Avoid accidentally triggering crop malnutrition, slow growth and pests and diseases
You may not realize it, but you already know a lot about redox. Redox deals with the transfer of electrons from one material to another in a living organism. The material that loses electrons is oxidized; the one that accepts them is reduced. Since the processes are intertwined, we call it redox.
From your daily life you know ingredients, the causes and the effects: electrical current, anaerobiosis (life in the absence of oxygen) and disease susceptibility of a crop — all parts of the theory of redox — are familiar phenomena to farmers, even if they can’t define the terms or describe how these concepts relate to each other. The center of redox biology is the energy metabolism of an organism, which is its way of breathing to “combust” its food.
Going through the hardship of understanding the inner wiring of redox biology can pay off for farmers, though, since understanding this concept can help growers avoid accidentally triggering unwanted phenomena, like crop malnutrition, slow growth and diseases and pests.
Electric Current and pH
The first thing to understand about redox is that every organism needs electric current. Organisms source electrons from their environment and use them to drive the assembly of organic material and other processes. The transfer of electrons (i.e., current) powers other processes we associate with life — e.g., perception and reaction to the environment. This means that the driving force behind material growth is current. Without electrons flowing, all the nutrients you provide to your crop just linger around in the soil and aren’t of any use.
If possible, this electron flow is maintained by photosynthesis in a green organism. A “feeding” organism will strip its food of electrons, use their energy to drive its bio-machinery, and ultimately dispose of them into its environment. The preferred final target is oxygen, as reaction with oxygen will yield the most energy compared to other acceptors. This so-called aerobic breathing can be compared to cold combustion, where the energy that is lost as heat and light to the environment is caught and used to accomplish work in a living being. The processes together — photosynthesis and breathing — form a large cycle of electrons throughout the entire system.
An electron carries negative charge and is — in the minimal possible combination of one electron and one proton — part of a neutral hydrogen atom. If you strip hydrogen of its negative charge to make use of it, a positive charge is left behind and — voilà — you generate acidity. You suddenly emerge in the world of pH, which is defined as the density of protons in water. If you strip a whole water molecule of electrons (which can occur in photosynthesis), you end up with acidity (protons) and oxygen.
The same thing happens the other way around: if you transfer an electron into a molecular acceptor, you “quench” acidity. Alkalinization results. This is why redox processes in living systems, where water is omnipresent, never come alone; they are always interlinked with pH. This is also why waterlogging — driving your soil into anaerobiosis — leads to soil alkalinization. Oxygen is consumed and the microbiology begins to reduce the environment.
Eh-pH-EC
The impact of pH on crop health is widely accepted; now it is time to look at its twin: redox, measured as Eh.
One idea to keep in mind is that redox chemistry always involves a transfer of electrons. The material you source electrons from is by definition oxidized and the material you “dispose” them “onto” is reduced. Just think of the sugar from cabbage bubbling as oxidized CO2 out of your fermentation jars. Or — closing the circle — carbon dioxide being reduced into sugar in photosynthesis.
The speed at which electrons can flow through this watery system is the electrical conductivity (EC). This is the factor that determines the overall speed that life can operate at, if the other components are present — and it is the factor in a cropping system that determines the amount of output per season.
Taken together, this setting of a specific combination of electron availability, acidity and conductivity is non-negotiable for an organism. When these factors do not fit the metabolic wiring of an organism, it simply cannot exist.
Children often learn this lesson the hard way with an animal they enclose in a jar, without holes in the lid. We as adults could learn this lesson the other way around — that is, that a disease-causing organism like Phytophtora, which depends on reduced (“low”) redox values, vanishes with proper soil aeration in neutral to alkaline pH conditions — but usually we do not attribute it to the logic that is so obvious in the jar example. We usually think of the waterlogged soil just as a facilitator for zoospore mobility.
Our direct perception of redox in our day-to-day life is fooled by two mechanisms. One is that there are organisms with the capacity to switch metabolic gears — like a muscle that has to turn to lactic fermentation when the circulatory system is not fit enough to provide the oxygen needed. These are usually microbial “smart devices” that can cope with a wide range of conditions and are accordingly widespread in environments with fluctuating conditions. This makes us assume that all organisms have this flexibility, which is simply not the case.
The other factor is that organisms manipulate their environment, if present in sufficient density or with allies — like a mixed microbial biofilm in soil, or a plant caught offhand by a million invaders that pull the internal processes out of balance with their excretions and knock it out by sheer number. But even in that case, getting the initial foothold in the plant is dependent on a fitting Eh-pH-EC invitation.
Our human perspective on life is that of an organism capable of redox regulation. We do not drop dead immediately if we rinse our skin with whiskey or hydrogen peroxide; from the perspective of a single microbe, though, its relation to the redox environment is as drastic as that of one of your brain cells during a stroke. A few minutes of oxygen deprivation leading to brain-cell injury make all the difference between a fully functional human body or a future in a wheelchair.
Nutrient Availability
One element of redox chemistry that is of major importance in farming is nutrient availability.
The chemical processes in living beings depend critically on enzymes to work in the direction needed, at the speed needed. If you let butter sit on a table, it will eventually go rancid — it becomes “oxidized.” If you “burn” your body fat in a cold combustion with oxygen, while on a diet to satisfy your energy needs, this process should be faster and with a controlled outcome. Enzymes harness this oxidation process to produce CO2 and water as an end product of the fat’s degradation, without you even noticing the switch from breakfast to consumption of your “problem zone.”
