Introduction
When people started experimenting with growing crops faster using electricity, one of the most popular approaches was using direct-current power supplies as the means of producing electricity.  So, around the time of the turn of the 19th century, experimenters used an adaptation of early battery technology to generate electric current.  Early battery designs are very similar to the batteries of today, consisting of two metals or other materials connected together in a container with an electrically conductive substance between the them.
How Earth Batteries Work
Instead of using a chemical battery, it was discovered that batteries can be built into the Earth directly.  They were constructed using conductive materials such as zinc, copper or graphite which were connected by a wire and implanted into the earth.  When implanted into the Earth, an electrical current flow is induced into the soil between the end caps the battery, also known as terminals or electrodes.
Below is a diagram that shows a conceptual view of the earth battery. This layout can generate 3V, enough to power many low-power consumer electronic devices.

A Multi-Cell Earth Battery Diagram

One person, an experimenter George Hull who came onto the electroculture scene some time after the earliest experimenters, pointed out that the widespread prevalence of the Earth battery was likely due to it’s cost – simply the cost of the metals themselves and nothing more.  Another advantage of the earth battery system was that is it was easy to install, making it the most easy-to-use system for trying out crop acceleration!
Today, a cousin of the Earth battery is used in science education toys – the concept of the “lemon clock”.  To the person buying the device, it seems as if it is the lemon that’s powering the clock, when it actuality, it is the two metal strips that come with the kit that are the main drivers that power the clock.  When zinc and copper are connected together through a substance that allows for electrical current to flow, a process called an electro-chemical reaction is initiated.

Without getting too much into the details, the zinc metal has a built-in property called it’s electrochemical equivalence (charge) which is equal to approximately 0.7 Volts.  Likewise, copper has a charge of about 0.4 Volts.  When combined together in suitable soil with the right conditions (e.g. moist, slightly acidic soil), a chemical reaction ensues producing zinc ions from the zinc metal which travel through the ground towards the copper terminal

The electrochemical reaction of a Zinc and Copper is shown graphically in the diagram below:
Source: Tutors4You
As you can see, the mechanism that causes electrons (e-) to flow through the connecting wire is the dissociation of zinc ions from the metal via the electrical potential difference setup between the zinc and the copper via the salt bridge. Since there is a net charge of approximately 1.1 volts across the metals, the copper electrode effectively acts like a vacuum that sucks the copper ions out of the solution and onto itself.  In the case of the earth battery, there is no salt-bridge.  The soil is the bridge connecting the two electrodes.  When the zinc ions separate from the zinc plate, they travel towards the copper plate.  creating a current flow.
When zinc ions are dissociated from the zinc metal, they flow through the water-filled pores in the soil via a process called electrokinesis.  In electro-horticulture, the desire is to have an electric field applied to the roots of the plants.  In terms of electroculture, the loss of zinc into the soil is an undesired side effect that’s inherent in this method of current generation.  While this is not a problem at the beginning since plants require zinc to grow (it’s one of the essential micronutrients for growth), over time, the amount of zinc that enters the soil rhizosphere builds until all of the zinc that was in the original electrode (depending upon it’s thickness) dissolves into the soil. One important thing to take note of is that at higher voltages, the ionic dissociation occurs at a faster rate.
Safe and Toxic Levels
The amount of zinc that’s considered to be safe varies by not only the particular type of plant grown, but also by its concentration in the soil.
For example, in the article, “Toxic levels of soil and plant zinc for maize and wheat” by P. N. Takkar and M. S. Mann, it states that for wheat, the level of zinc that is considered to be toxic is 7 parts per million (ppm), and 60 ppm for the soil in which wheat is grown.  In general, ut has very wide limits ranging from 64mg/L for sorghum to 2000mg/L for cotton.
For a more official answer to the question of how much is toxic, one can look to the following table of regulatory limits[1]  on heavy metals applied to soils:
Heavy Metal Annual pollutant loading rate      Cumulative pollutant loading rate
Copper 67 lb/A/yr 1340 lb/A
Zinc 125 lb/A/yr 2500 lb/A
One of the reasons why heavy metals such as copper and zinc are problematic is because they’re difficult to remove from soils because once metals are introduced into the environment, they remain indefinitely.  They don’t degrade like carbon-based organic molecules.
Note that when the zinc is electrically dispersed into the soil, it doesn’t necessarily stay within the limited area between the electrodes.  If the moisture level is low, then it is likely that the zinc will be limited to be within the area between the electrodes.
The amount of metal dispersed into the soil is based on a number of factors.  Low moisture conditions levels affects soil resistance, making it more resistive which results in a reduced current flow thus lowering the amount of zinc that gets into the soil.  On the other hand, when the soil is very moist, especially after a rainfall or a watering, the current flow is increased, releasing more zinc.  While it is likely that the amount of zinc in the soil in general will increase, it will more likely get lowered near the surface because the mass flow of water will push the zinc ions further down into the ground.  Yet once the rain stops, the amount of zinc in the soil will continue to build-up until the ground dries up or the anode complete dissolves.
When using earth battery systems, it is important to limit the amount of zinc that gets into the soil since it affects not only the plants and topsoil, but also the groundwater far below as well.  Toxic levels of metals in ground water are bad for everyone.
Factors Affecting Zinc Toxicity
As we touched on previously, the moisture content of the soil affect the amount of zinc that gets into the soil.  But there are other factors that affect this as well including:
  • Thickness of the plates
  • Electrode geometry
  • Soil acidity
If the electrode terminals are thick, they will provide current flow for a long period of time at the expense of greater amounts of zinc getting into the ground.  While the thickness of the plates affects the total amount of zinc that’s available, another factor that controls the rate of dissociation is the geometry of the plates.  Larger sized plates create greater amounts of current flow which are needed for use in larger-sized systems.  As we mentioned earlier, greater amounts of current equals greater amounts of zinc.  Lastly, if the acidity level of the soil is too high, then this will also increase the rate at which zinc is dissociated.
Effects on Soil and Plants
Zinc is an essential element for plant growth, playing an important role in several metabolic processes.  It activates enzymes, is involved in protein synthesis, is a part of normal metabolic activities.  However, when zinc is accumulated in excess, it causes alterations in vital growth processes such as photosynthesis and mineral nutrition.
Once toxic levels are approached, the following symptoms can be observed:
  • Stunting of the shoot
  • Curling and rolling of young leaves
  • Death of leaf tips
  • Reduction of biomass yield
  • Insufficient production of chlorophyll – paleness or yellowing of leaves

