Healthy Soil and Healthy Humans

With the WHO (under the One Health Initiative(1)) acknowledging that human health is affected by the quality of food and the quality of human living environments, this analysis summarizes the various direct and indirect pathways through which soil health affects human health. 

Save Soil is a global people’s movement (supported by the UNCCD, UNEP, IUCN, WFP and UNFAO) to raise mass awareness of the soil degradation crisis, and support governments in establishing soil health policies. The fundamental aim of the movement is to support policies that mandate a minimum 3-6% Soil Organic Matter (SOM) in agricultural soils(2). Increasing SOM brings alive the complex soil ecosystem that supports and provides the various macro and micro nutrients that plants need.

This paper looks at how SOM-rich soils, or in other words living and healthy soils, are foundational to many of the issues faced by human society:

- Malnourishment 

- SOM rich soils improve crop yields, thus increasing the amount of food produced.

- SOM rich soils also lead to better quality produce. The nutritional density of food produced in SOM rich soils is higher than in food produced in SOM deficient soils. Thus addressing the problem of hidden hunger (micronutrient deficiencies)(3).

- Mental Health 

- Nutrients that have been shown to support mental health are more present in food from soil with higher SOM. Nutrient rich foods from soil rich in SOM support a diverse gut microbiome, which in turn supports mental health and wellbeing. 

- Soil rich in SOM contributes to diverse living environments. 

- Increasing SOM in agricultural soils reduces crop failure and builds resilience in farms, which positively impacts the mental health of all the farmers who practice sustainable soil management or regenerative agriculture. 

1. Soil Health as Solution to Malnourishment

Concurrent with the degradation of soil health is the significant decline in nutritional values of various foods over the past 100 years, which has been attributed to soil mineral depletion, and loss of soil microorganisms along with changes in plant varieties(4). Food and nutrition security are intrinsically linked to healthy soil(5) rich in organic matter and teeming with diverse microbial life. One example of such life is mycorrhizal fungi, which can increase the supply of Cu, Fe, N, P, and Zn for plant nutritional needs in poor soils. Rhizobium and Bradyrhizobium species can fix atmospheric N2 into ammonium-based compounds for plant nutrition. Azospirillum can provide additional N to the roots of grasses such as sugarcane. It is estimated that N2 fixation provides between 10 and 20% of the N requirement for cultivated crops and between 25 and 40% of the entire annual reactive N in the world(6). Diverse microbial life in the soil thus provides the nutrients to grow healthy foods rich in these very nutrients, which in turn nourishes and supports the healthy functioning of the human body and mind. 

Human physiological and psychological functions require macronutrients, namely carbohydrates, proteins, and fats that provide the main source of energy for the human body, as well as micronutrients, or vitamins and minerals that are essential for the body’s many biochemical processes. Vitamins are organic compounds found in plants and animals obtained by the human body through ingestion. Minerals are inorganic compounds found in the earth that assist the body in energy production and other biochemical processes(7). As these micronutrients are sourced from the earth (and the plant and animal life it sustains), agricultural soils are required to be healthy, boosted by rich organic matter feeding a biodiverse soil microbiome, to be able to make these micronutrients available to plants and animals. Plants need at least 14 minerals for their nutrition. These include the macronutrients nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulphur (S) and the micronutrients chlorine (Cl), boron (B), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), nickel (Ni) and molybdenum (Mo).(8) Humans who ultimately consume these life forms can then also harvest these micronutrients that are crucial for proper physiological and psychological development. Conversely, degraded soils lacking enough organic matter content to support soil biology lead to weak and dysfunctional plant and animal life, and ultimately, human life. For example, calcium is essential for the transmission of nerve impulses, as a metabolic cofactor to release energy from macronutrients. Mild calcium deficiencies can cause heart palpitations, insomnia, irritability, nerve sensitivity, muscle twitching, mental confusion and a feeling of depression(9). 

Furthermore, a research review suggests that dietary nutrient deficiencies can lead to the development of depression. Research has linked zinc deficiency and folic acid (B9) deficiency to the severity of depressive symptoms. B vitamins are needed for the nervous tissue to function. For example, Vitamin B1, thiamine, is an important coenzyme in the synthesis of neurotransmitters, correlating inversely with depressive symptoms(10). Vitamin B6, pyridoxine, plays a role in the synthesis of neurotransmitters such as GABA, serotonin, and dopamine, and modulates neurobiological mechanisms along with magnesium(11). B9, folic acid, is involved in the synthesis of norepinephrine, among others. Research has found that lower serum folic acid levels during pregnancy relates to prenatal depression(12). Magnesium is important for neuromuscular coordination and nerve transmission, iron affects cognitive performance such as attention, memory and learning, and low zinc dietary intake is associated with depression in adults and elderly subjects according to one observational study(13). These are just a few examples of the countless ways micronutrients humans harvest from plants through consumption affect human neurological processes. 

