Authors: Ben Tyler & Greta Zarro, Unadilla Community Farm
As permaculture principles gain popularity, there is growing interest in “dynamic accumulator” plants and their potential as nutrient catch crops, “chop and drop” mulches, and fodder for home-brew liquid fertilizers. Dynamic accumulators are seen as a promising closed-loop nutrient management solution that converts common weeds into valuable nutrient sources, while reducing the need for purchased fertilizers and soil amendments.
However, up until now the term “dynamic accumulator” has largely existed in the realm of informal research, and in books on gardening and permaculture. This has led many to believe that dynamic accumulation is unproven pseudo-science, even though the accumulation of beneficial nutrients in the context of cover cropping has been extensively researched and accepted as fact. Likewise, the related field of “hyperaccumulator” plants has enjoyed over 40 years of enthusiastic research and discussion in peer-reviewed journals.
The thing is, literally speaking, hyperaccumulation and dynamic accumulation are two terms referring to the same biological process. But whereas the study of hyperaccumulation is specifically focused on the accumulation of toxic heavy metals, dynamic accumulation focuses on the accumulation of beneficial nutrients. In the context of agriculture, “dynamic” refers to the plants’ use of active transport, rather than normal diffusion, to transport a nutrient against the concentration gradient — in other words, to achieve a higher nutrient concentration in the plant than in the surrounding soil.
So, we know that mineral accumulation is real — we just need to establish clear criteria for the identification of dynamic accumulator species, as has already been done for hyper-accumulators. At Unadilla Community Farm in Central New York State, we recently wrapped up a 2-year study aiming to do just that. As first reported here through the Permaculture Research Institute in March 2020, our SARE-funded research set out to define what exactly qualifies as dynamic accumulation and investigate potential applications for these plants. Two seasons later, here are the results.
First, following the framework laid out by Robert Kourik and Dean Brown, the USDA-hosted “Dr. Duke’s phytochemical and ethnobotanical databases” were used to compile peer-reviewed nutrient concentration data across thousands of plant species. Concentration averages were calculated across 20 beneficial nutrients, and dynamic accumulator thresholds of roughly 200% the average were set. “High ppm” values were used, as these correspond with dried plant tissue samples, consistent with hyper-accumulator thresholds. This resulted in a total of 340 plant species that have been shown to achieve nutrient concentrations high enough to qualify as dynamic accumulators. You can view the full list of dynamic accumulators, along with all available peer-reviewed nutrient concentration data, in an easy-to-navigate online tool titled “Dynamic accumulator database and USDA analysis.”
Since the USDA databases receive regular updates as new plant tissue analyses make their way into peer-reviewed journals, the data set relied on for the study of dynamic accumulators is constantly growing. Nutrient concentration averages are constantly changing, too. This illustrates the “dynamic” nature of the USDA databases themselves, and the importance of stable nutrient concentration thresholds to assist in further studies and discussion of dynamic accumulators. The dynamic accumulator database will also need to be regularly updated to reflect the latest information on plant tissue nutrient concentrations and averages, and the dynamic accumulator thresholds themselves should be periodically reviewed as well, as is done in the field of hyper-accumulators.
We’re lucky that the study of hyper-accumulators is far enough along that we can follow in its footsteps. But the use of nutrient thresholds and curated databases hasn’t been perfected yet. For example, in the field of hyper-accumulators, researchers are still facing some challenges, such as the existence of multiple competing sets of thresholds, several databases with conflicting criteria for inclusion, and additional quality control issues such as the use of “spiked” growing medium or contaminated plant tissue samples giving inflated nutrient readings. But the implementation of nutrient thresholds and curated databases of hyper-accumulator species has gone a long way in facilitating the study of these plants. We hope that the creation of the dynamic accumulator database will spur on further study of dynamic accumulators as well.
The second step of our research utilized the dynamic accumulator database to select 6 promising species for 2 years of trials at Unadilla Community Farm: dandelion (T. officinale), lambsquarters (C. album), red clover (T. pratense), redroot amaranth (A. retroflexus), Russian comfrey (S. peregrinum), and stinging nettle (U. dioica). Crop yields and nutrient concentrations in the soil, dried plant tissue, and liquid fertilizer derived from these plants were measured. This data was used to assess the potential of these 6 species for a range of applications, including subsoil nutrient extraction, topsoil nutrient scavenging in buffer strips or fallow beds, home-brew plant-based liquid fertilizer production, and nutrient-rich mulch production (aka “chop and drop” mulch). You can access our full report on the field trials here.
