Nutrient properties

Characteristics of Biochar: Nutrient Properties

Biochar doesn’t just improve soil by changing its structure or water retention—it also contributes nutrients. But how much, and which nutrients, depends heavily on the feedstock and how the biochar was made. This chapter explains what determines the nutrient content of biochar, which parts are actually available to plants, and how production choices—from temperature to post-processing—affect that value. While the science is still evolving, a basic understanding of these factors can help producers and users make smarter decisions.

What Nutrients Does Biochar Contain?

At a basic level, biochar can contain all the essential nutrients plants need: nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg), and various micronutrients like iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu). But it’s not uniform. A biochar made from poultry manure will look very different, nutritionally, from one made from pine bark.

Manure- and biosolids-based biochars tend to be richer in N, P, S, Ca, and Mg. Crop residues and grasses are particularly good sources of K. Woody feedstocks—especially clean, untreated wood—generally have lower nutrient content across the board. These patterns reflect what’s in the feedstock to begin with. For example, animals excrete excess P and K, so their manures are rich in those elements. Grasses accumulate K in their stems. And protein-rich materials, like algae or insects, often carry more N.

Still, pyrolysis changes things. Volatile elements like nitrogen and sulfur tend to be lost as gases at higher temperatures. What remains may be chemically transformed or trapped in forms that are not easily available to plants. That’s why two biochars from the same feedstock can have very different nutrient profiles depending on how they were produced.

Pyrolysis Conditions: What Stays, What Goes

Among all the variables in pyrolysis, temperature is the one that matters most for nutrient content.

As the temperature rises, volatile nutrients (especially N and S) are more likely to be lost as gases. Nitrogen, for example, starts to evaporate above 300°C. For every 100°C increase, total nitrogen content tends to drop by about 0.1%. Some nitrogen remains, especially when it bonds into stable ring structures, but a lot of the easily available N can be lost.

Sulfur behaves similarly: about half of the S in straw can be gone after pyrolysis at 400–600°C. What stays may be locked in compounds that aren’t immediately available to plants.

By contrast, nutrients like P, K, Ca, and often Mg, tend to increase in concentration as temperature rises. That’s not because new nutrients are added, but because much of the feedstock’s mass is lost as volatile matter, concentrating the remaining elements. However, extremely high temperatures (above 800°C) may start to vaporize even these more stable nutrients or bind them in ways that reduce their availability.

Other process variables—heating rate, duration, and reaction atmosphere—seem to have more modest effects. Fast heating tends to reduce overall char yield, but has little impact on nutrient concentration. Long pyrolysis durations don’t change nutrient content much once the target temperature is reached. Minor adjustments in oxygen levels or pressure seem to matter more for surface chemistry than for total nutrient levels.

What’s Available to Plants?

Not all nutrients in biochar are usable by plants. Many are bound in minerals or organic forms that need to dissolve, decompose, or react before becoming bioavailable.

To assess what’s available, researchers extract biochar using water, salt solutions, or mild acids that simulate root zone conditions. In general:

Manure- and biosolids-based biochars have the highest concentrations of available nutrients, especially ammonium (NH₄⁺), extractable phosphorus, and calcium.
Crop and grass residue biochars are rich in available potassium.
Wood biochars, while often lower in nutrients overall, may still provide stable sources of carbon and contribute indirectly to nutrient retention.

The available nutrient content of biochar often tracks with the total nutrient content, but not always. For example, 20% of the total K in biochar is typically available. For micronutrients, the pattern is similar—manure-derived biochars have more available Fe, Zn, Mn, and Cu than those made from plant residues.

It’s worth noting that biochar is not a significant source of nitrate (NO₃⁻). That form of nitrogen is easily lost during pyrolysis. Some NH₄⁺ may be retained, and over time, microbes can convert it into NO₃⁻. But for crops like corn or wheat, which require large nitrogen inputs, the N contribution from biochar is usually small.

Surface Properties and Nutrient Behavior

The surfaces of biochar particles play a key role in nutrient retention and release. Biochar can hold nutrients in several ways:

Ion exchange: Nutrient ions swap places with others on charged surfaces.
Precipitation: Nutrients react to form solid compounds.
Complexation: Nutrients bind to organic molecules on the surface.
Pore trapping: Nutrients settle in the tiny spaces within the biochar structure.

These processes depend on the biochar’s surface area, porosity, and functional groups. Acidic groups (like carboxyls) give biochar negative charge and help attract cations (like K⁺ or Ca²⁺). Basic groups (like amines) or metal oxides can attract anions (like phosphate or sulfate).

Biochar from grasses and manures tends to have more of these functional groups than wood biochar. That makes them better at holding nutrients, especially through ion exchange. Hydrophobic surfaces—like those formed in high-temperature wood biochars—tend to repel water and reduce nutrient leaching, but also limit nutrient exchange.

The internal structure also matters. Nutrients can diffuse into the pores of biochar and bind inside. But not all pores are accessible, and the speed at which nutrients move in and out can vary widely.

Biochar pH and Its Effect on Nutrients

Most biochars are alkaline, especially those made from non-wood feedstocks at high temperatures. That’s because elements like Ca, Mg, K, and Na combine with carbonates and oxides during pyrolysis, raising the pH.

This liming effect can benefit acidic soils by neutralizing excess H⁺. Unlike lime, biochar also adds organic material and ash minerals. And it tends to buffer pH changes over time, helping to stabilize soil conditions.

However, high pH also affects nutrient availability. Some micronutrients (like iron or manganese) become less available at high pH. So while alkaline biochars can improve soil in many ways, they’re not a one-size-fits-all solution.

Can We Design Biochar with Better Nutrient Properties?

This is the big question—and the answer is: not yet, but we’re getting there.

Researchers know that feedstock type and pyrolysis temperature are the main levers for controlling nutrient content. They also know that physical and chemical treatments—before or after pyrolysis—can influence nutrient availability.

Examples include:

Grinding or sieving biochar to increase surface area.
Pre-treatment with acids or bases to change surface charge or remove metals.
Mixing feedstocks with different nutrient contents to create custom blends.
Adding metal salts to control phosphorus behavior or improve sorption properties.
Post-treatment with oxidants like hydrogen peroxide to increase functional groups and cation exchange capacity.

These techniques show promise, but they’re still being tested—mostly at laboratory or pilot scale. Cost, complexity, and environmental impact all need to be considered before scaling up.

What’s clear is that biochar’s nutrient properties are not fixed. They can be shaped—but doing so reliably and affordably across different production systems remains a work in progress.

Final Notes

Biochar brings more to the table than just carbon. Its nutrient content and availability depend on what it’s made from, how it’s made, and what happens to it afterward. For agricultural and environmental users, it offers a source of slow-release nutrients, a way to improve soil buffering, and a tool for managing fertility more sustainably.

But it’s not a fertilizer in the traditional sense. The nutrient contribution is modest unless the feedstock is particularly rich. And while the long-term stability of biochar is one of its strengths, that same stability means that most of its nutrients become available slowly, over time.

As the science improves, and as we gain more control over production variables, it’s likely that biochars with tailored nutrient profiles will become more common. For now, the best approach is to understand the general patterns and match the biochar to the needs of the soil and crop—rather than expect one material to do everything.