Source: Sustainable Agriculture, EDP Sciences.

Remediation
The expenses involved with traditional methods of cleaning toxic soils are usually very expensive.  On the other hand there are less expensive means for removing toxic contaminants from soil by using plants in a method called phytoextraction or phytoremediation.  In phytoextraction, the use of certain types of plants known as hyperaccumulators are used to adsorb metals present in the soil, moving the metals from the soil into the aboveground portions of the plant.  After the plants have grown for some period of time, they are harvested and incinerated or composted to recycle the metals.  In extreme cases, several growth cycles may be needed to decrease the contaminant levels to allowable limits.  If the plants (stalks, leaves and roots) are incinerated, the ash must be disposed of in a hazardous waste landfill.  A rule of thumb to keep in mind is the volume of biomass that would need to be grown and destroyed to remove metals from contaminated soil is about 1/40 of the mass of the contaminated soil [3].
Plants that can be used for phytoremediation include several plants in the genus Thlaspi (pennycress), Brassisa juncea (Indian mustard), and sunflowers.
Since earth batteries are used for growth acceleration, any plants used for phytoremediation in the area of desired vegetative growth will help to filter the soil in an accelerated manner.  Using plants and electricity together for the extraction of toxic contaminants in soil is called electrokinetic phytoremediation.  In most cases, this type of remediation is used for not only extracting contaminants at a faster rate, but for also pulling contaminants in from a greater portion of the underground volume towards the surface.
Note: Since the harvested plants used for phytoremediation contain a large amount of heavy metals, it is important to dispose of the plants through a qualified hazardous waste program.  Many municipalities have programs for the disposal of household hazardous waste.
Conclusion
Since it’s known that high-levels of metals (or any substance for that matter) will eventually become toxic to not only plants but to soil microorganisms as well, it doesn’t mean that you cannot try out this earth battery-based electroculture in your own garden, it just means that you need to exercise some care when doing so.   What that means is using minimally sized earth-battery geometries to minimize the current flow to the minimum amount that’s needed.  Furthermore, if attempting to use this technology in-the-ground vs in pots, you may want to consider growing some hyperaccumulator crops if not after harvest, but also during the main growing season, perhaps using intercropping techniques (e.g. http://www.intercrop.dk/General.htm).

Active Phytoremediation Using Intercropped Sunflowers

Since toxic metal buildup occurs over time, if you want to give earth-battery based electroculture a try, I suggest intercopping with sunflowers or any of the other relevant hyperaccumulators so the zinc ions can be safely removed while growing.  If inter-cropping is impractical or not desired, then consider planting a few sunflower plants at the opposite end of the field, next to the copper cathode, to help remove metals that make it the the far end over time.
While earth batteries have had a very successful role in the past for accelerating the growth of plants, increasing yield, and reducing some plant’s susceptibility to certain diseases, they have their limitations.  While many of the historical sources talk about the magnificent benefits that can be achieved through growing crops with earth batteries, perhaps one of the reasons why the technology worked intermittently was due to excess zinc accumulation in the soils.  Now, with greater knowledge of the effects of toxic metals upon plants and microorganisms, and the means to clean the soil at the same time of afterwards, perhaps earth batteries can once again be used to improve the growth rate of bumper-crops more on a more consistent basis.
If anyone is interested in trying out an earth battery system whether it’s in one’s own backyard, in an urban garden, or in an agricultural setting, please let me know – I would be happy to assist in any way I can.
References:
[1] (adapted from U.S. EPA. 1993. Clean Water Act, sec. 503, vol. 58, no. 32. (U.S. Environmental Protection Agency Washington, D.C.)
[3] http://www.aiswcd.org/IUMPDF/appendix/u03.pdf, p.4
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