Healthy, fertile soil is an invaluable resource that is getting increasingly scarce due to intensive agriculture, pollution, and climate change. Agricultural land takes up 37.6% of the world’s land area, which is more than a third of the planet’s terrestrial surface that is constantly disturbed and treated with various inorganic and organic compounds. Ironically, the topsoil, which is the most nutrient-rich and microbially active layer of the soil, gets depleted the most by tilling and wind and water erosion. These processes happen on almost all agricultural land, but they are particularly pronounced in areas where winters are cold and windy. Leaving fields in these climates barren over winter severely depletes soil fertility in the long term.
Soil pollution and depletion cause lower yields and biodiversity loss, as well as poor quality and nutrient content of the produce. These effects create a cascade of negative developments that threaten food security, human health, and the environment. Contaminated and nutrient-poor produce cannot feed the growing world population sustainably nor provide health and longevity. Let’s investigate the impacts of pollution and soil depletion in greater detail to understand what went wrong in food production and how our health is connected to it.
The causes of soil pollution are either natural or induced by human activities. Natural pollutants include a variety of toxic compounds that are naturally present in the soil or are introduced through processes like erosion, acidification, and salinization. These compounds include lead, mercury, nickel, arsenic, radioactive elements, asbestos, as well as zinc and copper in higher concentrations. However, anthropogenic pollution plays a far more significant role than natural pollution due to the large scale of industrial activities and significant waste generated in our homes. Mining, the chemical industry, the fashion industry, energy, transport, and agriculture are the most polluting, but we shouldn’t forget that urban waste also plays a significant role. Although more localized, urban sources of pollution like vehicle exhaust, leaking sewage, and decomposing materials in landfills can contribute substantially to soil pollution.
The most significant soil pollutants include heavy metals, organic pollutants, and pathogenic organisms.
The surge of industrial activities and wide use of fossil fuels over the last century greatly affected the quality of agricultural land. These factors increased the level of heavy metals in the soil and made this type of pollution widely distributed. The most common sources of heavy metal soil pollution are phosphate fertilizers, contaminated manure and irrigation water, smelter dust, sewage sludge, tailings, and other types of industrial waste.
What makes this problem even more difficult is that heavy metals tend to accumulate in the environment, and only a fraction of them gets degraded to less toxic compounds. In the process of taking up water and nutrients from the soil solution, plants (as well as other organisms, like fungi) also absorb toxic heavy metals like arsenic, cadmium, chromium, mercury, lead, copper, zinc, and nickel. Upon absorption, these elements get distributed in the plant tissues, ending up in roots, leaves, fruits, and seeds.
Depending on the plant species, its growth stage, growth rate, tolerance level, and the concentration of heavy metals in the soil, the level of contamination within the plants can vary. For example, some plant species are labeled as hyperaccumulators, which means that they are able to take up and store certain metals and metalloids within their tissues at levels that are far greater compared to most plant species. These plants are valuable assets in bioremediation – the process of removing toxic elements from the environment.
Most food crops are not hyperaccumulators, but there are some exceptions. Certain edible herbs, like Alyssum species and stonecrops, as well as elder and gooseberry trees (Phyllanthus acidus), can accumulate a significant amount of toxic metals. Most plants we use for food are sensitive to high heavy metal concentrations and usually contain these toxic substances in trace levels. However, we shouldn’t forget that heavy metals accumulate in the human body. The compounding effect of consuming these elements in trace levels can have significant health consequences in the long term.
The most common organic soil pollutants are polychlorinated biphenyls (PCBs), polychlorinated dibenzofurans (PCDFs), polychlorinated dibenzodioxins (PCDDs), polybrominated biphenyls (PBBs), polycyclic aromatic hydrocarbons (PAHs), pesticides, and fossil fuels like gasoline and diesel.
PCBs, PCDFs, PCDDs, and PBBs are persistent organic pollutants that come from a wide variety of industrial and consumer products. Their use and presence were ubiquitous before their toxic and carcinogenic properties were discovered during the second half of the 20th century. Even though the use of these substances was restricted or banned a couple of decades ago, their presence can be still detected in air, water, soil, wildlife, and human tissues. Their presence in the environment and soils is most commonly caused by waste incineration and decomposition, spills, and pesticide applications. PCBs, PCDFs, PCDDs, and PBBs enter the food chain through contaminated plants, especially in animal feed. Due to their chemical qualities, these compounds tend to accumulate in the tissues of living organisms, especially animals.
In order to battle unwanted and damaging organisms, plant growers use a variety of natural and synthetic organic chemicals we call pesticides. Although pesticide producers significantly pay more attention to the toxicity of the active ingredients compared to a few decades ago, there are still a lot of plant protection chemicals on the market that are not safe for humans or the environment. When a pesticide is applied to the plants, only a portion of it actively interacts with unwanted organisms. The rest gets washed with the rain and decomposes under the effect of UV light and other natural degradation processes. Some of it also ends up on the top layer of the soil during the application process, directly interacting with soil particles and its macro- and microbiome.
Growers prefer pesticides that have long-lasting effects since they don’t need to be applied as often and provide long-term protection. Because of this quality, these pesticides tend to persist and accumulate in the environment, sometimes even years after application. There are also some pesticides that degrade relatively quickly but produce persistent and toxic byproducts in the process. A variety of pesticides (herbicides, fungicides, insecticides, rodenticides, etc.) are applied every year, multiple times during the growing season.
