Mahalo nui to Dan and Rachel of Kulike Farms, who gave me a tour of their permaculture and agroforestry practices in action. I have sprinkled in photos from their site throughout this presentation.

See the HTML version of Part 1 here: https://www.easthawaiicacao.org/workshop/sarah/

What is Soil Biology?

Soil organisms not only facilitate nutrient cycling and availability in the soil, they also influence water availability and retention, increase nutrient stocks, improve disease suppression, and build the soil structure. These abilities all stem from organic matter on the soil surface. Soil organic matter (SOM) improves the soil's capacity to sustain life because it creates habitat, contributes nutrients, and prevents soil erosion.

Soil Organisms

Soil organisms can generally be grouped into 3 categories, the microfauna, mesofauna, and macrofauna. There is a much greater diversity within each of these 3 groups, but I have highlighted the most common organisms on this slide.

Microfauna

Starting with the microfauna, we have bacteria and fungi. Although small, they are not few. In one gram of healthy soil, there can be 1 billion bacteria. These important organisms help break down organic matter to contribute invaluable nutrients into the soil, capture Nitrogen from the atmosphere, and produce antibiotics that we use in modern medicine. Bacteria live in water films and therefore aren't as productive under drought conditions. Other microfauna such as fungi also help with decomposition. Fungi are unique because they can create multicellular branched structures called mycellium, which help the plant access nutrients and water in micropores due to a greater surface area of the rhizosphere. This creates a symbiotic relationship between the plant and fungi - the plant receives nutrients and water, and the fungi receives sugars released from the plant root. This is energetically more favorable for the plant, boosting resiliency of the plant. Given their Amazonian origination, cacao trees actually favor fungal-dominated growing environments.

Mesofauna

Nematodes and protozoa are considered soil mesofauna. There are beneficial and predatory nematodes. Nematodes eat bacteria as part of the soil trophic system, thereby releasing Nitrogen through their waste from the biomass of bacteria. Since nematodes can be predatory, they are commonly used as bio-control agents. We see indigenous nematodes being used, in East Hawaiʻi by the Big Island Invasive Species Committee as a bio-control method for the Queensland Longhorn Beatle. Protozoa also consume bacteria, and therefore also help stimulate Nitrogen release and play an important role in the soil food web.

Macrofauna

Macrofauna are crucial organisms to soil structure, an indicator of soil health. Earthworms are known as soil engineers because they naturally churn soil particles through the creation of their channels, a process known as bioturbation. Their waste is calcium rich, and this serves as a natural binding agent for soil aggregates to form, ultimately reducing compaction and aerating the soil. Insects contribute to the aboveground biodiversity and help pollinate plants. Burrowing mammals also contribute to bioturbation and contribute organic matter to the soil through their waste.

The Rhizosphere

The rhizosphere is "the most active portion of biogeochemical processes…the area around a plant root that is inhabited by a unique population of microorganisms influenced by the chemicals released from plant roots." (McNear Jr., 2013). The rhizosphere is the interface between the root and microbes. When you think about it, this is the direct link between the plant (root) and the soil (microbes).

Specific surface area refers to the total surface area (of the roots) per mass or volume (soil), essentially the quantification of contact the roots have with the immediate surrounding soil. The more root area, the more interactions with soil microbes. Thinking back to the feeder roots of the cacao tree, which are lateral and very fine, the specific surface area of these roots is greater than the tap root, indicating a very high number of biogeochemical reactions.

This interface is distinct from the bulk soil, as this is where we get diverse rhizodeposits (Carbon-rich) secreted from the roots that feed the soil microbes, so we see increased biological activity, nutrient and water exchange, increased pH, and better soil structure here.

Agroforestry

Agroforestry consists of many different designs, including Silvopastural Agroforestry, Alley Cropping, Windbreaks, Forest Farming, Riparian Buffers, and Home Gardens. They all serve various purposes ranging from contributing organic matter, mitigating soil erosion, and encouraging plant productivity (all stimulating microbes!). The Agroforest you design should be tailored to the specific resource concerns you have onsite.

Pictured above are Kulike Farms’ multi-layered Forest Farms with perennial peanut as a ground cover, taro, comfrey, chiya, catook, cinamon, mac nut, jaboticaba, and peach palms.

Agroforestry Fundamentals:

1. Having a diverse aboveground requires diversity below ground
   • Important to foster environments for diverse microbial communities to live in (treating the soil as a living ecosystem!)
2. To achieve species richness, try 10+ plant families. Agroforests have the potential to mimic the species richness of completely untouched forests!
3. Native, canoe, introduced plant species shift microbial relationships from parasitic to symbiotic
   • Won't take advantage of their own kind to compete in any way
4. To select plant families, we want to know what is needed in your system/agroforestry purpose
   • Ultimately, researching trees' functional and physiological characteristics (root architecture, nutrient cycling, microbial interactions, etc…) is extremely important.

