Exploring Relationships between soil health and Theobroma cacao
Presented: June 26, 2025
Topics Covered
- Soil Science Fundamentals
- E. Hawai’i Soil Types
- Theobroma cacao plant and root traits
- Applications to E. Hawai’i Soil Types
- What is Soil Health?
- Pest and Disease Mitigation Chart
- Cadmium in Cacao
- Fertilizing
In my own words, soil is the biological, physical, and chemical breakdown of geographically unique bedrock and organic material, in turn forming a living body that creates habitat, supports life, and contributes to nature's beautifully diverse landscapes.
The study of soil in its natural form is known as pedology, which is composed of 3 pillars: soil genesis, morphology, and taxonomy.
The Three Pillars of Pedology
Refers to the formation of soil. Soil scientists represent the development of soil using this equation to include the 5 factors of soil formation:
Soil is a function of climate, organisms, parent material (given bedrock and organic material that weathers to make soil), relief (topography), and time.
The scientific description of soil profiles. Soil scientists have derived standard procedures that analyze soil color, texture, structure, chemical composition, roots, and soil-forming factors, yielding unique horizonation of different soil profiles.
The classification of soil based on their morphological descriptions. USDA Soil scientists have derived a hierarchical classification system for categorizing soils - starting with 12 soil orders (most general, based on formation), all the way down to 25,000 soil series (the most specific, based on climate regimes and diagnostic features).
Soil Classification and Distribution
This is a map of soil orders around the world (although other countries have their own classification systems, these are the U.S types projected globally).
You can see a lot of the U.S. is made up of Mollisols (fertile grassland soils), and Ultisols (highly weathered, acidic soils). Inceptisols are also common globally, since they are recently developed with shallow profiles.
As you can see in the poster, all soil orders look completely different, from their color, to their structure, down to their horizonation.
Web soil survey: Web soil survey is a great application developed by the NRCS. You can get information on any soil type within the states and territories of the U.S and gain access to erosion, chemical, vegetative information after uploading or drawing out an area of interest.
Hawaii Soil Distribution
Most EHCA members are based up Hāmākua and Puna, and as you can see from this soil distribution map, most of Hāmākua is made up of Andisols (volcanic soils), and Puna is made up of Inceptisols, rock, andisols, and Histosols.
Hāmākua - Hilo Soil Series (Hawaii's State Soil!)
Derived from volcanic ash over lava flows, Andisol, rainy side of Mauna Kea. Well-drained soils, acidic, red color, high Iron and Aluminum content, typically 6ft from basalt bedrock, generally rich in OM and nutrients, prime ag land, low in P because it binds with high metal content. Prone to erosion due to high rainfall and sloping topography. Sugarcane plantations have caused soil up Hāmākua to be more compacted, causing issues to agricultural producers in this region.
Puna
Shallow soil profile with lava layers, rich in nutrients due to volcanic ash parent material and histosols (organic soils). Inceptisols (weak profile development) on top of lava flows.
Theobroma Cacao Root Architecture
Understanding the specific traits of plants equips us to better feed them and ultimately boost plant health and productivity. For example, Theobroma cacao has two different types of roots that make up the root architecture:
A network of very fine roots. They are the main collectors of nutrients and water because they have such a high surface area of root-to-soil contact (known as the "Rhizosphere"). The health of the feeder roots can be used as an indicator of soil health.
Grows vertically 0.8-1.5m below laterals. The taproot collects water and nutrients as well, but less than the feeder roots since it has less surface area contact with the soil. They provide structural support to the tree. Issues: compaction preventing taproot growth, waterlogged conditions reducing structural integrity.
Soil Characteristics for Cacao Growth
Hāmākua Soil Characteristics:
Well-drained, good soil structure, low bulk density for root growth, deep soil profiles for taproot. Low Phosphorus (P) since tropical soils are high in metals and those typically bind with P. Hilo soil series is vulnerable to erosion (high rain, slopes), requiring a need for a leaf litter layer to protect topsoil.
Theobroma cacao is also native to the Amazon rainforest, so it is best adapted to environments in which other tree species are growing and depositing leaf litter (Agroforestry systems). Additionally, since the feeder roots and microbes of the cacao tree reside in the topsoil, it is crucial to feed them by leaving organic matter on the soil surface, mimicking the natural environment in which the tree is adapted to, ensuring the tree is fed according to its unique traits.
There is not one source of mulch that would be best, as microbes, like humans, are happiest when fed a variety of food and energy sources. Leaving mulch on the ground also provides friction for when raindrops fall, reducing the splashing onto cacao tree trunks and ultimately reducing risk of rot. Remember to prune to promote some aeration.
Puna Considerations:
Shallow profile, which makes it difficult for taproot to grow. Very important to build soil profile overtime by keeping soil covered with organic matter, planting diverse species, and promoting soil health practices. Remember one of the soil forming factors at the beginning I touched on is time. Improving growing conditions for cacao will take time, as soil develops across the slow geologic time scale, but anything and everything helps.
Adding organic matter and keeping the existing soil profile covered will help feed the microbes, who break down the organic matter and create soil, and also minimize soil erosion of topsoil → contributing to deeper profiles.
Selecting cacao varieties with shorter taproots could also be beneficial.
Understanding Soil Health
Ultimately, soil health is deeply connected to soil biology. While in the past, research was typically centered around soil chemistry and mechanical manipulation of the soil to "boost productivity." However, recent research has revealed that soil biology and the life within the soil accounts for 90% of the soil's productivity.