Perhaps most importantly, we found that plant tissue nutrient concentrations are relative to soil nutrient concentrations. Dynamic accumulators are well-suited to extract specific nutrients from fertile soil, but they aren’t going to create nutrition that isn’t there. As shown in our field trials, when grown in poor, unamended soil, all 6 trial crops possessed nutrient concentrations lower than those measured in previous studies. This confirms similar findings made by researchers of hyper-accumulators on the correlation between growing medium and plant tissue concentrations. For this reason, it is helpful to report nutrient concentrations for both plant tissue and the growing medium used. With these two data points, bioaccumulation factors can be calculated, by dividing plant tissue concentrations (in ppm) by “background” concentrations in the soil (also in ppm). It is only by reporting bioaccumulation factors for a plant species across a range of growing conditions and growing media that we can better understand how to effectively use dynamic accumulators in a larger permaculture system.
That said, even when grown in poor, unamended soil, two species surpassed dynamic accumulator thresholds. Dried lambsquarters foliage was found to possess potassium concentrations that exceeded dynamic accumulator thresholds (40,715 ppm), and liquid fertilizer made by steeping lambsquarters foliage in water for 5 days contained the highest potassium concentrations of all the trial crops (903 ppm).
Likewise, Russian comfrey foliage surpassed dynamic accumulator threshold concentrations for both potassium (52,959 ppm) and silicon (513 ppm), with similarly high potassium concentrations found in the resulting liquid fertilizer (889 ppm). This is particularly exciting because, while Russian comfrey has been known to be a dynamic accumulator of potassium, this may be the first study to reveal it’s a dynamic accumulator of silicon as well.
While the other four species studied did not surpass dynamic accumulator thresholds when grown in our field trials, there were some interesting findings. In particular, we found stinging nettle foliage to possess the highest calcium concentration of all trial crops, as well as the highest bioaccumulation factor for calcium. Liquid fertilizer derived from stinging nettle foliage proved to be very nutrient rich, possessing the highest concentrations of P, B, Ca, Cu, and Mn after 5 days of steeping compared to all other trial crops, as well as the highest nutrient carryover rates for all of these nutrients plus K and Mg, meaning stinging nettle’s nutrients are particularly soluble and well suited for liquid fertilizer.
Chopping and dropping with stinging nettle also produced some exciting results. Calcium concentrations more than doubled in the 0-6” and 6-12” soil horizons, while dropping to 63% in the 12-24” soil horizon. This is consistent with the widely held belief that dynamic accumulators enrich the topsoil by extracting nutrients from the subsoil. Overall, stinging nettle proved to be very well suited to virtually every aspect of these field trials: it thrived under low-maintenance food forest growing conditions; formed a thick, weed-suppressing ground cover; produced large yields of calcium-rich foliage with multiple commercial uses; displayed excellent potential as a source of highly soluble liquid fertilizer; and showed promise as a nutrient-rich mulch as well.
Redroot amaranth (also known as pigweed) is one species you probably don’t want to intentionally plant. But if you already have it growing as a weed, you might want to try brewing some liquid fertilizer out of it. Our trials showed that liquid fertilizer derived from its foliage possessed the highest concentrations and the highest nutrient carryover rates of iron and sulfur compared to all other trial crops. But due to its invasiveness, great care should be taken to harvest before it sets seed.
Dandelion possessed the highest concentrations of phosphorus and sodium of all the trial crops, both in its leaves and in liquid fertilizer made by steeping its foliage in water for 3 days. This isn’t terribly good news. Being an accumulator of phosphorus is a good thing, but while a little sodium has been shown to be beneficial for plants, many growers grapple with excess sodium in their soil. Also, possibly because of its small size and low yields, dandelion didn’t affect the surrounding soil nutrition very much, meaning it probably wouldn’t be very effective as a nutrient catch crop.
Finally, while the sixth species studied, red clover, did not surpass dynamic accumulator thresholds in our field trials, its dried plant tissue foliage did exhibit the highest concentration of iron out of all trial crops, as well as the highest bioaccumulation factor for iron. This makes sense, since red clover has been shown in previous studies to surpass the dynamic accumulator threshold for iron. However, liquid fertilizer derived from red clover did not possess particularly high nutrient concentrations or carryover rates for any nutrient.
As our 2-year study on dynamic accumulators comes to a close, it is clear that more research is needed. Our analysis of Dr. Duke’s databases and calculation of dynamic accumulator thresholds, resulting in the creation of the new dynamic accumulator online tool, help lay the groundwork for further study of these plants. Like cover crops and hyperaccumulators, dynamic accumulators are proven mineral accumulators. But our field trials showed mixed results, with some species surpassing dynamic accumulator thresholds, while other plants did not live up to previously recorded nutrient concentrations. This underscores the importance of soil health in achieving high nutrient content in plants, and the need for more data on bioaccumulation factors when studying dynamic accumulators, not just plant tissue nutrient concentrations alone. Simply put, plants aren’t going to produce something out of nothing, but by utilizing plants that are known to accumulate specific nutrients, we can selectively draw up nutrients that are present in the soil — making dynamic accumulation a valuable tool within a larger permaculture system.
This material is based upon work supported by Sustainable Agriculture Research and Education (SARE) in the National Institute of Food and Agriculture (NIFA), U.S. Department of Agriculture (USDA), under Award No. 2019-38640-29877. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.
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