Research Evidence

Study: Ngaba, M. J. Y., Mgelwa, A. S., Gurmesa, G. A., Uwiragiye, Y., Zhu, F., Qiu, Q., Fang, Y., Hu, B., & Rennenberg, H. (2023). Meta-analysis unveils differential effects of agroforestry on soil properties in different zonobiomes. Plant and Soil, 496(1–2), 589–607. https://doi.org/10.1007/s11104-023-06385-w

Researchers compiled a meta-analysis on the impacts agroforestry has on physical, biological, and chemical soil properties compared to monoculture operations. A meta-analysis includes results from various independent studies. They are compiled to gain more accurate and reliable conclusions, typically more telling than one individual study. These researchers looked at various climates, including temperate, Mediterranean, and tropical.

Focusing on how agroforestry systems impacted biological soil properties in the tropics, researchers found that nutrient stocks, microbial respiration, and microbial biomass were all greater in agroforestry systems than monoculture systems. Let's focus on just the microbial respiration property. Microbial respiration is quantified as the flux of carbon dioxide from the soil to the atmosphere, measured with lab instruments and procedures. This is generally the most indicative biological soil property of microbial activity, as it measures how much the microbes are breathing. Just as humans produce a lot of carbon dioxide when we exercise, the more carbon dioxide fluxing from the soil, the more productive the microbes are.

Mulch/Soil Organic Matter

The second soil health practice to boost biological productivity is the contribution of mulch or Soil Organic Matter (SOM) to the feeder roots right at the soil surface. Pruned material can be left on the surface to help prevent erosion and recycle nutrients. Previous studies revealed that the productivity of soils can decline by up to 50% as a result of erosion (Diby, L. N.). Leaf litter acts as physical barriers to soil erosion, typically caused by wind or water.

Some discussion was had regarding the heating of county mulch prior to application on the soil. Members found the mulch more activated after the county applied heat.

Hugel Mounds

I learned about this third soil health practice from Dan and Rachel of Kulike farms who have a large hugel mound on their property. A hugel mound is a permaculture solution to highly acidic, compacted, or altered soil that is not ideal for plant growth.

The word Hugel has Germanic origin from the word "hügelkultur" which translates to hill culture. Large mounds/raised beds are made out of foundational wood sources and other natural materials that would otherwise go to waste to create more fertile soils. This is a great idea for folks with Hāmākua sugarcane-impacted soil or in Puna with lava flow layers.

Wood is the foundation of these mounds and is the main Carbon source, enabling microbial activity to take place. A shallow pit can be dug beforehand, if possible. For structural support, you can create contours using branches to stabilize the sides of the mound. "Nail" them using small pieces of wood. Add mulch and compost on top and plant immediately. The organic material laid on top contributes material for the organisms to break down, contributing to soil development and nutrient cycling. This helps raise the pH from being acidic to more neutral/basic. The natural materials also act as a sponge and retain water, overall creating a suitable growing environment for plants.

Here in Hawaiʻi, you can use material from invasive trees (such as waiwi or albizia) to fill/shape the mound. Ensure that the mounds are not engineered at the bottom of a slope, as too much water runoff could cause rotting of the wood.

Permaculture Association has some great resources for practices like this, with videos showcasing the process of creating hugel mounds.

Using Nutrient Deficiencies to Design Agroforests

Now, I'll provide a few examples of how to identify common nutrient deficiencies, and how you can develop a framework to cater your agroforest around your site's individual needs.

The most common nutrient deficiencies in cacao trees in Hawaii include: Nitrogen (N), Phosphorous, Potassium (K), Zinc, Boron, and Iron. Being able to identify these deficiencies correctly can guide you to designing the agroforest/hugel mounds, since you have specific nutrient/purpose in mind.

Nitrogen Deficiency

This picture on the left was taken of a yellowing cacao tree at Kulike Farms. The leaves are experiencing chlorosis. The yellow color indicates a sign of N deficiency since N builds chlorophyll, and gives the leaves its green color.

If you see signs of chlorosis, you can look into leguminous trees that will fix N into the atmosphere such as Acacia family (Acacia koa), Gliricidia species (good forage species), or Inga species (such as ice cream bean). You can contribute organic material from the leguminous species to the topsoil or soak their leaves to create a foliar spray. There are multiple ways to contribute N to the soil, and planting the tree species close to the species with a N deficiency is not the only way. Ice cream bean is known to be a very aggressive grower, so ensuring it has plenty of space in the agroforest is crucial.

