Wood Ash as a Potassium Fertilizer Alternative Increases Crop Biomass by 3.31 Times

Japanese Study Highlights Promising Potential with Environmental Caveats

According to a recent study conducted in Japan and cited by AgTechNavigator, wood ash significantly boosts plant growth and soil quality, positioning itself as a viable alternative to conventional potassium-based fertilizers.

🔹 Substantial Biomass Increase

Researchers tested the effects of wood ash as a potassium fertilizer in a pot experiment on komatsuna (Japanese mustard spinach). A treatment containing just 1% wood ash resulted in a dramatic increase in potassium uptake—from 123 mg/kg to 559 mg/kg—leading to a 3.31-fold increase in crop biomass.

The above-ground dry weight of the plants rose 2.86 times compared to the control. The study concluded:

“Wood ash application led to a substantial increase in crop biomass, underscoring its potential as an effective potassium fertilizer.”

The wood ash used contained high levels of potassium, measuring 168 g/kg.

🔹 Soil Improvement and pH Enhancement

The treatment also significantly increased the soil solution pH from 4.5 (control) to 7.2–7.4, improving the crop-growing environment.

Key soil benefits observed:

• Increased organic carbon content
• Enhanced microbial activity
• Stimulated nitrogen mineralization

This liming effect was attributed to the presence of calcium and potassium carbonates, improving nutrient availability. In addition to potassium, elevated concentrations of calcium, magnesium, phosphorus, and molybdenum were recorded both in the soil and the plants.

🔹 A Circular Economy Solution

Wood ash is a byproduct of biomass-based energy production and is gaining popularity as a renewable energy source. In the U.S. alone, 3–5 million tonnes of wood ash are generated annually, primarily from pulp and paper mills.

Yet, more than 80% of boiler ash in the U.S. is landfilled. The study notes:

“Rising disposal costs and limited landfill availability have sparked interest in alternative uses for wood ash.”

🔹 Heavy Metals: The Double-Edged Sword

While wood ash improves soil fertility, it also affects heavy metal dynamics in the soil and plant uptake.

Decreased concentrations in soil and crops:

• Zinc (Zn)
• Manganese (Mn)
• Nickel (Ni)
• Cadmium (Cd) — notably reduced from 0.709 mg/kg to 0.057 mg/kg, below Codex standards.

This was primarily attributed to the elevated soil pH, which reduces heavy metal solubility.

Increased concentrations in soil:

• Copper (Cu)
• Chromium (Cr)

However, uptake of these metals by plants remained low, likely due to their low bioavailability in the ash matrix.

Despite these benefits, researchers emphasized the need for caution due to the presence of arsenic, cadmium, copper, chromium, and nickel, which may pose leaching risks.

🔹 A Major Adoption Barrier

The potential contamination with heavy metals remains a key barrier to widespread adoption.

“Although wood ash can enhance soil fertility and crop growth, careful management is essential to mitigate potential risks associated with heavy metal contamination.”

Moreover, reducing the availability of micronutrients like Zn, Mn, Ni, and Cd could unintentionally lead to nutrient deficiencies in plants.

🔹 Final Conclusions and Outlook

The researchers concluded:

“Wood ash holds promise as a sustainable alternative to conventional potassium fertilizers, especially in acidic and potassium-deficient soils.”

However, they stressed the need for:

• Further research to optimize application rates
• Environmental risk mitigation strategies
• Long-term field studies to assess cumulative effects on soil health, crop yield, and environmental sustainability

(Photo: Freepik)