In spite of this knowledge of the essential role micronutrients play in proper human development and well-being, micronutrient deficiencies are still prevalent across the globe. According to a 2022 study, more than 50% of children under the age of five and about 66% of women between the ages of 15–49 worldwide are micronutrient-deficient and the other two-thirds of the population, such as school children, younger adolescents, older adolescent boys, men, and older adults also face similar deficiencies(14). 

Faced with such alarming statistics, concerted effort must be taken now to address this global health issue at its roots: rebuilding soil health.  Returning soil to its rightful health can restore sound ecosystems for all the microorganisms and macroorganisms that live in symbiosis with plants, by supplying  plant-available micronutrients in exchange for the sugary exudates released by plants. Soil organic matter is foundational to bring back the soil -food -web ecosystem. This oil organic matter content can be returned to agricultural soils around the world through widespread adaptation of regenerative agriculture. 

Conventional agricultural practices like excessive tillage and injudicious use of chemical fertilizers and pesticides have led to alarming loss of soil microbiota and consequently nutrient leakage from soil(15), leading to the loss of bioavailable nutrients for plant uptake and ultimately, human uptake. The loss of soil life negatively affects plant health, food nutrient-density(16) and human health(17) as discussed above. Consequently, it also limits states of human mental well-being. The rise of nutritional psychiatry and the greater shift in interest towards dietary change as forms of intervention in mental illness point to the significant impact nutrient dense foods have on mental health(18). 

2. Soil Health as a Solution to Mental Health

2.1 Soil Organic Matter as Food for Thought: Soil Health’s Impact on Human Mental Health

The World Health Organization states mental health is “a state of mental well-being that enables people to cope with the stresses of life, realise their abilities, learn well and work well, and contribute to their community. It […] underpins our individual and collective abilities to make decisions, build relationships and shape our world. Mental health is a basic human right(19).”

The connection between the decline in mental health around the world and the simultaneous degradation of global agricultural soils has been proven by numerous scientific studies(20,21,22),,, which all point to the great extent soil can influence not just mental health, but overall human well-being, such as gut health, nutrition, the living environment, all of which will be explored in this review. Therefore, according to the above WHO definition, if mental health is a basic human right, then it follows that soil health is also a basic human right. Soil health and mental health also go hand in hand beyond just the individual to underlie the greater community and global socio-economic fabric.  

The WHO lists some determinants of mental health, such as substance use and genetics, exposure to unfavorable social, economic, geopolitical and environmental circumstances – including poverty, violence, inequality and environmental deprivation(23).

Similarly, the Healthy People 2030 framework of the US Department of Health and Human Services identifies 5 key social determinants of health, or SDOHs: (1) economic stability, (2) education access and quality, (3) health care access and quality, (4) neighborhood and built environment, and (5) social and community context(24). These SDOH’s are made up of many underlying factors that rely directly or indirectly on soil health. For example, healthy soil impacts farmer economics and food security in the arena of economic stability, access to nutrient-dense foods that support healthy eating patterns in the arena of health care access and quality, clean air, water, and natural spaces and greenspaces, and quality of housing in the arena of environmental conditions(25).  

2.2 Soil and Gut Health

Humans share many of the same bacterial groups with soil (26). Most of the microbiomes of the human body are spread in the human intestines and skin. In this way, the human body is covered inside and out with these microbes. The amount of human contact with the soil affects the diversity (and therefore health) of the human gut microbiota and gut health.  

A 2012 study of the phylogenetic composition of the gut microbiome across various regions found the lowest bacterial diversity in the urban US citizens compared to rural Amerindian and Malawian populations. The highest bacterial and genetic functions diversity ever reported in a human group was found in a secluded population of hunter-gatherers in the Amazon jungle(27). This shows soil contact affects short-term and long-term changes of the microbiome on an individual and community level, and that great gut microbial diversity is to be found where humans live in closer contact with soil.

