1.7: Soil Health, Plant Health and Human Health

Of the literally tens of thousands of species of soil organism, relatively few cause plant diseases. And the same is true for human diseases, with examples such as tetanus (a toxin produced by a bacterium), hookworm (a nematode), and ringworm (a fungus). But the physical condition of soil can also affect human health. For example, people in the path of dust storms, which pick up fine particles from bare soils, may have significant respiratory problems and damaged lung tissue. In general, soils with a high degree of biological diversity, good soil structure and continual cover with living plants will be healthier for people as well as the plants growing in them. In fact, frequent contact with soil and farm animals early in life results in fewer allergies and stimulates the immune system, helping it to better respond to infections as one grows older.

We discuss soil degradation in this chapter because protecting soil’s productivity and limiting environmental impacts are important objectives in and of themselves. However, there are ongoing debates around the world about whether improved soil health also translates into better-quality food and human health outcomes. Soils are the primary source of minerals for humans and animals, but can soil degradation eventually lead to nutrition and health problems? Also, is organically produced food healthier than conventional foods?

To answer these questions we need to understand the two main components of the food chain: how soil health affects plant health and how plant health subsequently affects human health. Together, this is the soil-plant-human health connection. For our discussion we’ll ignore the impacts of intermediate steps of food processing, diets and food sourcing, although these can also have significant impacts. Soils provide plants with nutrients and water, but this doesn't always happen in an optimal way. Healthy plants require essential nutrients like nitrogen, phosphorus, potassium and other major and minor elements discussed in Chapter 18. Other elements are not essential but are considered beneficial because they have a positive effect on plant growth or help the uptake of other elements. These are typically taken up by plants in trace amounts. A third category is toxic elements that are detrimental to plants at certain concentrations. Sometimes, elements are essential or beneficial at low concentrations and may become toxic at high concentrations, like copper and iron.

Nutrient Deficiencies

When crops are grown over many years, nutrients in soil are steadily absorbed by plants. In natural ecosystems the nutrients in plant material are mostly cycled back to the soil, but agricultural systems generally remove many of these nutrients from the farm when the harvested crops are sold, with variable amounts of nutrients remaining on the farm in residues, depending on the crop. (We discuss cycles and flows in Chapter 7). With the use of synthetic fertilizers some nutrients, notably nitrogen, phosphorus, potassium and calcium, are being replenished, but the minerals needed in small or trace amounts generally don’t get replaced. This is especially the case in developing countries where farmers often don’t analyze their soils and they apply standard fertilizer blends. Sometimes this is aggravated by compaction problems, when the minerals may be present in deeper soil layers but are not root accessible. In some cases soils are naturally deficient in essential elements that may affect plants, animals or humans. For example, selenium is naturally low in the northeastern and northwestern United States. It does not affect plants much but can cause problems with animals and humans.

Toxicities

Many elements in soil can become toxic to plants, animals or humans. The most egregious cases tend to be associated with some type of pollution from human activities. For example, heavy metals may have accumulated from atmospheric deposition of industrial smokestack emissions or from acid deposition from coal-fired power plants. In other cases agricultural activities themselves cause problems, like the long-term use of fertilizers containing high levels of cadmium. An unusual case involved the introduction of tube wells in Bangladesh to irrigate rice. The groundwater source contains naturally high levels of arsenic, which accumulates in the rice grains, causing serious health concerns with local populations. (A common occurrence in regions of grain crop production is the over application of nitrogen fertilizer, which can lead to high concentrations of nitrate in drinking water, which adversely affects the health of rural residents. Although this problem is not a result of direct consumption of plants, it is directly related to how we grow crops.)

Another issue is that crops growing on soils low in biodiversity, in which plant disease organisms flourish, are generally treated with pesticides (fungicides, insecticides, nematicides). These chemicals, as well as herbicides, may find their way into the foods we eat, sometimes into the groundwater we drink. There has been a link established between a number of pesticides in the environment and human diseases.

Human Health Effects

It is difficult to scientifically prove effects of soil health on human health, in part due to the complexity of diets and ethical considerations around clinical trials involving humans. The most significant effect of soil degradation relates to the reduced ability to produce sufficient nutritious foodstuffs to meet peoples’ basic caloric and protein needs. Especially in isolated rural areas in developing countries people depend on crops and animals raised on their own farms with little opportunity to buy additional food. Degraded soils and weather extremes can cause crop losses and significantly impact the food supply, with especially high concerns for the long-term impacts to children.

A secondary problem associated with soil degradation is deficiencies of essential minerals, especially in soils that are naturally of low fertility. Again, this may be a problem in regions with mineral mining and heavy dependence on local grain-dominated diets. In developed societies nutritional deficiencies are rare because people obtain food from diverse sources. For example, regional soil selenium deficiency does not impact people when they also eat nuts from other regions. (In developed societies, the concern is increasingly about unhealthy diet choices and the affordability of healthy food.)

Humans also benefit from organic plant compounds that may be indirectly linked to soil health, like the protein content in grains (related to nitrogen in soil), or so-called secondary metabolites that have beneficial health effects, like antioxidant activity (for example, phenolics and anthocyanins). A question is whether we can link the benefits of better soil management to actual higher human health outcomes. For example, organic management requires certain practices that enhance soil health because it involves integrated nutrient and organic matter management through better use of rotations and organic amendments. But will it also improve food quality and human health? Many people choose organic foods due to concerns about pesticides (which is a real potential health issue that we should be aware of) or because they believe it tastes better. Or they feel strongly about supporting farmer livelihoods and reducing environmental impacts. There is no evidence that nutrients from organic sources affect human health differently than those from synthetic or processed sources, because either way plants take up the nutrients almost exclusively as inorganic forms. Some studies have shown that organically produced food can positively impact some indicators such as increased levels of antioxidants. But due to many other confounding factors (people who eat organic food typically have better diets, healthier lifestyles, and are wealthier), no study has been able to definitively correlate those with positive human health outcomes.