Herbicide use is very common in mainstream agriculture. Despite the documented environmental harm caused by herbicides, and the findings by the World Health Organisation (WHO) International Agency on Research on Cancer (IARC) that glyphosate (Roundup) causes cancer, herbicide use remains rampant and unabated!
In 2020 the global use of herbicides reached nearly 1.4 million metric tons, compared to fungicides at around 606,000 metric tons and agricultural pesticides at 2.66 million metric tons.
It’s no coincidence that farmers who use herbicides also employ ecologically unsound farming methods that damage and degrade soils. If you’ve ever wondered why they don’t use methods that improve soil health such as regenerative farming practices, the answer is simple. They just can’t, because herbicides do not work very well in healthy soils!
In this article we’ll explain why this is the case, and the science behind it.
Healthy Soils Grow Healthy Plants
Even through mainstream agriculture treats soils as nothing more than an inert wettable substrate to plant crops into and add synthetic chemical fertilisers to, nothing could be further from the truth.
Soils are in fact very complex living ecosystems that plant roots co-inhabit alongside many other forms of soil life, interconnected in an interdependent and web of life. Soil microorganisms and fungi live in complex beneficial relationships and associations with the root systems of plants and trees.
Healthy soils not only provide the water needs of plants, but also supply nutrient to the roots in a slow and sustained way. As with all ecosystems, they provide a balanced and stable environment which all living things require to grow and thrive. Healthy soils also have a good soil structure that strikes a balance between adequate moisture retention to meet plant moisture needs, and sufficient drainage to allow oxygen to reach plant roots and prevent root rot diseases. The beneficial soil organisms also keep the soil-borne pathogens (soil diseases) in check, protecting plants from root diseases.
To put it another way, healthy soils are required to grow healthy plants. Unhealthy soils cannot produce health plants, they can only produce weak, sickly and struggling plants.
How Mainstream Agriculture Forces Productivity Out of Unhealthy Soils
Unhealthy plants are naturally in a weakened state and cannot protect themselves very well from pests and diseases, which ultimately serve the ecological function of eliminating them so that stronger plants that are better suited to that location can take their place.
If mainstream agricultural practices damage soils and heavily depend on herbicides, how do they manage to produce crops?
The approach of industrial agriculture, known as agribusiness, is simply to continue the philosophy of working against nature by forcing things to grow and killing anything which threatens the crops that nature seeks to kill off. This is done through the following practices:
- Using plenty of water to irrigate bare, unprotected, unmulched soils that lose plenty of water to evaporation (cheap government-subsidised water helps).
- Using synthetic chemical fertilisers, many of which are mineral salt that are highly water soluble and force plants to grow when they take up water, producing fast, soft, sappy growth that encourages pests. They also leach readily into waterways causing eutrophication (uncontrolled algae growth that suffocates and kills all water creatures) and into the water table, contaminating groundwater. These mineral salts also ‘salt’ the soil, killing the soil microorganisms whose ecological role is to provide plants with soil nutrients in the form they can use and to support plant health.
- Using routine scheduled applications of highly toxic pesticides and fungicides to stop the pests and diseases that come to wipe out the vulnerable force-fed monocultures of crops. These toxic chemical controls wipe out the more vulnerable beneficial predatory insects which control the pests first, causing explosions of pest populations afterwards, and leaving toxic pesticide residues in the food which are harmful to human health. Many pesticides affect bees and other non-target species, can persist in the soil, or be carried by water off site to other locations.
These mainstream agricultural practices seriously degrade soils and damage soil health, but there is no attempt made to reverse the damage as the herbicides routinely used in these farming methods are incompatible with healthy soils that raise healthy plants.
The Problem of Agricultural Herbicide Persistence in Soils
When toxic chemical herbicides are used in agriculture, the intention is to control weeds or exert their herbicidal effects in the season that they are applied, but not after that.
Herbicide-treated soils can’t be replanted with the next round of crops until all the applied herbicide has broken down and is no longer active in the soil. If the herbicides don’t degrade and persist in the soil, they will kill or damage the next season’s crop when it’s planted, which would be a very serious problem!
What Factors Determine How Long Herbicides Persist the in Soil?
The factors that affect herbicide persistence in soils include:
- Soil composition – the relative amounts of sand, silt, and clay in the soil, as well as the levels of organic matter content.
- Soil pH – the acidity or alkalinity of the soil.
- Soil microbial activity – the types and quantities of soil microorganisms present in the soil.
- Soil tillage – the mechanical manipulation of the soil (such as forking, digging and plowing) for preparation of soil for planting and the cultivation of soil after planting.
Soil Composition and Herbicide Persistence
The persistence and activity of herbicides in the soil is affected by the degree to which herbicides bind to soil particles (adsorption), are carried by water and washed down into the soil (leaching) and are converted into a vapour form which is lost to the air (volatilisation).
Soils that are high in clay or organic matter have a greater capacity to bind herbicides to the soil particles, and also reduce herbicide losses from the soil through leaching and volatilisation.
This tie-up of herbicides in the soil reduces the amount available for uptake by weeds through their roots, reducing herbicide’s overall effectiveness. More of the bound-up herbicide ends up being held in reserve in the soil, increasing soil herbicide persistence and carryover, potentially causing injury to susceptible crops planted at that site in the future.
Fine-textured soils which contain high amounts of clay and silt usually tend to contain higher levels of organic matter compared to coarse-textured sands or sandy loams. The organic matter content of sandy soils may be less than 1%, loams (which consist of fairly equal proportions of sand, silt and clay) may contain 2% to 3%, and clay soils contain from 4% to more than 5% organic matter.
