Biochar and bacteria increase plant-accessible potassium in acidic soil

A laboratory study has demonstrated that combining rice-straw biochar with potassium-solubilizing bacteria can significantly enhance potassium availability in acidic topsoil. The study found a 52.5% increase in water-soluble potassium and a 49.8% rise in available potassium.

The integrated treatment also improved soil pH, increased organic matter, and reduced potassium leaching into the subsoil. These findings suggest that carbon-based amendments, such as biochar, along with biological products, can help retain nutrients within the crop root zone. However, these results are based on an incubation experiment, and the study is currently a preprint under public discussion and peer review.

The research, led by Saba Babar, was published on EGUsphere, the preprint platform for SOIL, an open-access journal of the European Geosciences Union. The manuscript was submitted on June 1 and entered public discussion on July 17.

In the experiment, researchers placed 850 grams of acidic soil into columns with a 0–15 centimeter topsoil layer and a 15–30 centimeter subsoil layer. Potassium fertilizer was applied only to the top layer to assess retention near the surface and movement downward under irrigation.

Five treatments were tested in four replicates: an untreated control, potassium fertilizer alone, potassium with 1% biochar by soil weight, potassium with a bacterial inoculum, and potassium with both biochar and bacteria.

Compared to potassium fertilizer alone, the combined treatment increased topsoil pH by 13.5% and soil organic matter by 53.9%. Water-soluble potassium rose by 52.5%, and available potassium increased by 49.8%.

The study does not suggest that growers can reduce potash application by the same percentage. Instead, it shows that the combined treatment altered potassium release and retention in the experimental soil.

Potassium in soil exists in several forms. Some are dissolved in soil water or held on exchange sites accessible to plant roots, while a larger portion may be trapped within minerals or fixed in forms not immediately available to crops.

Potassium-solubilizing bacteria release organic acids and other compounds that mobilize potassium from minerals. The study suggests that biochar supports this process by retaining some of the newly solubilized nutrient.

Biochar’s porous structure, surface area, and cation-exchange capacity provide adsorption sites for potassium ions. In the experiment, these properties limited downward movement as bacteria increased potassium solubility.

This interaction is particularly important in acidic soils, which may have limited potassium adsorption capacity. As a result, dissolved potassium can move below the main root zone during irrigation.

The combined treatment increased soil enzyme activity and altered the bacterial community in the topsoil. Researchers observed increases in groups such as Actinomycetota and Bacteroidota, which they linked to improved nutrient cycling and soil biological activity.

These changes indicate that the treatment’s effects extended beyond potassium concentration. Higher soil pH and organic matter may have improved conditions for microbial activity, and the biochar may have provided habitat for the bacterial inoculum.

The researchers concluded that the integrated treatment improved potassium bioavailability by supporting bacterial solubilization and increasing the soil’s capacity to retain the released nutrient.

Potassium is one of agriculture’s three main fertilizer nutrients, along with nitrogen and phosphorus. Improving potassium retention could benefit regions reliant on imported potash and irrigated systems where nutrient loss below the root zone reduces efficiency.

The findings suggest a complementary nutrient-management strategy rather than a direct replacement for mineral potash. Biochar only supplies potassium if the feedstock contains it, while potassium-solubilizing bacteria mainly mobilize existing reserves in soil or mineral amendments.

A commercially viable program would need to consider soil potassium reserves, crop removal rates, biochar composition, inoculant performance, and fertilizer application rates. The economic case depends on whether improved nutrient retention offsets the costs of producing, transporting, and applying biochar and microbial products.

The controlled study design allowed researchers to track potassium at different soil depths but did not include crops, root uptake, or variable weather conditions. Each column contained less than one kilogram of soil, and the 1% biochar rate by weight could require substantial field application depending on incorporation depth and soil bulk density.

Microbial inoculants may perform differently across soil types, climates, and crop systems. Competition with native microorganisms, moisture changes, and storage or application conditions can affect whether introduced bacteria establish and remain active.

Multi-season field trials are needed to determine if increased potassium availability leads to greater crop uptake, higher yields, or reduced potash requirements. Researchers must also establish practical application rates and compare costs with conventional fertilizer programs.

Until further research is completed, these findings demonstrate a potentially beneficial interaction between biochar and potassium-solubilizing bacteria, but do not yet support a recommendation for commercial potassium management.