Potassium Deficiency

This is a cacao leaf with a K deficiency. The margins of the leaves have deteriorated as if the leaf has been "burned." If you see signs like this, maybe you'll want to include bananas in your plantings to increase K in the soil. Cacao husks tend to accumulate a lot of K, and when harvested, this causes the removal of double the amount of N removed.

Potassium is a critical nutrient for cacao trees because they facilitate root development, enhance photosynthesis and make conversion of sunlight into sugar more efficient for the plant (in turn creating naturally sweeter beans). K also helps increase disease resistance through the activation of enzymes.

CTAHR has a very useful photo album of common cacao disease and nutrient deficiency signs (linked here).

To wrap this up, follow this approach when it comes to other resource concerns, such as erosion. If you are observing soil erosion on your site, research deep rooted trees/grasses (vetiver, lemon grass, pigeon pea) to plant as the understory of your agroforest or along steep banks.

Of course, taking an educated guess at what deficiencies are occurring is great, but testing the soil and leaves could be another layer of security to ensure you have a holistic understanding of the problem at hand.

Nutrient Deficiency Guide

This is a general guide to general signs of nutrient deficiencies in plants. (Note: although signs are similar among plants, signs may slightly differ for cacao leaves, images are of corn leaves).

How Does Soil Biology Improve Cacao Production and Resiliency?

• Leaf litter/soil cover prevents topsoil erosion caused by precipitation and wind, protecting a vital soil layer, thereby conserving nutrients in the subsoil.
• Greater SOM added to the soil introduces more nutrients to the system, improving the nutritional quality of the cacao.
• Deeper and developed microbial networks can tap into nutrient and water reserves deep in the soil profile. This reduces drought severity for the trees.
• Diverse microbial communities bring functional diversity, which strengthens the immunity and responses of the tree to pests and diseases.

Relevant Studies

Allen, S. L., Robayo, L. A., Martin, C. D., & Ganem, J. L. (2024). Productivity, Soil Health, and Tree Diversity in Dynamic Cacao Agroforestry Systems in Ecuador. Land, 13(7), 959. https://doi.org/10.3390/land13070959

Diby, L. N. (2024). Towards a Sustainable Soil Health Management in the West African Cocoa Production System. CABI. https://doi.org/10.1079/soilsciencecases.2024.0001

Feliciano, D., Ledo, A., Hillier, J., & Nayak, D. R. (2018). Which agroforestry options give the greatest soil and above ground carbon benefits in different world regions? Agriculture, Ecosystems & Environment, 254, 117–129. https://doi.org/10.1016/j.agee.2017.11.032

Ngaba, M. J. Y., Mgelwa, A. S., Gurmesa, G. A., Uwiragiye, Y., Zhu, F., Qiu, Q., Fang, Y., Hu, B., & Rennenberg, H. (2023). Meta-analysis unveils differential effects of agroforestry on soil properties in different zonobiomes. Plant and Soil, 496(1–2), 589–607. https://doi.org/10.1007/s11104-023-06385-w

Wang, X., Wang, T., Huang, Y., Liu, A., Li, Q., Wang, Y., Li, M., Fan, F., & Tang, Z. (2024). Effect of biochars on the immobilization and form of Cadmium (Cd) in simulated Cd deposition of iron rich soils. Ecotoxicology and Environmental Safety, 272, 116045. https://doi.org/10.1016/j.ecoenv.2024.116045

References

• Ahenkorah, Y., Halm, B.J., Appiah, M.R., Akrofi, G.S. and Yirenkyi, J.E.K. (1987) Twenty years' results from a shade and fertilizer trial on amazon cocoa (Theobroma cacao) in Ghana. Experimental Agriculture 23(1), 31–39. DOI: 10.1017/S0014479700001101
• Allen, S. L., Robayo, L. A., Martin, C. D., & Ganem, J. L. (2024). Productivity, Soil Health, and Tree Diversity in Dynamic Cacao Agroforestry Systems in Ecuador. Land, 13(7), 959. https://doi.org/10.3390/land13070959
• Chocolate, L. (n.d.). Unlocking Sustainable Cocoa Production: How Agroforestry Systems are Key to Achieve Our Triple Impact Model. Luker Chocolate. Link
• Hugel mounds | Permaculture Association. (n.d.). Link
• Plant nutrient management in Hawaii's soils. (n.d.). https://www.deepdirtcacao.com/plantnutrientmgt.html
• Soil biology. (n.d.). Link
• The Rhizosphere - roots, soil and everything in between | Learn science at Scitable. (n.d.). Link
• Webmaster. (2021, September 6). Nutrients needed in cocoa production. GrandSur. Link