Researchers have demonstrated that soil nutrient density can alter concentrations of some kratom alkaloids, but not all.
Alkaloids are a class of organic compounds found naturally in a variety of plants. Caffeine, for instance, is the alkaloid that provides the energizing properties of coffee. Similarly, researchers believe that select alkaloids are responsible for kratom's effects. But unlike coffee, kratom contains over 40 alkaloids, with concentrations varying from leaf-to-leaf. And as a result, the plant's effects aren't always consistent between users and products. (Todd et al., 2020, pg 8)
Thankfully, recent studies have strived to improve our understanding of kratom alkaloids and how we can maximize their concentrations. The latest was published on December 21st, 2020. In it, established kratom researchers assessed how soil nutrient density affects kratom alkaloid levels.
The researchers began by growing 68 kratom trees from a single mother stock and dividing them into four groups. They then treated each group with varying fertilizer levels as recommended by the manufacturer, with one group receiving none. Finally, the researchers harvested kratom leaves from each group on a bi-weekly basis and extracted their alkaloid contents for analysis.
Only three alkaloids responded to fertilization: speciogynine, corynantheidine, and isocorynantheidine. (Zhang et al., 2020, pg 10) Low-to-medium levels of fertilizer increased levels of all three alkaloids, while extremely low or high amounts of fertilizer decreased concentrations. Many readers might be unfamiliar with these alkaloids: for context, the authors described corynantheidine and isocorynantheidine as "opioid antagonists and a muscle relaxant" (pg 8)
Strangely, fertilization didn't affect mitragynine (pg 7) and 7-hydroxymitragynine (pg 7), two of kratom's primary alkaloids. But the authors theorized that it might not matter since mitragynine might not be a reliable predictor of kratom potency to begin with. "[Other compounds] may play a much more significant role in reported medical effectiveness of kratom than what is currently understood," they stated. (pg 7)
As you might expect, increased fertilization also increased the physical size of the authors' kratom trees. The high fertilization group produced the tallest and widest plants, while the non-fertilized group was the smallest.
Highly-fertilized plants also grew more leaves and "yielded more dry leaf mass overall." (pg 5) As a result, the authors noted that high fertilization might be best for creating kratom extract products. (pg 9). In contrast, low fertilization seemed to be preferential for producing traditional kratom products: each harvest would yield less plant mass, but each leaf would likely have higher alkaloid concentrations overall.
Although the researchers illuminated the relationship between soil nutrient density and alkaloid levels, they emphasized that there's much more work to be done.
"Despite a long history of cultivation in southeast Asia, kratom production practices are largely undocumented and thus confound application of established management practices that aim to maximize plant growth and alkaloid synthesis," wrote the researchers in their conclusion. (pg 7)
"Additional research examining relationships between production environment, plant growth, and alkaloid synthesis is necessary to further understand kratom and to develop reliable, efficient management practices." (pg 7) The authors also stressed that replicating this study's setup on a longer timeline could yield clearer results.
You can read the study in its entirety here.