I know many members here use a wide range of bagged soils or include a variety of rock dust (Azomite, basalt, greensand, etc) for trace minerals, each of which contain "high" levels of aluminum and other heavy metals depending on the origin. After recently using Azomite (11% Al) while repotting a few cacti, I stumbled on
this interesting thread discussing the increased bioavailability of aluminum with chelating agent acids. Predominantly in question is humic/fulvic acid (many forms: pure, worm castings, humus, Recharge, etc), but also incorporates other acids like acetic and citric commonly used to lower water pH.
Normally, aluminum is only made bioavailable in a very acidic (<5 pH) soil environment. At that point, there's way more to worry about re: plant health than aluminum toxicity. So what am I talking about then?
Azomite's FAQ contains a question about aluminum content being a concern: "No. The alumina in AZOMITE® is not biologically available. It is bound to the silica and is an aluminosilicate." That's where a chelating agent (fulvic/humic acid) comes into play. According to my understanding, it would break apart the alumina-silicate bond in Azomite and "retain the minerals in a bio-available form for cell penetration or uptake" independent of soil pH.
The foundational chemistry/logic makes sense, but I haven't seen any literature or aluminum toxicity personal stories in my online searches. That helps dispel some of my fears and more towards a belief this interaction is most applicable with excess Azomite repeated over the course of many years. I'm still curious to get input from this knowledgeable community since I used materials in soil and nutrient delivery that contain humic/fulvic acid.
Thoughts?
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ADDENDUM
I reached out to the founder (Zak) of
Big Foot Myco, a company that makes a mycorrhizae fungi product I use regularly that contains Azomite + humic acid, for additional clarity. He has a direct relationship with the manufacturer of Azomite, so he contacted them for more info. Below is their response.
As a side note, Zak shared a few additional thoughts: "Azomite is OMRI certified. I'm sure they take their process very seriously when it comes to the approval process. In the Big Foot products, we use a small percentage of each ingredient, not enough for any type of large scale ph change or heavy metals pollution. Also fun fact, mycorrhizal fungi actually uptake heavy metals and store the metals in their mycelium structure. The fungi can't lose the host plant, so the fungus will do everything in it's power to make sure the plant survives."
Response from Azomite manufacturer:
Per your question, it’s not quite as straight forward as the Humic substances chelating with the “aluminosilicate” alone and precipitating out aluminum molecules that would then be taken up in a singular fashion by plant roots.
AZOMITE falls within the class of materials that are termed, Aluminosilicates. More specifically, AZOMITE is a Hydrated Sodium Calcium Aluminosilicate, and within that, it’s classified as a Dacitic (Rhyolitic) Tuff Brecia.
This means it’s made up of what was once molten igneous rock and other volcanic ash, that spewed into a cold-water body, and caused the molecules to become permanently disarranged, with many nutrients scattered in an amorphous material of highly porous, small volcanic glass particles, within an aluminosilicate connecting structure. In essence, there’s some aluminosilicate component within it, but a lot of other metals and micronutrients that would be adsorbing and chelating or intercalating the Humic substances at the same time, as the aluminosilicate connecting framework.
By comparison, both sand and quartz are aluminosilicates, but they are not nutritive, and more highly compressed crystalline forms, so they would not interact with humics and fulvics the same was as AZOMITE. In essence, Aluminosilicates is a class of materials that vary greatly across the earth. Each mined material in the class should be investigated individually, as they may react differently depending on their other ingredients and physical characteristics.
In the case of AZOMITE, it is likely the humic acids and other weak acids in the soil fraction, along with acids and enzymes from plant roots, beneficial bacteria and mycorrhizal fungi would end up making, Mineral Associated Organic Matter Complexes. (MAOM). This are where microbes are involved both in creating the MOAM, transferring and translocating the nutrients within plant roots, and using MOAM to form the most stable carbon components in soils, and it is likely what contributes to the great growth enhancement of using these products together in growing programs.
To try and answer the question more directly, I’ve included a couple studies on the subject that hopefully shed light on what happens with humics/fulvics and aluminosilicates (in general).
Evidence of humic acid-aluminium‑silicon complexes under controlled conditions
Abstract
The chemistry of silicon (Si), the second most abundant element in soil after oxygen, is not yet fully understood in the soil-water-plant continuum. Although Si is widely accepted as an element that has little or no interaction with natural organic matter, some data seems to show the opposite. To identify a potential interaction between natural organic matter and Si, batch experiments were achieved at various pH and Si concentrations, involving also Al3+ as a common ion in soil and using humic acid (HA) as a typical model for natural organic matter. Several complementary techniques were used to characterize the possible complexes formed in the dissolved or solid phases: molecular fluorescence spectroscopy, 29Si solid-state NMR, Fourier transform infrared spectroscopy, quantification of Si, Al and organic carbon, and nanoparticle size distribution. These tools revealed that humic acid indeed interacts, but weakly, with Si alone. In the presence of Al, however, a ternary complex HA-Al-Si forms, likely with Al as the bridging atom. The presence of Si promotes the maintenance of both Al and dissolved organic matter (DOM) in solution, which is likely to modify the result or the kinetics of pedogenesis. Such complexes can also play a role in the control of Al toxicity towards plants and probably also exists with other metals, such as Fe or Mn, and other metalloids such as As.
The Effect of Dissolved Humic Acids on Aluminosilicate Formation and Associated Carbon Sequestration
Excerpt
At pH 6 HA is soluble, deprotonated, and thus able to complex with Al, which otherwise polymerizes rapidly [12]. Organics are known to inhibit Al polymerization, depending upon the affinity of the ligand for Al [46–48]. Because HA has a strong affinity for Al, polymerization is reduced, and the formation of amorphous Al phases is promoted [49, 50], as indicated by the thermodynamic modeling of the initial compositions of the experimental solutions. As Si alone also reduces Al polymerization, and the number of Al-HA binding sites is limited, Al-(oxy) hydroxide polymerization is further reduced in the presence of both Si and HA.
Precipitates in HA-bearing systems contain a significant portion of organic matter, suggesting insoluble metal-organo complexes. In pH 5-6, Si-free, Al-HA solutions, this can be attributed to chelation, sorption of humics on Al(OH)3 flocs, and/or coprecipitation mechanisms [51]. The addition of Si and subsequent detection in the precipitate suggests that these mechanisms involve aluminosilicate material. Results from experiments with only Si and HA show no independent interaction of these two components. This is because Si prefers to be tetrahedrally coordinated and so forms weak complexes with HA carboxylic functional group oxygens, compared to the stable five-membered chelate rings that readily form with Al [52]. Because Si-HA complexation is negligible, any interaction of Si with HA is likely to proceed indirectly. Thus, the precipitation mechanisms for Si may involve insoluble Al-bridged Si-Al-HA complexes.
Humic Substances and Their Potential for Improving Turfgrass Growth
Excerpt
Application of HA has also been shown to reduce aluminum toxicity to plants by chelating available aluminum and rendering it unable to compete with P uptake, thus increasing P availability in acidic soils [Tan and Binger, 1986). Kreij and Basar [1995) reported that humic substances lowered the uptake of manganese, zinc and copper for several herbs with the response being more pronounced at low pH. Reduced uptake at low pH may be due to increased complexation by humic substances and lower availability in complexed form.
