crop needs without synthetic help. The farm is addicted to the input.

But here's what Gabe Brown and others who've done this transition show: if you feed the biology, with compost, with cover crops, with organic matter, with leaving residues in place, the soil biology rebuilds. It takes time. It takes patience. But it comes back, and when it does, the soil's natural fertility starts to reassert itself. Yields stabilize and then, in many cases, improve.

That's the decay cycle doing what it has always done. It just needs the conditions to do it.

I don't use synthetic fertilizers. Not at my Houston garden, not at the regenerative agriculture project in Needville, Texas. The reason is not ideological purity, it's practical. I've seen what living soil does when you feed it right. I've seen earthworm populations explode in beds where they used to be absent. I've watched compost turn gray dead dirt into dark, aggregated, alive soil in a single season.

When you use compost as your sole amendment, you are feeding the biology, and the biology feeds the plants. That relationship has been running for 400 million years. It doesn't need synthetic help. It needs organic matter, moisture, and a little bit of patience.

The question isn't really whether synthetic fertilizers kill soil microbes in a binary sense. The better question is this: what does your soil biology look like after five years of synthetic fertilizers versus five years of compost? That answer is clear. And it'll change the way you garden.