Predicting Sandalwood Heartwood Yield: How Soil and Climate Shape Santalum album in Karnataka’s Semi-Arid Alfisols

Author: Dr. B.N. Divakara PhD (Forestry), Scientist - Institute of Wood Science and Technology

Why Heartwood Matters for Sandalwood Growers

Indian sandalwood (Santalum album L.) is famed for its aromatic heartwood, a prized commodity in the global perfume, cosmetic, and carving industries. With heartwood oil containing 66–80% santalol, Indian sandalwood fetches premium prices—often Rs. 7.9–12.5 million per ton—making it a lucrative crop for farmers and a vital export for India. However, heartwood yield is notoriously variable and unpredictable, especially in the semi-arid regions of Karnataka, which holds the largest area under sandalwood cultivation in India.

Understanding what drives heartwood formation—especially the role of soil and environmental factors—can help agronomists and farmers maximize both yield and quality. This article summarizes a comprehensive study conducted by B.N. Diwakara and G.R. Maruthi Sankar, which analyzed how soil and tree parameters interact to predict heartwood development in sandalwood across four agro-climatic zones (ACZs) of Karnataka.

Study Overview: Mapping Sandalwood Variability Across Karnataka

Scope:
The study covered 63 sandalwood plantation sites across four major ACZs (ACZ-3, ACZ-4, ACZ-5, ACZ-6), each with distinct rainfall, temperature, and soil characteristics. Researchers collected soil samples from the top 0–30 cm and measured key tree parameters: height, diameter, heartwood diameter, and heartwood percentage.

Key Soil Parameters Measured:

• pH: Acidity/alkalinity of soil
• Electrical Conductivity (EC): Indicates soil salinity
• Organic Carbon (OC): A proxy for soil fertility
• Nutrients: Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), Sulphur (S), Iron (Fe), Manganese (Mn), Zinc (Zn), Copper (Cu)

Tree Parameters:

• Height (m)
• Diameter (cm)
• Heartwood Diameter (cm)
• Heartwood Percentage (% of total wood)

Key Findings: How Soil and Climate Drive Heartwood Formation

1. Environmental Variability Across Zones

• Rainfall: Ranged from 377 mm (ACZ-3) to 468 mm (ACZ-5), with high variability (coefficient of variation up to 56.9%).
• Temperature: Average 25.2°C, with less variability (CV ~4.4%).
• Tree Age: Averaged 9.5–11.5 years across zones, reflecting plantations at different growth stages.

2. Soil Fertility and Nutrient Status

pH: Mostly neutral (6.5–7.2), but ranged from slightly acidic (5.5) to alkaline (8.7) in some sites.

Organic Carbon: 0.36–0.59%, indicating moderate fertility.

Nutrient Ranges:

• Nitrogen: 127–214 kg/ha
• Phosphorus: 18.7–38 kg/ha
• Potassium: 334–661 kg/ha
• Calcium: 1,428–8,078 ppm
• Magnesium: 309–612 ppm
• Sulphur: 21–103 ppm
• Micronutrients (Fe, Mn, Zn, Cu): Varied widely, reflecting underlying soil diversity.

3. Tree Growth and Heartwood Yields

• Height: 8.2–10.3 m
• Diameter: 8.8–10.4 cm
• Heartwood Diameter: 4.4–5.4 cm
• Heartwood Percentage: 25.1–27.7%

Relationships and Predictors: What Drives Heartwood?

A. Weather-Tree Interactions

• Rainfall: Positively correlated with heartwood percentage in ACZ-5—more rain, more heartwood.
• Temperature: Negatively correlated with heartwood in ACZ-5 and with heartwood diameter in ACZ-6—higher temperatures seem to reduce heartwood formation.

