Dissertation on Potassium Experiment

Potassium (K) is one of the most influential macronutrients in coffee cultivation, influencing processes from photosynthesis and carbohydrate transport to seed filling and aroma formation. The Poma Potassium Experiment (S2E7) conducted on the Centroamericano H1 variety demonstrates how targeted potassium fertigation during the seed development stage alters both the physical and sensory attributes of coffee. This study investigated five potassium application levels (0, 50, 100, 150, and 200 kg K₂O/ha) and measured yield, seed density, and cupping quality. Results revealed a marked improvement in yield and cup score with increasing K supply up to 150 kg K₂O/ha, beyond which diminishing returns occurred. The highest performance was observed at 150 kg K₂O/ha, with seed density of 801 g/L and a Specialty Coffee Association (SCA) score of 89.0. These findings emphasize potassium’s role not only in agronomic productivity but also in enhancing the sensory quality of coffee, suggesting that nutrient management can serve as a deliberate aroma-modulating strategy in specialty production.

1. Introduction

Coffee (Coffea spp.) quality is shaped by a combination of genetic, environmental, and nutritional factors. Among mineral nutrients, potassium (K) holds a crucial role in regulating plant physiology and, consequently, seed development. As the most abundant cation in plant tissues after nitrogen, potassium governs a range of physiological processes. Stomata regulation, enzyme activation, carbohydrate translocation, and osmotic balance—each of which directly impacts yield, seed filling, and metabolic composition.


In the specialty coffee sector, there has been very little focus on understanding how nutrition affects not only productivity but also the sensory characteristics that define cup quality. Traditionally, fertilization strategies have emphasized yield optimization through Nitrogen applications. However, new research indicates that diverse nutrient management, especially the timing and intensity of potassium application among micronutrients such as Zinc, Boron and Manganese, can profoundly influence aroma complexity and intensity.


Building onto our research on foliar nutrition and the effect of micronutrients on coffee physiology and quality, The Potassium Experiment represents a targeted attempt to link potassium fertilization levels to measurable changes in both physical and sensory parameters of coffee seeds. Using the Centroamericano (H1) hybrid—a cross between T5296 and Rume Sudan—the study explored how precise potassium supply during seed development alters density, yield, and aroma expression.

2. Background: Potassium and Coffee Physiology

2.1. Physiological Role of Potassium

Potassium functions primarily as a regulator of metabolic and transport processes within the coffee plant. Unlike nitrogen or phosphorus, potassium does not form structural components of biomolecules but serves as an essential activator of more than 60 enzymatic reactions. It modulates ATP production, carbohydrate metabolism, and photosynthate transport from source leaves to developing fruits.


During the critical seed development stage, adequate K availability facilitates efficient phloem loading and translocation of carbohydrates into the coffee fruit and seeds. This not only promotes seed filling and increases seed density but also contributes to the accumulation of aroma precursors—amino acids, sugars, and organic acids—that define the cup profile.


2.2. Influence on Coffee Quality

Previous agronomic studies have demonstrated that potassium deficiency leads to irregular fruit ripening, low seed density, and a dull cup profile characterized by weak sweetness and muted aromatics. Conversely, balanced potassium supply enhances seed uniformity and improves sensory qualities such as intensity, sweetness, and body.
The physiological connection between potassium and coffee aroma arises from its role in osmotic regulation and carbohydrate mobilization. Enhanced K uptake improves sugar concentration in the seeds, which during roasting undergoes Maillard and caramelization reactions responsible for desirable sweetness and aroma complexity.


2.3. Targeted Fertigation as a Nutrient Management Strategy

Conventional field fertilization often distributes potassium non-uniformly throughout the crop cycle, but fertigation—precisely timed delivery of nutrients through irrigation—offers the advantage of synchronizing nutrient availability with specific phenological stages. In the Potassium Experiment, potassium was administered during the seed development and filling period, aiming to maximize its physiological and sensory impact.

3. Experimental Setup

3.1. Experimental Design

The experiment was conducted on Centroamericano H1 coffee plants cultivated at a simulated altitude of 1,800 meters above sea level in a climate controlled greenhoes in Odense, Denmark, under identical environmental and management conditions. The only experimental variable was the potassium supply during the seed development stage, delivered through water-soluble potassium fertigation.

Five treatments were applied at potassium levels of 0, 50, 100, 150, and 200 kg K₂O/ha. The experiment measured three primary outcomes:

• Yield (kg green coffee per tree)
  
  

• Seed Density (g/L)
  
  

• Cup Quality (SCA score, 0–100 scale)
  
  
  
  

3.2. Analytical and Sensory Evaluation

After harvest, the seeds were processed under our standardized “cold washed” conditions to maintain consistency across treatments. Sensory evaluation followed SCA protocols with a trained expert panel.

