Biofertilizers: What they are, how they work, and what the research shows

Fertilizer prices keep climbing, regulations on synthetic inputs keep tightening, and the question of what to do about soil health is no longer a fringe concern. Biofertilizers come up in almost every conversation about alternatives, but the term is often misused or misunderstood. Many people assume that if a label says 'bio,' the product must be a biofertilizer, but the prefix covers several different categories of inputs.

This article walks through what biofertilizers actually are, the categories you'll encounter, and what controlled field research has shown when farmers put them to work. The goal is to give you a working understanding without overselling what these products can do.

What biofertilizers are

Biofertilizers are products whose active ingredient is a living microorganism, usually bacteria or fungi, sometimes actinobacteria or algae. The microbe is selected because of how it interacts with plant roots and soil chemistry.

These microbes don't fertilize plants the way urea or DAP does. Instead, they make existing nutrients available. Some fix atmospheric nitrogen and convert it into a form plants can absorb. Others release phosphorus that's locked up in the soil. Others stimulate root development, improve water uptake, or help plants cope with stress.

In the scientific literature, microbes that perform these functions are grouped under the term Plant Growth-Promoting Microorganisms, or PGPMs. You'll see it often once you start reading about this category.

How biofertilizers differ from chemical fertilizers

The two work on completely different principles, and the comparison only makes sense if you understand that they're not interchangeable.

Active ingredient

• Chemical fertilizers: Synthesized nutrient salts (urea, DAP, MOP)
• Biofertilizers: Living microorganisms

Mode of action

• Chemical fertilizers: Direct nutrient supply
• Biofertilizers: Facilitate nutrient access through biological activity

Response time

• Chemical fertilizers: Fast, often within days
• Biofertilizers: Slower, requires a colonization period

Effect on soil biology

• Chemical fertilizers: Generally neutral to negative over time
• Biofertilizers: Adds to and interacts with the existing soil community

Shelf life

• Chemical fertilizers: Long, stable
• Biofertilizers: Limited; viability drops with poor storage

Application rates

• Chemical fertilizers: High (kg/ha)
• Biofertilizers: Low (g/ha to L/ha)

Variability of result

• Chemical fertilizers: Predictable
• Biofertilizers: Depends on soil, climate, and crop conditions

The last row is the one most worth dwelling on. A bag of urea behaves the same whether you apply it in a clay soil in Buenos Aires or a sandy soil in Karnataka. A biofertilizer doesn't. Its performance depends on whether the microbe survives the journey from the factory to your field, whether your soil conditions allow it to colonize, and whether the crop you're growing responds to that particular species or strain.

The main types of biofertilizers

Most products on the market fall into one of these categories.

Nitrogen-fixing biofertilizers. Microorganisms in this group capture atmospheric nitrogen and convert it into ammonia, which plants can use. Rhizobium species form symbiotic associations with legume roots and are the workhorse of soybean, bean, and pea production worldwide. Azotobacter and Azospirillum are free-living or associative nitrogen fixers used in cereals and grasses.

Phosphate-solubilizing biofertilizers. Phosphorus is one of the most common nutrient bottlenecks in agricultural soils, not because it's absent but because much of it is bound to other minerals and unavailable to plants. Certain Bacillus and Pseudomonas species secrete organic acids and enzymes that unlock that phosphorus. They're often paired with nitrogen fixers in commercial formulations.

Potassium- and zinc-solubilizing biofertilizers. A smaller but growing category. The same principle applies: microbes release nutrients from forms the plant can't access directly.

Mycorrhizal fungi. These form physical associations with plant roots, effectively extending the root system. They improve uptake of phosphorus and several micronutrients, and can help plants tolerate drought. Glomus and related genera are the most commercially developed.

Application methods vary. Seed coating is the most common for crops planted from seed. Soil drench, in-furrow application, and foliar sprays are used depending on the product and the stage of the crop.

Why farmers are looking at them

Three forces are converging.