B. Soil-Tree Interactions

• Organic Carbon & Nitrogen: Strong positive correlation across all zones—soils richer in organic matter and nitrogen support better tree growth.
• Calcium & Magnesium: These nutrients often moved together, especially in ACZ-3, ACZ-4, and ACZ-5, indicating similar sources or soil processes.
• Electrical Conductivity (EC): Linked to both calcium and pH, especially in ACZ-3 and ACZ-6, suggesting that soil salinity and nutrient availability are connected.
• Sulphur: Associated with nitrogen, organic carbon, and zinc—highlighting the importance of balanced micronutrient management.
• Tree Parameters: Heartwood percentage was consistently and positively correlated with tree diameter and age, reinforcing the need for patience and good silviculture.

C. Key Takeaways for Farmers

• Bigger, older trees yield more heartwood—but only if soil fertility is maintained.
• Balanced nutrition (especially N, P, K, Ca, Mg, S, and micronutrients) is crucial for maximizing heartwood.
• Rainfall is beneficial, but excessive heat can be detrimental to heartwood formation.

Principal Component Analysis: Simplifying Complexity

What is Principal Component Analysis (PCA)?
PCA is a statistical tool that reduces a large set of variables to a smaller set of “principal components” (PCs) that explain most of the variability in the data. For farmers, this means identifying the most influential soil and tree factors for heartwood production.

Results by Zone:

• ACZ-3: Key drivers were pH, EC, OC, N, P, Ca, S, Fe, and Cu.
• ACZ-4: EC, OC, N, Mg, S, Zn.
• ACZ-5: Ca, Mg, S, OC, N, EC, P, Fe, Mn, Zn.
• ACZ-6: pH, OC, N, P, Ca, Fe, EC, Mg, K, Zn, Mn.

The first principal component in each zone explained the majority of the variance (up to 85% in ACZ-6), meaning that a handful of soil properties can be used to predict heartwood yield with high confidence in most zones.

Predicting Heartwood: Regression Models for Farmers

Regression models were developed to predict heartwood percentage using soil parameters for each ACZ:

Interpretation:
Models for ACZ-3 and ACZ-4 are highly reliable, allowing farmers and extension workers to forecast heartwood yield based on simple soil tests. In ACZ-5, predictions are moderately accurate, while in ACZ-6, other unmeasured factors may be more important.

Practical Recommendations for Sandalwood Growers

1. Regular Soil Testing
Monitor key nutrients (N, P, K, Ca, Mg, S, Fe, Zn, Cu) and maintain soil pH near neutral for optimal tree growth.

2. Focus on Organic Matter
Boost organic carbon through composting or green manures to improve nitrogen supply and overall fertility.

3. Manage Water Wisely
Irrigation or rainwater harvesting can help in low-rainfall years, especially in ACZ-5 where rainfall is closely tied to heartwood yield.

4. Patience Pays
Allow trees to mature—older, larger trees consistently yield more heartwood.

5. Use Predictive Models
Work with local extension services to utilize regression models for your specific zone, optimizing fertilizer and irrigation inputs for maximum heartwood return.

Conclusion: Science-Driven Sandalwood Farming

This study provides a robust, data-driven foundation for predicting and enhancing sandalwood heartwood yields in Karnataka’s semi-arid alfisols. By understanding and managing the complex interplay between soil fertility, climate, and tree growth, farmers can make informed decisions that boost both productivity and profitability. For agronomists, these findings offer a blueprint for targeted soil management and extension advice, paving the way for a sustainable and thriving sandalwood industry.

Further reading

Vetiver Essential Oil Distillation: Methods, Yield, and Quality Factors

Herbs - Spices

How to Grow Vetiver for Oil Production

Patchouli cultivation guide – How to grow Patchouli Commercially for oil

Distillation of patchouli for essential oil – Patchouli essential oil extraction

Commercial Cultivation of Sweet Basil (Ocimum basilicum) for oil production

Distillation of Lemongrass for Essential Oil

This article has been created based on the following research paper:

Assessment of variability and relationships of soil and tree parameters for predicting heartwood of sandalwood under semi-arid alfisols 

B.N.Diwakara¹ and G.R.Maruthi Sankar²

¹Institute of Wood Science and Technology, 18th Cross Road, Malleshwaram, Bengaluru-560003, Karnataka, India.

²ICAR-Central Research Institute for Dryland Agriculture, Santoshnagar, Hyderabad-500059, Telangana, India.