The compound effect of such applications leads to a high concentration of these toxic chemicals in the environment, especially in the soil. The interactions and the effects of these chemical cocktails have not been fully described and studied yet. A wide variety of physical and chemical properties of pesticide active ingredients, as well as the diverse structural and biological content of different soils, make the study of these interactions very difficult and expensive. However, what we know for certain by now is that the presence of pesticides in the soil negatively impacts its biological diversity, which is an essential component of soil fertility.
Diesel fuel and gasoline are the two most widely used fossil fuels in transport and, as such, are some of the most common organic soil pollutants. They are a complex mixture of a variety of organic molecules, as they are made by processing and mixing crude oil with additives that improve their qualities for specific purposes. Each additive has certain toxicity and displays adverse effects on living organisms at a certain level. Aside from a variety of synthetic compounds, both gasoline and diesel contain PAHs and organic molecules of natural origin, which are also shown to display negative effects on soil organisms and human health. However, there are certain differences between these two types of fossil fuels, with diesel being more toxic due to the presence of a wider variety of additives.
Soil pathogens can be present in the soil naturally, or due to the soil being exposed to contaminated water and animal remains. These organisms may not grow in the soil, but lie dormant, waiting for contact with their hosts. Soil pathogens include fungi, bacteria, viruses, nematodes, protozoans, and worms. Some of the most common species include Escherichia coli, Salmonella spp., Shigella spp., Clostridium tetani, and C. botulinum, Campylobacter spp., hepatitis A and E, and different polio and adenoviruses. Depending on the qualities of the soil and climate conditions, these organisms can persist over shorter or longer time spans within the biologically active layer. The threat of them contaminating the plants we grow for food also depends on the plant species – root crops are more likely to be contaminated compared to crops that grow on trees.
Aside from the direct health threat posed by soil contaminants, there are more subtle negative effects of inadequate soil quality. The most notable one is the poor nutrient content caused by soil depletion. Soil depletion occurs when soil resources are extracted or removed faster than they are replenished. It is caused by various processes, including:
- erosion – removal of the microbially active and nutrient-rich topsoil by wind and water
- degradation – acidification, salinization, and alkalinization
- leaching – when nutritive elements and compounds are drained into deeper layers of the soil, becoming unavailable to plant roots
- compaction – caused by prolonged effects of agricultural machinery, vehicles, and non-human and human animals that disrupt the flow of water and air through the soil by creating a consolidated layer of soil particles and anaerobic conditions favored by various pathogenic organisms
- excess water – caused by flooding and excessive irrigation
- overcropping – continuous cultivation of crops without proper fertilizing and allowing the soil to restore its nutrient content naturally
- leaving the soil barren – most commonly due to crop removal and overgrazing
- loss of soil biodiversity – caused by soil pollution and all the aforementioned degradation processes
Poor soil management is the main contributing factor to soil depletion on agricultural lands and has a far-reaching effect on human health. We are what we eat after all, and consuming nutrient-poor foods depletes our bodies of essential elements and compounds. The quality of the food we eat greatly affects our health in multiple ways. Aside from the “fuel” we get from fats, proteins, and carbohydrates – the macronutrients, our body also needs micronutrients to run properly. Micronutrients are compounds that we require in minute concentrations, but they are nevertheless crucial for the normal functioning of our bodies. They include hydrosoluble and liposoluble vitamins, as well as trace minerals.
The hidden hunger
‘The hidden hunger’ is an umbrella term that includes any lack of essential vitamins and minerals. Nutrient deficiency is not only a problem that affects plants – but it also affects us. You don’t need to be vegan to feel the effects of soil depletion. The animals we eat also rely on plant food, and if there are not enough nutrients in the food they eat, their meat, eggs, and milk won’t provide enough nutrients to satiate the needs of our bodies. This isn’t a problem that affects only the developing world, but also the countries that employ the most advanced agricultural practices.
Being healthy is not equal to caloric intake. The calories are just “the amount of energy released when your body breaks down (digests and absorbs) food.” Our bodies need more than just energy – they need a variety of other compounds that are directly or indirectly derived from soil nutrients. Even if crops are grown in hydroponic or aquaponic environments if there aren’t enough essential elements in the substrate, the plants will suffer from deficiencies that will eventually affect our health. We might not feel the effects of nutrient-poor produce immediately, hence the term “hidden hunger,” but consuming such foods will catch up with us in the long run. For that reason, it is essential to keep track of the qualities of plant substrates, adopt proper fertilizing practices and avoid contaminants that can be detrimental to our health.
“The food you eat can be either the safest and most powerful form of medicine or the slowest form of poison.” – Ann Wigmore
- Rodríguez-Eugenio, N., McLaughlin, M., Pennock, D. (2018). Soil pollution: a hidden reality. FAO.
- Brevik, E. C. & Burgess, L. C. (2014). Soils and Human Health. Boca Raton, Florida: Taylor & Francis Group.
- Steffan, J. J., Brevik, E. C., Burgess, L. C., Cerda, A. (2017). The effect of soil on human health: an overview. European Journal of Soil Science.
- Biesalski, H. C. (2013). Hidden hunger in the developed world.