The active component in these enzymes is often charged metal atoms that will accept or transfer an electron from the material they work with. Iron or manganese are prominent examples of these charged metal atoms — ions — in the plant world. Manganese is a critical component in the photosynthetic apparatus because it helps to split water: to strip it of electrons and thus generate the oxygen we breathe.
In soils that have been mixed with oxygen, as happens during tillage, these metals oxidize. They form “rust” and react with other components in the soil to become insoluble. One possibility is that an oxidized iron ion, carrying three positive charges instead of two, will associate with the negatively charged phosphate from NPK fertilizer and precipitate as iron-phosphate, which is not plant available.
The job of the grower at this point is (1) to set conditions that render the material soluble again so it can be transferred into the plant, and (2) to reverse the loss of electrons to the oxygen in order to make it functional again.
In a compacted soil, the material becomes soluble again as soon as the soil becomes waterlogged. The soil microbiology runs out of the favored oxygen as acceptor and needs other “wastebins” for the disposition of the electrons stemming from their energy metabolism. Depending on the makeup of the parent material, and in absence of other more favorable acceptors, manganese and iron are present and are used; as they accept these electrons, they lose positive charges and regain solubility.
The downside of this process is that the plant in this case is suffering asphyxia in the rootzone — no oxygen for microbes also means no oxygen for the root! This is a great stress, stopping growth and predisposing the plant to soilborne diseases. On top of this, it is flooded with metal ions of all kinds — undesirable ones like aluminum, as well as the necessary ones.
The superior way to go is promoting an aggregated soil, where the soil surface is in contact with oxygen while it is consumed in the inside by microbes. This way, in the inside of an aggregate, the reduction of metals can take place and they become accessible through microbial trade networks to the plant — in correct dosage, on demand, triggered by root exudation, in cooperation with a functional, cooperative root microbiome. That is: if the plant has not been accidentally mutilated in its capacity to communicate through a long breeding history optimizing it for nutrition via soluble mineral salts! (The ability to interact with bacteria is usually intact, but not necessarily to fully cooperate with established mycorrhizae).
In the case of iron, redox can produce an interesting phenomenon — the “chlorosis paradox.” If the plant (the following does not apply to grasses) has to take up the non-reduced iron with three positive charges, it will transport it right up into the leaves, where it will precipitate around the leaf veins in the cell walls with a phosphate-storage compound. What you won’t see is the hidden hunger for iron hampering performance; what you might see is apparent chlorosis due to lack of iron in advanced stages, even though in a tissue analysis, iron will show up as plentiful.
To make use of this iron, the plant needs to receive functional iron with two positive charges as an investment credit, which can be integrated in enzymes directly; it can then run photosynthesis at a higher rate, generate the potential to reduce materials through this process and eventually dissolve and make use of this internal deposit.
Plants Out of Balance
Once conditions get out of balance, the internal redox balance of the plant is also affected. Processes usually tip after photosynthesis rates drop for a while. This can be due to malnutrition (the apparatus is simply not large enough or is too slow for the demand), downregulation (growth stops and demand stops) or simply prolonged periods of shading (planting density reaching a closed canopy), overcast weather or lack of water.
The lack of photosynthesis, which is the source of reduction in the plant, will move the internal balance toward oxidation. This can be monitored with properly conducted Brix readings. If Brix readings are continuously low, trouble is ahead.
The direction of deviation from the small “space of health” on the redox-pH-EC map determines which illness or pest will move in — which organism fits best, as it is given the capacity to “breathe” in the prevailing conditions in the plant, to make use of the nutrients present and thrive in the climatic conditions that apply.
Many of the chemical weapons (natural or synthetic!) we use to assist plants in their defense unfold their toxic potential as an oxidizing effect and therefore work best when they are oxidized. That means sprayed in little bubbles that are well aerated. These chemicals are effective to support the initial emergency reaction of the plant — the “oxidative burst.” This production of reactive oxygen species inside the plant shifts the redox balance temporarily out of the region that is acceptable for the invader. Biomolecules get destroyed by random oxidation.
Think of wound disinfection with hydrogen peroxide. The oxidative burst is costly and dangerous for the plant, as prolonged oxidative stress also damages the bio-machinery and structure of the plant itself. This means that the plant needs to return to its own equilibrium after the defense reaction by means of antioxidant reserves or production. The antioxidants, which again stem from the photosynthetic source, are the fire extinguishers that quench the oxidative burst. After reduction has set in, an invading organism — depending on an oxidated environment — cannot simply “breathe” and combust its food source.
What we do not yet know is to what degree a plant can draw on the reserves of electrons stored in the soil, when transferred by the soil microbes onto stable soil organic matter after consumption of root exudate. But it is very tempting to speculate that if this battery is full, it can be a supply “charge” in case the solar panels of the plant are down.
Finally, I would like to discuss one example of “green-thumb magic.” One trick of Korean natural farming consists of multiplying yeasts that live on plant surfaces just by providing a food source and the temperature conditions they prefer in a jar. These are applied to plants in case of viral infections that occur after prolonged periods of rain.
In the frameset of redox thinking, a plant in these conditions will suffer starvation, resulting in oxidation, which is the perfect setting for viruses to develop. Yeasts will reduce their environment when feeding on the plant surface — we all know that and have explored this trait in our long association as humans with yeasts in the brewing process. Apparently, the pulse of reduction after application of the yeasts as foliars can be enough to stop viral disease progression.
And here it is again: on the fundamentals of a functional soil. A plant that is so unhealthy that it needs to be coaxed along to stay alive will not be able to take advantage of such a mild correction.
Harriet Mella is a PhD biologist specializing in botany, mycology and microbiology. She teaches a course on carbon microcycling on kindharvest.ag and can be on LinkedIn or KindHarvest.