One study stated the main factors that determine the human intestinal microbiome are (i) host genetics and metabolism (heritage), (ii) lifestyle (environment) in particular, and (iii) diet and nutritional habits(28). It showed that soil not only impacts the quality of the environment and food, but also overpowers genetics in shaping the composition of the gut microbiome. A recent study of the gut microbiome of baboons showed that neither host genetic ancestry, host genetic relatedness nor genetic distance between host populations were predictors of baboon gut microbiota. Instead, gut microbiota was predominantly affected by the baboons' environments, especially the soil's geologic history and exchangeable sodium. This study concluded “soil effects were 15 times stronger than those of [host genetics], perhaps because soil predicts which foods are present, or because baboons are terrestrial and consume soil microbes incidentally with their food (29).” 

Another large-scale study was conducted with over 1,000 healthy individuals, which showed no similarity in the gut microbiomes of relatives not sharing a household, while genetically unrelated individuals sharing a household showed significant similarity. This suggests that genetics play only a small role in shaping the gut microbiome, with an overall microbiome heritability below 8%, while non-genetic factors such as environment, lifestyle and diet, play a predominant role (30). 

Soil health is then a major contributor to gut health, but its contribution to human well-being does not end there. The gut microbiome influences human mental states through the gut-brain axis, a network of connected chemical, neuronal, and immunological pathways that enable bidirectional communication between gut bacteria and the brain. Through these, gut microbiota can influence behaviors like mood, social interaction, and even neurodevelopmental disorders like autism, neurodegenerative diseases like Parkinson's, and mood disorders like depression and anxiety (31).

The enteric nervous system, which orchestrates gastrointestinal function, is regarded as the human second brain. It produces and uses more than 30 classes of neurotransmitters which are also present in the central nervous system. Serotonin is a brain neurotransmitter, yet more than 90% of the body’s serotonin is synthesized in the digestive tract (32). In addition to serotonin, the gut produces and stores about 50% of the body’s dopamine (33). Gut microbiota regulates the level of serotonin and dopamine in the human body and increasing evidence proves that they also produce or consume other neurotransmitters, such as norepinephrine, gamma-aminobutyric acid (GABA), histamine and acetylcholine (34).

Interruptions in these neurotransmitters can lead to several mental disorders. Preclinical and clinical evidence associate depression with disturbance in serotonin, dopamine and norepinephrine transmission in the central nervous system. Decreased serotonin and increased norepinephrine levels are common in the pathophysiology of bipolar disorders. The serotonin network is the main target for most common classes of antidepressants. Many studies show  dopamine deficiency is associated with depression and Parkinson’s disease, while numerous other experiments and clinical data show decreased GABA function accompanied by depressed or manic mood states. Sleep disorders and drug and alcohol addiction have been related to irregular GABA function in the brain. Furthermore, research has shown that patients with anxiety disorder and depression have more gastrointestinal symptoms than the healthy controls (35). 

Soil influences human mental health to a great extent through the gut microbiome, but to address the definition and determinants of mental health put forth by international health authorities, various other ways in which soil can alter human mental states must be addressed. It is established that the gut is instrumental in producing and regulating the neurotransmitters that affect human mental states, but human macro- and micro nutritional needs must be met for the body to properly carry out these functions. 

2.3 Soil Health and the Living Environment: Forests, Green Spaces, Healthy Neighborhoods and Mental Health

Soil health is the foundation of ensuring natural and built environments are habitable. One key aspect of this is ensuring air quality, as healthy soil is a sink for air pollutants via nutrient cycling and release of gasses by diverse soil microbes. It supports diverse plant life that regulates air quality through O2 release and the trapping of air pollutants (36). Additionally, healthy soil can absorb, store and filter water. For example, a grassland in Miyagi Prefecture, Japan absorbed the majority of a snowmelt, and the minor amount of runoff it released contained lower concentrations of radioactive caesium than the standard limit value of drinking water in Japan (37). Similarly, pollutants from pharmaceuticals and personal care products were removed from wastewater by forests before reaching the groundwater and watershed outlet (38). 

Healthy soils can also cool the land through their structures that retain moisture and store heat, stabilizing the surroundings from temperature changes better than poor soils that have drier and looser structures. For this reason, cities in which plants and soil are scarce can become “heat islands” and warmer than other nearby areas (39). Transpiration from photosynthesizing trees and plants significantly cools the plant leaves and the surrounding air. The air temperature above green transpiring landscapes is typically cooler by a mean difference of 2.3 and 11.7 degrees Celsius, compared to bare soil or pavement respectively (40). 

