Pyroligneous Acid and Biochar Cuts Fertilizer Need by 50%
Introduction
In an era of increasing environmental consciousness and sustainable agricultural practices, researchers are exploring alternatives to synthetic chemical inputs that often harm human health and the environment. A promising solution emerges in the form of pyroligneous acid (PA) and biochar, both by-products of the biomass pyrolysis process. A recent study conducted by researchers from Wageningen University & Research in the Netherlands and collaborators from Japan and Canada has revealed the impressive potential of these natural materials for sustainable crop production.
Pyroligneous acid, also known as wood vinegar or wood distillate, is a liquid material with condensed and highly oxygenated organic acids arising from reactions between volatile elements generated during thermal decomposition. It consists of 80-90% water and more than 200 organic compounds, including acids, alcohols, phenols, aldehydes, and esters. In agriculture, PA is recognized as a valuable antimicrobial agent, bio-insecticide, and bio-herbicide due to antioxidant activity provided by its various constituent materials.
Experimental Design and Application
The researchers investigated the effect of combined applications of pyroligneous acid, biochar, and chemical fertilizer on the growth of Komatsuna (Brassica rapa var. perviridis, Japanese mustard spinach) in a greenhouse pot experiment. The experiment examined three factors:
- Two dilution rates of PA (200-fold and 800-fold)
- Four levels of chemical fertilizer rates (100%, 75%, 50%, and 0%)
- Biochar addition (presence or absence)
Pyroligneous acid was derived from Japanese pine wood (Pinus thunbergiana), while biochar was produced from Japanese cedar (Cryptomeria japonica). Both materials were generated through pyrolysis at temperatures between 400-500°C. The PA was diluted with water (either 200-fold or 800-fold dilution) and applied as a one-time soil amendment before planting, with 200 mL of the diluted solution mixed directly into the soil. For the control treatment, the recommended rate of chemical fertilizer for the region was applied. In treatment plots, various percentages of inorganic fertilizer were replaced with biochar and/or PA.
To characterize the chemical profiles of the organic inputs, four spectroscopic measurements were utilized: excitation-emission matrix (EEM), size exclusion chromatography-high-performance liquid chromatography (SEC-HPLC), ion chromatography, and gas chromatography-mass spectrometry (GC-MS).
Key Findings
Chemical Composition of PA
The spectroscopic analyses revealed that the PA contained several beneficial compounds for plant growth:
- Biostimulants: Tryptophan (a well-known biostimulant that enhances nutrient uptake), tyrosine and phenylalanine (aromatic amino acids considered biostimulant materials), and humic-like substances (which can induce ATPase pumps in root tissues, promoting root elongation through an auxin-like effect).
- Organic acids: Acetic acid was the major component, making up 41% of the total PA and 73.8% of all detectable oxide acids. Pyruvic acid (13%), succinic acid (1.1%), and malic acid (0.4%) were also detected.
- Phenolic and aromatic compounds: GC-MS identified acetic acid, 2-hydroxyethyl acetate, cyclopentanone, O-guaiacol, vanillin, 2-methoxy-4-methylphenol (creosol), 2'-hydroxy-5'-methoxyacetophenone, and levoglocosan. Creosol is not only often present in liquid smoke but is also a well-known disinfectant and antiseptic.
Effects on Plant Growth
The application of PA alone at both dilution rates (800-fold and 200-fold) significantly increased plant growth when neither fertilizer nor biochar was applied. This effect was particularly evident under nutrient-deficient conditions, suggesting that PA may trigger enhancement of plant defense mechanisms for stress mitigation via the production of reactive oxygen species involved in secondary metabolism.
Interestingly, the 800-fold PA dilution produced slightly more total fresh biomass than the 200-fold dilution, indicating that high concentrations of PA may negatively impact plant growth compared to lower concentrations.
While biochar application alone did not have a significant impact on biomass, the combination of biochar, 200-fold PA dilution, and 100% chemical fertilizer produced the largest fresh weights among all treatments.
Effects on Plant Nutrition and Soil Properties
Regarding plant leaf nutrients, significant differences were recorded in levels of calcium and phosphorus. The highest concentration of these minerals was observed following treatment with biochar and 800-fold PA dilution without chemical fertilizer. The increased phosphorus content in plant leaves may be attributed to high levels of organic acids in PA, which can solubilize soil phosphorus, releasing usable phosphoric acid for plant uptake.
Biochar application significantly reduced soil sodium content, possibly due to nutrient absorption by the biochar's large surface area and high porosity. This property could be advantageous for remediating soils with high salinity.
Implications
This research offers compelling evidence for the potential of PA and biochar as environmentally friendly alternatives to synthetic fertilizers and pesticides. The study demonstrates that PA not only enhances plant growth but also improves nutrient uptake and potentially stimulates plant defense mechanisms, particularly under stressful conditions such as nutrient deficiency.
For farmers and agricultural practitioners, several application strategies emerge from this research:
- PA application alone (at 800-fold dilution) can significantly boost plant growth in nutrient-deficient conditions
- Combined application of PA with biochar can increase plant nutrient accumulation
- The combination of PA, biochar, and reduced chemical fertilizer rates can produce yields comparable to full fertilizer application
These findings suggest that PA and biochar could play important roles in transitioning toward more sustainable agricultural systems by reducing reliance on synthetic inputs while maintaining or enhancing crop productivity and quality.
Future studies are needed to further explore the physiological processes driving these effects, such as altered carbohydrate metabolism and secondary metabolism, for a better understanding of the underlying mechanisms of PA on plant growth.