Wood Vinegar Improves Nutrient Uptake in Citrus Rootstocks
Introduction
Citrus cultivation in arid and semi-arid regions with highly calcareous soils presents significant nutritional challenges for growers. In southern Iran, these soils typically contain high amounts of carbonates, have alkaline soil reactions, and low organic matter content, leading to various nutritional difficulties for citrus trees. Recent research has focused on exploring how organic amendments and chemical fertilizers might interact to affect plant growth and nutrient uptake in such challenging soil conditions.
A team of researchers from Shiraz University in Iran has investigated how the application of wood vinegar (pyroligneous acid) and potassium fertilizer affects soil properties, nutrient availability, and the growth of two common citrus rootstocks: sour orange (Citrus aurantium) and Mexican lime (Citrus aurantifolia). These rootstocks are widely used in southern Iran due to their adaptability to various soil conditions and resistance to alkaline and saline environments.
Experimental Design and Application
The researchers conducted a factorial greenhouse experiment using a completely randomized design. They applied wood vinegar at three different concentrations (0%, 0.2%, and 0.4%) and potassium at two levels (0 and 100 mg kg⁻¹ as K₂SO₄) to soil planted with either sour orange or Mexican lime seedlings.
The experiment was carried out in plastic pots containing 5.0 kg of highly calcareous soil (60% calcium carbonate equivalent) with a sandy texture, slightly alkaline reaction (pH 7.87), and low organic carbon content (1.33%). The wood vinegar, produced from the slow pyrolysis of fruit woods at 400°C under limited oxygen conditions, had an acidic reaction (pH 3.54) and contained various nutrient elements.
For the potassium treatment, the required K₂SO₄ was thoroughly mixed with the soil before planting. The wood vinegar was diluted with water and added to the pots at three intervals: 3, 6, and 9 weeks after rootstock sowing. Soil moisture was maintained near field capacity with distilled water throughout the 90-day growth period.
Key Findings
The application of wood vinegar at 0.2% and 0.4% concentrations significantly decreased soil pH from 7.86 to 7.79 and 7.76, respectively. This pH reduction, though seemingly small, is noteworthy in calcareous soils that have high buffering capacity against pH changes. Similarly, potassium application decreased soil pH from 7.84 to 7.76.
Wood vinegar at 0.4% increased soil electrical conductivity (EC) from 0.40 to 0.44 dS m⁻¹, while potassium application increased EC from 0.38 to 0.45 dS m⁻¹. The increase in soil EC can be attributed to the dissolution of minerals, particularly calcium and magnesium carbonates, and the addition of soluble compounds from both wood vinegar and potassium fertilizer.
The study revealed that wood vinegar increased the availability of calcium, sodium, and manganese in the soil but had no significant effect on the availability of other nutrients. The 0.4% wood vinegar treatment increased soil calcium by 17% compared to the control. Potassium application increased the availability of soil potassium (by 45%), calcium (7%), manganese (4%), and zinc (9%).
The research showed striking differences in how the two rootstocks responded to treatments. Sour orange exhibited greater overall growth than Mexican lime (9.24 vs. 6.90 g pot⁻¹ dry weight). However, wood vinegar application at rates of 0.2% and 0.4% decreased dry weight of sour orange by 12% and 16%, respectively, while having no significant effect on Mexican lime.
The most dramatic effect was observed when potassium and 0.4% wood vinegar were applied together, decreasing the dry weight of sour orange and Mexican lime by 20% and 24%, respectively. This indicates that the combined application of these amendments at higher concentrations may have adverse effects on plant growth.
Implications
This research provides valuable insights for citrus growers dealing with calcareous soils. The findings suggest that rootstock selection should be considered carefully, as sour orange and Mexican lime show different capacities for nutrient uptake and responses to soil amendments. Sour orange appears to have a greater ability to mobilize nutrients from calcareous soil, particularly phosphorus, iron, and manganese, but is more sensitive to wood vinegar application than Mexican lime.
For practical applications, the researchers recommend using lower rates of wood vinegar in calcareous soils with potassium and phosphorus deficiency. They also emphasize that wood vinegar application for citrus plants should be tailored to the specific rootstock variety, as sour orange showed greater sensitivity to wood vinegar than Mexican lime.
The study highlights the complex interactions between organic amendments, chemical fertilizers, soil properties, and plant nutrient uptake mechanisms, underlining the need for careful management of soil amendments in citrus cultivation on calcareous soils.