Cities are socio-ecological systems in which urban soils provide important ecosystem services such as climate regulation, stormwater management, habitat for life, and social services such as pollution control and food security (41). These ecosystem services increase community resilience in dense urban areas when natural disasters strike. They also contribute to the level of citizen health and quality of life. Studies found that in the US, ozone exposure has been significantly higher in community parks located in disadvantaged communities compared to affluent community parks, while urban tree canopy, which could mitigate these effects, is positively related to household income (42). In addition, urban air pollution exposes residents, especially vulnerable populations, to unhealthy air that is worsened by the micro-climatological effects of buildings and other infrastructure and the associated decrease in vegetation due to limited soil resources (43). The clear way to mitigate these urban challenges is to introduce more spaces with healthy soil, which will support greater tree cover and vegetation as a means to improve urban air quality, bring cooler temperatures, and uplift disadvantaged, marginalized communities. With healthy soil and rich plant life, clean air, clean water, and stable temperatures can contribute to a more comfortable and pleasant living environment, eliminating stress on human mental well-being. 

Many studies have also proved that interaction with forests and green spaces established on healthy soil or direct contact with soil through horticultural activities can invite a wide range of positive mental states. Nature-based interventions to mental disorders have shown reduced anxiety, depression(44), and stress(45), as well as improved cognitive function(46). For example, the Psycho-Evolutionary Restoration Hypothesis (PERH) focuses on exposing people to old-growth forests or restored ecosystems to assess the influence of nature on mental health. These ecosystems are rich in phytoncides, which are volatile organic compounds that may reduce depression and lower cortisol levels (47,48,49).,, A review study investigated the effects of forest bathing on levels of salivary or serum cortisol as a stress biomarker in order to understand whether forest bathing can reduce stress. Results showed that cortisol levels were around 16% lower in groups exposed to forests compared to those exposed to urban areas.(50)

Another study measured reactive oxygen metabolite (d-ROM) levels and biological antioxidant potentials (BAPs) associated with psychological stress. Fifty-nine patients with chronic stroke were randomly assigned to either a forest group or urban group and scores on the Beck Depression Inventory (BDI), Hamilton Depression Rating Scale (HAM-D17), Spielberger State-Trait Anxiety Inventory (STAI), d-ROMs and BAPs were evaluated both before and after the treatment programs. BDI, HAM-D17 and STAI reduced significantly and BAP increased significantly by around 11% for the forest group. For the urban group STAI increased significantly and no significant change on BAP was measured(51). Three studies were conducted where participants spent time in nature, in urban areas or watched videos of natural and urban settings. In all three studies, exposure to nature significantly increased connectedness to nature (based on the Connectedness to Nature Scale (CNS)), attentional capacity, positive emotions, and ability to reflect(52).

The act of soil mixing can affect pleasant brain response through the inhalation of volatile organic compounds, or VOCs, from soil bacteria. Studies have found that olfactory stimulation can reduce mental stress, bring about relaxation and improve cognitive function. A fragrance molecule, when inhaled through the human nose, binds to the olfactory receptor in the nasal cavity and activates the olfactory receptor cell. It then sends an electrical signal to the olfactory bulb via the neural network to stimulate the hypothalamus and cerebral cortex through the limbic system of the brain to modulate brain functions like memory, thoughts, and emotions(53). Fragrance molecules further act on the neuroendocrine system, neurotransmitters, and neuromodulators, influencing psychological behavior and body function. In this way, fragrance molecules can influence human emotions, thoughts, and memory by influencing spontaneous brain function through the olfactory system. 

One study compared the physiological effects in adults during 5-minute soil mixing activities based on the presence or absence of the soil bacterium Streptomyces rimosus. Blood samples were drawn from 30 adult participants and metabolomic analysis revealed that serotonin was around 90% higher in the treatment group compared to the control group(54). The study found that S. rimosus produces VOCs that give soil the earthy-musty smell. Exposure to these VOCs increased serotonin levels and decreased CRP levels over a short period of time. CRP is an inflammatory biomarker, and a high CRP level is related to depression and euthymia. 

The effect of natural spaces and greenspaces on child mental health also needs further attention, especially when more and more children and adolescents globally are living with mental illness. According to UNICEF’s State of the World’s Children 2021 report (55), 1 in 7 adolescents globally (more than 13%) live with a mental disorder - that’s about 166 million adolescents, or 89 million boys and 77 million girls globally. One simple, cost-effective, way to alleviate this alarming statistic is to strengthen their contact and connection with natural spaces, especially healthy soil. A study conducted in Barcelona, Spain tracked the greenspace exposure of 350 school children using satellite imagery and geographic information systems from birth till age 20. The study found a positive association between exposure to green spaces throughout childhood and the structure of the brain in young adults. The findings suggest that spending time in nature during childhood may have long-term benefits for brain health and cognitive function(56). 

