Use Pyroligneous Acid At Low Doses For Soil Application

Introduction

As the agricultural sector increasingly seeks environmentally friendly alternatives to synthetic pesticides and fertilizers, pyroligneous acid (PA) has gained attention as a natural option. Also known as wood vinegar, PA is a by-product of charcoal production obtained through the condensation of gases during the pyrolysis process. This ancient agricultural input has been traditionally used for controlling soil and plant pests and diseases, but its effects on soil chemistry remained poorly understood until recently.

A groundbreaking study conducted by researchers from the Universidade Estadual Paulista (UNESP) in Brazil has revealed important insights into how PA applications influence soil chemical properties, fertility, and nutrient movement in soil profiles. The research provides critical data for understanding the environmental implications of using this alternative agricultural input, particularly in clayey Oxisols, a soil type common in tropical and subtropical regions.

Experimental Design and Application

The researchers designed a sophisticated experiment to evaluate how different concentrations of PA affect soil chemistry and nutrient leaching. They constructed detachable soil columns using PVC rings, filled with a clayey Oxisol composed of 226 g kg⁻¹ sand, 223 g kg⁻¹ silt, and 551 g kg⁻¹ clay.

Five concentrations of PA (0%, 1%, 2%, 4%, and 8% v/v) were applied to the soil columns, followed by water infiltration equivalent to 1.5 times the soil's total pore volume 24 hours after treatment. This infiltration simulated irrigation or rainfall conditions, allowing researchers to study potential nutrient leaching.

Key Findings

One of the most significant findings was that higher concentrations of PA (4% and 8%) substantially decreased soil pH in the top 20 cm layer. At 8% concentration, pH dropped from 5.7 to 4.8 in the 0-10 cm layer and from 5.7 to 5.3 in the 10-20 cm layer. This acidification was expected due to the acidic nature of PA (pH 2.9).

Correspondingly, the potential acidity (H+Al) increased significantly in the top soil layers with 4% and 8% PA applications. The application of PA significantly affected nutrient distribution throughout the soil profile. While P typically has low mobility in clayey soils, PA concentrations of 4% and 8% reduced P concentration in the top 20 cm layer. This suggests PA may compete with P for soil adsorption sites or dissolve mineral structures, increasing P movement to deeper layers.

The team collected soil samples from four different depths (0-10, 10-20, 20-30, and 30-40 cm) and analyzed the leachate (water that drained through the soil) to measure changes in pH values, organic matter content, nutrient concentrations (P, K, Ca, Mg, S), total cation exchange capacity, base saturation, and potential acidity.

Higher PA concentrations caused significant decreases in exchangeable cations in the upper soil layers. With 8% PA, K decreased by 58% in the 0-10 cm layer, Ca decreased by 45%, and Mg decreased by 73%. These decreases indicate that PA induced leaching of these nutrients to deeper soil layers.

Analysis of the leachate provided further insights. The leachate pH decreased linearly with increasing PA concentration, though values remained higher than the original PA solutions, suggesting soil buffering effects. Electrical conductivity of the leachate increased with higher PA concentrations, indicating greater ion movement through the soil.

Implications

This research reveals important considerations for agricultural practitioners using pyroligneous acid. PA at concentrations up to 2% caused only minor changes in soil chemistry, suggesting these levels may be suitable for soil applications without significant adverse effects on fertility.

Concentrations of 4% and 8% significantly altered soil chemistry, potentially leading to nutrient losses through leaching. These higher concentrations should be used with caution, particularly in areas prone to heavy rainfall or irrigation. The enhanced mobility of nutrients induced by PA applications may necessitate adjusted fertilization practices to compensate for potential nutrient losses.

Conclusion

The study demonstrates that while pyroligneous acid offers potential benefits as an alternative agricultural input, its application must be carefully managed to prevent adverse effects on soil fertility and the environment. Lower concentrations (up to 2%) appear to be safer, causing minimal disruption to soil chemistry, while higher concentrations significantly alter soil properties and enhance nutrient leaching.

These findings provide valuable guidance for sustainable agriculture practitioners seeking to incorporate PA into their management systems. Further research is warranted to investigate the long-term effects of repeated PA applications and to develop optimal application protocols for different soil types and agricultural practices.

Article based on: Togoro, A. H.; Silva, J. A. S.; Cazetta, J. O. Chemical changes in an oxisol treated with pyroligneous acid. Ciência e Agrotecnologia, 38(2):113-121, 2014.