Control and fertigated samples were roasted to the same agtron on the same roast curve, and brewed using a standardized mineral water profile designed in collaboration with Apax Lab to ensure consistent extraction and aroma representation.

4. Results

4.1. Quantitative Findings

4.2. Observed Trends


A clear positive correlation was observed between potassium supply and both yield and seed density up to 150 kg K₂O/ha. At this level, plants achieved 2.1 kg green coffee per tree and the highest seed density (801 g/L). Quality scores followed a similar trend, peaking at 89.0 SCA at 150 kg K₂O/ha before slightly declining to 88.5 at 200 kg K₂O/ha.


These data suggest an optimal potassium threshold beyond which additional application yields marginal or even slightly negative returns, likely due to nutrient imbalances or osmotic stress effects at excessive K concentrations.

5. Discussion

5.1. Potassium’s Contribution to Yield and Seed Density

The increase in yield with rising K levels is consistent with potassium’s known role in enhancing photosynthetic efficiency and assimilate transport. By facilitating carbohydrate partitioning to developing fruits, potassium enables more uniform and complete seed filling. This mechanism explains the observed rise in seed density from 744 g/L in the control group to 801 g/L at the optimal K rate.


Dense seeds are typically associated with better structural integrity, uniform roasting behavior, and higher potential for complex aroma development. Therefore, the density increase observed here not only reflects improved physiological health but also serves as a reliable proxy for superior cup quality.

5.2. Sensory and Chemical Implications

The sensory analysis reported distinct qualitative differences between control and potassium-treated samples. The control lot exhibited mellow apricot and caramel tones with a toffee-like finish, reflecting lower sugar accumulation and reduced aromatic complexity. In contrast, the potassium-fertigated lot expressed brighter florals, autumn honey sweetness, and layered fruit notes such as elderflower and cascara, indicating enhanced metabolite diversity and sugar-derived aroma precursors.


These sensory shifts are physiologically consistent with elevated sugar and amino acid synthesis under optimal K nutrition. Potassium’s regulation of phloem loading promotes the transport of photosynthates to developing seeds, enabling higher soluble solids accumulation that translates into perceptible sweetness and complexity after roasting.
Furthermore, potassium’s influence on enzymatic activity may improve the metabolic conditions for secondary metabolite synthesis—terpenes, esters, and phenolic compounds—responsible for floral and fruity aromatic notes in coffee.

5.3. The Diminishing Returns at 200 kg K₂O/ha

The slight decline in both seed density and quality score at the highest potassium level (200 kg K₂O/ha) suggests an upper limit to the beneficial effect of potassium. Excessive K can lead to nutrient antagonism, particularly reducing magnesium and calcium uptake, both essential for chlorophyll stability and cell wall integrity.


Such imbalances can result in marginal reductions in photosynthetic efficiency and metabolic stress, explaining the plateauing of yield and the minor dip in quality beyond 150 kg K₂O/ha. Therefore, precise nutrient calibration is essential emphasizing that “more” potassium does not always necessarily mean “better” quality.

5.4. Implications for Sustainable Coffee Production


Beyond sensory benefits, potassium optimization has sustainability implications. Efficient fertigation ensures that nutrients are applied only when physiologically needed, minimizing waste and reducing environmental runoff. The observed yield gains at moderate K levels imply that farmers can achieve higher productivity per tree without resorting to excessive fertilization, improving both economic and ecological outcomes.

5.5. Practical Recommendations


Field implementation should consider soil testing and localized adjustment, as base K availability, soil texture, and leaching rates can vary widely. Integrating fertigation with periodic leaf nutrient analysis can help maintain potassium within an optimal physiological window, supporting both productivity and cup excellence.


Contact our research and advisory lead Rasmus Madsen at
rama@pomacoffee.com to learn more about how to apply potassium to get the most out of your orchard.

6. Conclusion

The Potassium Experiment (S2E7) provides compelling evidence that potassium plays a dual role in coffee cultivation enhancing both plant physiology and cup quality. Controlled K application during the seed development stage can significantly improve yield, seed density, and sensory attributes.


The study underscores potassium’s fundamental role in carbohydrate transport, seed filling, and aroma compound synthesis, mechanisms that translate into perceptible differences in sweetness, brightness, and aroma complexity. Importantly, it highlights the diminishing returns at excessive K rates, affirming the need for balanced nutrient strategies based on soil and leaf analysis.


As the specialty coffee industry increasingly seeks to fine-tune flavor through agronomic precision, potassium management emerges as a powerful tool. By aligning plant physiology with sensory objectives, producers can move beyond yield-centric fertilization to an era where nutrient management becomes a deliberate, scientific act of aroma creation.