As part of our retreat offerings, we provide what we believe is the best Airbnb experience Hawaii can offer. We conduct tours of our orchards and gardens, showcasing all of the bountiful life on our property. We would not be the greatest Airbnb experience Hawaii provides, though, if we were not blessed with such healthy, rich soil. Our soils are rich with indigenous microorganisms that feed our plants and protect them from disease.

But our soils, and soil the world over, is under constant threat. Why? The use of fossil fuels, toxic paints, plastics, and a plethora of other products has led to the accumulation of toxic heavy metals in soil everywhere. This week we explore the nature, effects, and potential remediation of toxic heavy metals.

What Are Toxic Heavy Metals?

A heavy metal is an element that possesses a specific gravity five times that of water. Examples of heavy metals include arsenic, cadmium, iron, lead, chromium, copper, zinc, nickel, and mercury. Plants, animals, and humans require some heavy metals in small quantities. Iron, for example, is critical to the proper delivery of oxygen to the brain and muscles, and zinc and chromium ensure proper adrenal and metabolic function. However, heavy metals can be toxic in high quantities. Heavy metals that we either do not need or need very little of can be toxic in even tiny doses.  When we are incapable of metabolizing and excreting these toxic compounds, they build up in soft tissue.

Heavy Metals in Our Soil

Heavy metals are everywhere—in our food, our air, and even our water. It is not so much the presence of heavy metals that we must be concerned with as agriculturalists, but the concentrations. All heavy metals are present in soil, but increasing levels of heavy metals, specifically lead and arsenic, have rendered our soils more and more toxic in the last century. In fact, most heavy metal ingestion is actually the result of plant consumption. Once the soil contains these compounds, it can be difficult to eliminate them. And they won’t just disappear on their own. Lead, for example, has a half-life of 53,000 years.

The typical culprits of metal-laden soil are many. Lead toxicity is often the result of paint in nearby structures, pesticides comprised of lead that were used in the first half of the 20thcentury, leaded gasoline, and the proximity of both former mines and fuel-fired electrical plants. Culprits of other heavy metal contamination include the use of fossil fuels, which release heavy metals into the air that travel to invade our soils; the use of fertilizers and pesticides containing heavy metals; battery production; toxic sewage; and municipal waste disposal.

Heavy Metals and Growing Food

Different plants will metabolize heavy metals differently. In general, heavy metals are more likely to contaminate green vegetables and root crops. The fruiting parts of plants, such as berries, apples, beans, and squash, are less susceptible to heavy metal toxicity. It is important to keep in mind, though, that heavy metals are present on the outside of some plants as well. Lead dust, for example, will accumulate on the leaves of plants, so you should always clean your greens.

Certain levels of heavy metals are non-permissible for growing food. For example, soil that contains over 1,200 ppm of lead is not considered appropriate for growing food and should only be used for ornamentals. As lead and some other metals typically reside in the topsoil, you may want to test your soil for heavy metals before tilling, which can transfer toxic compounds deep into the soil.

The Health Risks of Heavy Metals

Heavy metals can wreak havoc upon our health and wellbeing. Mercury, lead, and cadmium rank among the most harmful heavy metals—a particularly frightening fact since lead poisoning is the most common form of heavy metal poisoning attributable to soil contamination. When we can’t excrete these compounds, they build up in our kidneys, blood, brain, liver, bones, and tissues.

Different metals are associated with different health conditions. For example, mercury threatens neurological functioning, while cadmium can cause kidney damage and weak bones. Lead is particularly dangerous for children, as both their digestive tracts and blood-brain barriers are highly permeable. Heavy metals even contribute to Alzheimer’s and Parkinson’s disease.

Acute heavy metal poisoning is incredibly dangerous and requires immediate medical attention. But toxic heavy metals can be harmful even in low doses. Most of us face a certain degree of exposure to toxic heavy metals. Indeed, heavy metals are in food, cosmetics, plastics, paints, children’s toys, medications, and more. Chronic, low-level exposure to toxic heavy metals is conducive to digestive complications, exhaustion, depression, painful joints, impaired reproductive health, and poor blood sugar regulation. Those with certain nutrient deficiencies are particularly susceptible to heavy metal toxicity; when we lack certain compounds, heavy metals take their place. Toxicity resulting from heavy metal exposure has become so pervasive that today, some babies are born with over 200 toxic substances, including heavy metals, in their blood.

Heavy Metals and Plants

Heavy metals contained in soil used to grow food crops are detrimental to human health. But they are also incredibly detrimental to the health of the plants themselves. The presence of toxic heavy metals in the soil negatively impacts soil microbiology and biochemistry in many ways. The degree of damage is largely based on soil pH, organic matter, soil temperature, and inorganic anions and cations. In general, toxic heavy metals reduce the health and productivity of plants.

The introduction of these compounds into the soil can significantly reduce the number and diversity of soil microbes. As we explained in last week’s post, these organisms form communication networks with the root systems of plants. Plants communicate their nutritional needs to IMO’s through these networks, and the organisms in turn provide nutrients to the plant. Thus, the elimination of these beneficial microbes severs the plant from key sources of nutrition.  It also inhibits the decomposition of organic matter into nutrient dense material, further starving plants.

There are several other ways that heavy metals impact the health of plants. They can interrupt Photosynthesis, depriving plants of the energy they need to grow. They also inhibit the activity of certain plant enzymes, generating oxidative stress that damages cell structures.

In compromising crop yields and plant nutrition, heavy metals do not only damage human health. They also threaten to intensify food scarcity and further starve our population of the nutrition necessary to beat the intensifying crisis of inflammatory disease. Thus, intensifying heavy metal concentrations in our soil is becoming a crisis.

How to Address Heavy Metals in Your Soil

Some use physical or chemical methods to remove toxic heavy metals from their soils. The alternative approach, bioremediation, is far more Permacultural because it entails the use of natural and/or indigenous materials. It’s also more economical, costing 50-65% less than other methods. Certain forms of bioremediation remove or reduce toxic heavy metals, whereas others render the metals less absorbable and, sometimes, less toxic. The proper approach will depend upon the specific heavy metals with which your soil is contaminated.

Biostimulation is a form of bioremediation that indirectly helps to remove toxic heavy metals. Biostimulation consists of adding nutrients in the form of manure or other organic amendments that function as a carbon source for indigenous microorganisms. By introducing these organic compounds, farmers encourage the proliferation of indigenous microorganisms which, in turn, can reduce the toxicity of heavy metals.

Some farmers actually introduce mycorrhizal fungi from the local ecosystem into the soil to achieve the same effect. These fungi not only feed indigenous microorganisms, but they also help plants become more resistant to disease. You can even introduce amendments, like compost, to increase the pH of the soil to above 6.5. At this pH level, the bioavailability of heavy metals declines.

Additionally, there are two forms of phytoremediation that farmers use to address heavy metal toxicity. The first, phytoextraction, involves introducing a plant that will absorb the metal. The plant is then removed and disposed of. The second, phytostabilization, entails introducing a plant that impairs the mobility of heavy metals, preventing their uptake by other plants.

The most successful approach to reducing heavy metal concentrations is to combine methods of biostimulation and phytoremediation. For example, employing phytoextraction and also introducing compost will produce more positive results than phytoextraction alone.

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