In the face of the myriad ecological and social services that soils provide, healthy soil and diverse plant life should garner more attention in the city-scape and in schools. Especially for marginalized communities, increasing spaces with healthy soil and rich tree cover and vegetation can be a cost-effective way to significantly enhance the quality of life, and of course, the mental well-being of residents of all ages. 

2.4 Soil Health and Farmer’s Well-being 

Soils affect individual human health as well as greater social well-being, but the plight of one group of society has gone largely ignored: Farmers. Farmers are the social group that spends the most time in direct contact with soils, yet face greater suicide rates than the general population, according to studies from India, Sri Lanka, USA, Canada, England and Australia (57). A systematic review on farmers’ mental health cites four main influences on farmers’ mental health: pesticide exposure, financial difficulties, climate variabilities/drought, and poor physical health/past injuries (58). Small-scale farmers within a study cited their values were to grow resources for the family, have good health, enough money, enough to eat, and a clean environment (59). For these farmers, a healthy soil nourished through regenerative agriculture can be the one simple solution to achieve all these aspirations. 

Farmers in India rely heavily on rainfed agriculture and are vulnerable to the effects of climate change like drought and heat waves(60). Climate change along with unfavorable economic circumstances are contributing factors in farmer suicides not only in India(61,62), but also US (63) and Australia (64). Healthy soils underlie climate resilient agriculture in the face of climate change (65). All these factors can create favorable socio-economic situations for farmers and positively affect their sense of well-being (66). 

  Data from multiple studies conducted by the Soil Health Institute demonstrates increased net farm income resulting from adopting sustainable soil management practices. For example, eliminating tillage and the usage of cover crops reduced farm expenses for corn and soybean farmers in 9 states across the US Corn Belt (67). A reduction in farm expenditures can lead to less financial stress on farmers’ mental well-being. In addition, studies from 1999 to 2020 conducted with over 2,000 farmers from different countries found that organic farmers were less depressed, sad and angry and had fewer neurological symptoms and behavioral symptoms than conventional farmers (68). Another set of studies found organic farmers  held positive feelings for the transition from conventional to organic farming, such as freedom, pride, pleasure. On the other hand, conventional farmers held negative attitudes towards their farming methods due to livestock methods and pesticide use (69). A 2014 study found organic farmers and their families had better health status than conventional farmers and their families (70).

Another example of climate change-inflicted farmer casualties is the positive correlation between groundwater availability and farmer suicides, cited by one study, in the Indian state of Maharashtra, where mass farmer suicides have captured the world’s attention (71). In Australia, there is a strong correlation between droughts and farmer suicide (72). Healthy soil structure allows for great water holding capacity, infiltration and percolation abilities, which can recharge groundwater supplies and provide water for crops and livestock even during dry seasons. 

However, studies have not discovered conclusive evidence of psychiatric causes to farmer suicides. There is evidence from Karnataka, Kerala, Tamil Nadu, and other Indian states that socioeconomic stress may play a greater role in farmer suicides. The NCRB (National Crime Records Bureau, India) 2014 data indicate that almost 75% of the 5,650 suicides recorded during 2014 were by small and marginal farmers exposed to severe socioeconomic hardship (73). The same NCRB report attributed bankruptcy, indebtedness, crop failure, and other farming-related issues to more than 60% of the suicide cases. Farmer's income from crops depends on the fluctuating market situation, greedy middlemen, and selling logistics, and others that are beyond the farmer's control. Insufficient government support and marginal profits force farmers to take loans, mostly from private lenders with high interest rates. When they are not able to pay off the loan, they have to take fresh loans, and are thus caught in debt traps. An article form the Indian Journal of Psychiatry suggest that the state should play a greater role in farmer suicide prevention rather than mental health professionals (74). 

Soil is the farmer’s base for livelihood, but soil care is not and cannot be the concern of farmers alone. If soil health is to take its rightful place at the forefront of global mental health, then it will begin with farmers and the soils they nourish.

Conclusion

Healthy soil provides ecological services and cultural services, defined as the “non-material, and non-consumptive outputs that affect the physical and mental states of people (75).” The different pathways through which soil health influences mental health is testament to the central and pervasive role soil has in human well-being. In addition, many of the UN SDGs rely on healthy soil to realize zero hunger (SDG 2), human health and well-being (SDG 3), clean water (SDG 6), sustainable cities and well-being (SDG 11), climate action (SDG 13), and life on land (SDG 15). It touches and informs human experience of life in more intimate ways than is publicly known or accepted. Soil is the convergence point of many environmental and socio-economic issues the world faces today. Therefore, regeneration of soil health merits global attention, resources, and effort. 

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