Synthetic fertilizer costs have become unpredictable. Growers who used to absorb price swings now find them harder to plan around, and any input that reduces the absolute amount of urea or DAP needed per hectare is worth a closer look.

Regulation is shifting in the same direction across multiple regions. The European Union's restrictions on certain synthetic inputs and the growing list of residue limits in export markets push producers toward inputs that leave less of a chemical footprint.

And then there's soil. Decades of intensive fertilization have left many production soils with degraded structure, reduced microbial diversity, and declining organic matter. Biofertilizers don't reverse that on their own, but they're part of a broader shift toward managing soil as a living system rather than a substrate.

What the research actually shows

This is the section worth reading carefully, because the gap between marketing claims and peer-reviewed results is usually wider than people expect.

Brazil: soybean inoculation with Bradyrhizobium

Brazil has built the most extensive biological nitrogen fixation program in the world for soybean. Decades of strain selection have produced Bradyrhizobium inoculants efficient enough that the country's soybean crop runs almost entirely on biologically fixed nitrogen, without synthetic nitrogen fertilization. A 2015 study by Chibeba and colleagues tested Brazilian and US Bradyrhizobium strains in Brazil and Mozambique. In Brazil, inoculation produced grain yield gains of 4 to 5% over uninoculated controls. The percentage looks modest, but at the scale of the country's soybean area it represents a significant contribution.

Brazil: maize and wheat with Azospirillum brasilense

The story here is more complicated, and it's worth telling honestly. The strains Ab-V5 and Ab-V6 were authorized as commercial inoculants in Brazil in 2009, after field trials by Hungria and colleagues showed maize yield increases of 24 to 30% in initial experiments. Those numbers are real, and they were the basis for one of the largest biofertilizer markets in the world.

But a meta-analysis published in 2022, covering 60 studies and 103 field trials in 54 locations across Brazil, found an average maize grain yield increase of 5.4%. The response was strongest in lower-yielding fields (around 21% increase when baseline yields were under 3,000 kg/ha) and smallest in already-productive fields (1.5 to 6.2% at higher yield levels).

The takeaway isn't that the early results were wrong. The takeaway is that biofertilizer performance varies with conditions, and a national average tells you more about what to expect than a single trial does.

Colombia: Azotobacter chroococcum on cotton

Colombian researchers at Corpoica tested two strains of A. chroococcum on cotton in a 2017 study published in the Argentinean Journal of Microbiology. The result that gets quoted often is that co-inoculation allowed a 50% reduction in synthetic nitrogen without compromising plant growth.

One detail to keep in mind: this was a greenhouse study, not a field trial. Greenhouse conditions are controlled in ways that field conditions are not, and translating these results to commercial production requires field validation. The result works as a proof of concept, not as evidence of yield performance across multiple seasons and locations.

Should you try biofertilizers?

Before changing anything in your fertilization program, a few practical points.

Know your starting point. A soil test that includes basic chemistry and, if possible, some indicator of biological activity, will tell you whether the input you're considering is solving a problem you actually have. A phosphate-solubilizer makes sense in a phosphorus-limited soil. It will do less in a soil with abundant available phosphorus.

Trial before you commit. Apply the product to a defined plot or strip, keep an untreated control next to it under the same management, and measure the difference. This is the single most useful thing you can do, and it's the step most often skipped.

Pay attention to product handling. Biofertilizers are living products. Storage temperature, exposure to sunlight, expiration dates, and compatibility with other agrochemicals (especially fungicides on seeds) all affect viability. A product that arrives dead at the field will not perform regardless of how good the strain is.

Ask what strain you're buying. Generic labels like "nitrogen-fixing bacteria" or "PGPR consortium" give you no information about what's actually in the bottle. Reputable manufacturers will tell you the species, the strain identifier, and the concentration.

Where this is heading

Biofertilizers are not a replacement for everything chemical fertilizers do, and the research doesn't suggest they will be. What the field is moving toward is integration: using biological inputs to reduce the synthetic load, to extract more from the nutrients already in the soil, and to maintain soil function across years rather than seasons.

The results are real, the variability is real, and both deserve attention.