The Power of Soil Biology
One teaspoon alone of healthy soil can contain up to 1 billion microbes. Imagine the potential for plant and soil health if we feed and invest in those microbes!
They build the soil aggregates (forming soil structure), and good soil structure promotes water drainage, root channel growth, aeration, stores Carbon, etc… Therefore, it is in our best interest to feed them, as investing in the soil ecosystem has been proven to correlate to plant pest and disaster resilience, crop taste, and nutritional value.
Soil Health Practices for Cacao
Lay on topsoil and therefore directly feed into cacao's feeder roots. Other benefits include reduced erosion, increased nutrient cycling, absorption of raindrops. Something to keep in mind is to add a variety of wood and plant sources. Wood provides Carbon and plant/food compost provides Nitrogen. 15:1 is the ideal C:N to feed microbes and promote effective breakdown material.
Theobroma cacao is native to the Amazon, so practicing agroforestry emulates their natural environment and "feeds" the tree how it prefers. CTAHR claims intercropping bananas with cacao can help increase midge (the primary cacao pollinator on Hawai'i) presence, boosting pollination of cacao flowers. Midges lay their eggs in leaf litter, rotten cacao husks, and banana pseudostems/stalks. Banana material will also increase [K+] in soil.
Practicing agroforestry is important because these systems help the retention of cultural and ecological knowledge of diverse species. They also ensure alternate sources of income for the producer. Diverse tree roots will help grow Mycorrhizal fungal networks, essential for nutrient and water uptake.
Practice that focuses on relationships between plants, soil, and organisms to tackle environmental issues from holistic management. Inspired by core beliefs of aboriginals of Tasmania. The usage of "trap crops" to distract pests of the primary crop or to encourage beneficial insects is a common practice.
Great resource from CTAHR:
Replicating the biodiversity and processes that would naturally be in an environment to boost resiliency, ecosystem health, productivity, and minimize unnatural additives. Suggests not importing mulch discusses using bioaccumulator comfrey as mulch. Increases water-holding capacity, soil structure, C:N.
Recipes and more resources can be found at the Natural Farming Hawaii website.
Research Findings
STUDY: Impacts of Shade Trees on the Adjacent Cacao Rhizosphere in a Young Diversified Agroforestry System Fungal diversity as measured by the Shannon index had the greatest influence on dry bean and healthy pod yields, while bacterial diversity and composition had the greatest influence on infected pod yields.
Quick note: A cacao producer in Mountain View who tried nematode injections observed they have helped reduce populations of QLB. More on this: CTAHR
Heavy Metals and Cadmium Concerns
Tropical soils within the cacao belt are high in metals, such as Iron, Aluminum, Arsenic, and Cadmium (Cd), due to the rain binding with weaker charged ions, leaching them down the soil profile. This leaves behind more resistant metals and metalloids listed above.
Cacao trees are known as Cd bioaccumulators. Cd is toxic that can be naturally occurring in the soil profile or introduced through human products such as paints and batteries. Bioaccumulation within the tree follows the order: root > stems > leaves > pod husks > beans.
Mitigation Strategies:
1. Soil amendments: Liming to raise pH and lower metal content has variable effects. Zinc (Zn) products (2+ charge) have been found useful because the charge is the same as Cd (tree takes up Zn, an important micronutrient, instead of Cd).
2. Genetics: Certain varieties bioaccumulate less Cd. The ones that bioaccumulate the greatest amounts are found in Latin America.
3. Fermentation: Helps to lower the Cd levels in beans. Combining beans from varieties of high Cd bioaccumulation with lower Cd helps to lower final Cd levels in chocolate for EU limits.
Current research: Into using biochar to immobilize Cd.
Fertilizer Recommendations
Overall, fertilizing will become less needed as soil health improves. When soil health practices are used, we can slowly eliminate the usage of soil amendments because the microbes become better "fed", productive, and the natural soil ecosystem is being re-built.
If fertilizing is a management preference of yours, here are some recommendations.
UH Hilo lab testing: https://hilo.hawaii.edu/analab/service.php#fn:1
The most important thing is to test both the soil and leaves, and fruit if possible to assess nutrient cycling, identify deficiencies, and fine-tune fertilizer applications.
I would recommend getting the Mehlich test done for micronutrients such as Copper, Zinc, and Manganese as those are typically overlooked in nutrient management plans but are key to crop and human health.
The Extractable Phosphorus test would also be good because as we learned about the Hilo soil series, tropical soils are rich in metal cations that bind with P easily and form organometallic complexes.
Refer to the chart above for P requirements based on extraction test.
Would also not hurt to try the Soil Organic Matter (SOM) test to gauge if more mulch would be needed or not (typically healthy soils have 1-5% SOM).
Once you receive the results back, compare the concentrations to the nutrient requirements for cacao to calculate the rate of fertilizer application.
Some mitigation tips: make sure to check the forecast and not apply fertilizer before rain or during wind. Foliar has been proven successful for cacao. Foliar test after foliar applications have been made. It is recommended to take foliar samples after leaves have flushed.
Mahalo nui for caring about the soil, plants, and organisms. You showing up to learn about soil health is a great step in helping to build resilient island ecosystems and agricultural operations.
Big mahalo to Lynda Carey, Raven Hanna, and Sarah Bharath for meeting with me, showing me their cacao trees, allowing me into the world of cacao, and encouraging me throughout the preparation of this presentation.
I would also like to thank the EHCA Board for inviting me to give this talk!