Herbicide carryover, where herbicides persist in the soil from a previous application, can occur in any soil type under the right circumstances, but is typically more common in soils with higher clay and organic matter content, such as medium and fine textured soils with more than 3% organic matter. These soils have the greatest potential to bind or hold herbicides and to injure sensitive rotation crops. Soils with a coarse to medium texture and less than 3% organic matter are less likely to retain herbicides and cause carryover problems.
Soil Microbial Activity and Herbicide Persistence
One of the most important pathways for the degradation and breakdown of herbicides in the soil is through the activity of soil microorganisms. The rate of decomposition is determined by the various types of microorganisms present in the soil, such as bacteria, fungi, actinomycetes, protozoans, etc. and their numbers.
Microorganisms are most active, grow (multiply) fastest, and as a consequence, break down herbicides the fastest, when they have the required environmental conditions. They proliferate in environments which are warm, dark, and moist, with a soil pH close to neutral, as well as a good supply of oxygen and mineral nutrients, such as is found in fertile, well-aerated soils.
This would suggest that healthy soils promote microorganism activity which breaks down herbicides faster. There’s a catch though, some herbicides have a negative impact on soil microorganisms!
Glyphosate (Roundup) is one of the most widely used herbicides in agriculture. This herbicide works by inhibiting the plant enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) in the aromatic amino acid (AAA) biosynthetic pathway, also known as the shikimate pathway. This biochemical pathway can be found in plants, fungi and bacteria.
Studies have demonstrated that glyphosate (Roundup) has an overall negative effect of glyphosate on soil fungal communities, causing a decrease in their biomass. They have also shown that this herbicide has a negative effect on certain types of soil bacteria, causing decrease in the numbers in response to glyphosate exposure. One such group are the Acidobacteria, a soil bacterial phylum whose members are widespread and can be found distributed across nearly all ecosystems. It is understood that this class of bacteria play a dynamic role in vital ecological processes, such as the regulation of biogeochemical cycles, decomposition of biopolymers, exopolysaccharide secretion, and plant growth promotion, and a decrease in their abundance could lead to significant changes in nutrient status of the rhizosphere.
Soil pH and Herbicide Persistence
The soil pH can affect the persistence of some herbicides.
At high (alkaline) soil pH, the chemical and microbial breakdown of herbicides is often slower.
The breakdown of most members of the triazine and sulfonylurea families of herbicides slows as the soil pH increases, particularly when the pH is above 7.0, so they persist longer in the soil. In alkaline soils, less of the herbicides bind to the soil particles, increasing availability for uptake by plants. With these two factors combined, more of the herbicide is present in the soil and available to be taken up by plants, which can lead to herbicide carryover and injure the next crop planted.
At low (acidic) soil pH, some herbicides break down faster, while others become bound up in the soil, preventing their degradation.
The triazine and sulfonylurea families of herbicides break down more rapidly when the soil pH is below 6.0.
Atrazine binds to soil particles in acidic soils, making it unavailable for uptake by plants so it can’t damage them, and the acidic soils chemically degrade the herbicide more quickly.
With some herbicides, such as the imidazolinone family of herbicides, imazaquin (Scepter) and imazethapyr (Pursuit), a low soil pH increases their persistence in the soil. As the soil pH drops below ph 6.0, the herbicides imazaquin and imazethapyr bind more strongly to soil particles (adsorption), reducing their availability to the soil microorganisms that are the primary mechanisms of their degradation. Despite the greater level of adsorption in acidic soils, these herbicides can still be released back into the soil several months later to make them available for plant uptake and potentially injuring any sensitive crops that follow afterwards.
Soil Tillage and Herbicide Persistence
The amount of tillage, the mechanical manipulation of the soil (such as forking, digging and plowing) following herbicide application can affect its persistence in the soil. Tillage indirectly encourages herbicide decomposition by increasing microbial and chemical breakdown.
Incidentally, the practice of tilling, which involves turning over the top 15-20cm (6-10″) of soil before planting new crops, effectively disrupts the soil ecology, and kills off many of the fungi and bacteria and other soil microorganisms that are critical to soil health (and herbicide breakdown). Long-term use of deep tillage destroys healthy soil, leaving an inert, lifeless growing medium that is dependent on synthetic chemical fertiliser, pesticide, herbicide and fungicide inputs to remain productive.
Sustainable and regenerative farming practices such as minimum-till and no-till, which reduce soil disturbance, preserve the soil structure, protect the soil ecology and reduce soil erosion don’t work well with herbicides because the herbicides tend to accumulate at greater concentrations near the soil surface. When these herbicides are persistent in the upper zone of the soil, they can affect susceptible shallow-planted crops.
To avoid potential herbicide carryover problems and dilute the herbicide levels through the soil, industrial agriculture is required to use ecologically destructive plowing methods which increase soil erosion, which in itself is a huge problem, being a major environmental threat to sustainability, productivity and food security.
The whole philosophy of industrial agriculture is rather backwards, as choosing farming methods to support herbicide use is a case of having the tail wagging the dog! It’s likely those 1.4 million metric tons of herbicide used globally in a single year and rising factor into this.
Perhaps a better approach to long-term food security is to choose the most productive AND sustainable methods, such as regenerative agriculture, and then select the tools and methods to utilise to achieve the desired outcomes!
For more information on herbicides and alternatives, see these related articles:
- How to Neutralise Glyphosate (Roundup) Herbicide Contamination in Soil
- How to Identify and Treat Herbicide Contamination of Commercial Soil, Compost and Manure
- Is Tree Stump Killer Herbicide Safe Around Ponds?
- How to Kill a Tree Stump Without Poisonous Chemicals
- How to Kill Weeds Without Digging or Toxic Chemicals
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