A concept that lives with many is professionals in the agronomic sector is that "healthy soils" will produce healthy and nutritious food that in turn will ensure healthy human beings and animals (Albrecht 1975; Hornick 1992). The notion of soil as a fundamental basis for everything on a farm is a core idea of the original organic pioneers. Practicing according to this idea was abandonment after World War II when food quantity rather than quality was most essential. At present more and more attention is paid to the nutritional quality of crops and “soil health” in particular as important influential factor.
(i) The absence of direct toxic effects such as radioactive or chemical contamination, either naturally occurring or man-made. Heavy metal pollution, e.g. Cd, is an example of adverse chemicals that can be in soils and or are added by cheap fertilizers, e.g. phosphate fertilizers.
(ii) From (horticultural) research we know that a balanced soil nutrient content is most likely to provide optimal growth and a good supply of nutrients to the plant. One has to be aware of the dilution effect, i.e. better growth that might reduce nutrient content of specific nutrients (§5.1.7). It might therefore be useful in particular cases to “over supply”, i.e. increase certain nutrient concentrations in the soil above the threshold than needed for optimal growth (Figure 5.2). Examples of nutrients that might be increased by fertilisation are e.g. Zn (§5.2.4) or Se (§5.2.5). Note however that Zn is a heavy metal and should not be overfertilized. Moreover, as shown in Figure 5.4 Zn will reduce the uptake of iron. Selenium fertilizers will interact species dependent manner with sulfur (§5.2.5), i.e. in some plant species Se and S are symbionts and in some antagonists.
(iii) There might be a general positive effect of soil health itself on plant and animal health, but this is hard to quantify and is often more a gut feeling than a science-based fact. As discussed in chapter 2 and 3 we know that increasing vegetable and fruit consumption benefits human health. Phytochemicals might partly cause these beneficial effects as indicated for several specific chemicals by studies on model species like rats. However, increased amount of these phytochemicals in plants is hard to relate to an improved human health. Besides, it is more likely that adverse soil conditions instead of a healthy soil, will generate stress responses and thereby increase all kinds of phytochemical levels. One of the components of a healthy soil is a well-functioning soil ecosystem. Literature on the beneficial effects of the soil ecosystem on plant health show many inconsistent findings. Therefore, the effects of soil organisms on plant growth and health remains an ongoing and active field of research (Reeve et al. 2016). However, the impact can be big as the number of microbial cells inhabiting the plant rhizosphere greatly outnumbers plant cells, and the community of organisms present at this interface is often referred to as the second genome of the plant, or its microbiome (Berendsen et al. 2012). Although plants are perfectly able to grow under completely sterile conditions, in open field agriculture the plant microbiome has a big impact on plant health. This is because open field conditions are far from optimal for plant growth and by increasing access to water and nutrients (e.g. mycorrhiza) and modulating resistance to pests and disease microbes can really be beneficial in open field agriculture. They can improve both soil and plant health, which in turn might be related to human health by future research, but at present evidence is scarce and ambivalent. More research in this domain is needed to clearly make the relation between soil health and human health. Nevertheless, the (i) absence of toxic chemicals and the (ii) presence of balanced soil nutrient levels can already be scientifically related to human health as discussed throughout this document.
Albrecht WA. 1975. The Albrecht Papers (J Charles Walters, Ed.). Raytown, Missouri: Acres U.S.A
Hornick SB. 1992. Factors affecting the nutritional quality of crops. American Journal of Alternative Agriculture 7: 63. DOI: 10.1017/S0889189300004471.
Reeve JR, Hoagland LA, Villalba JJ, et al. 2016. Organic farming, soil health, and food quality: Considering possible links. Elsevier Inc. DOI: 10.1016/bs.agron.2015.12.003
Berendsen RL, Pieterse CMJ, Bakker PAHM. 2012. The rhizosphere microbiome and plant health. Trends in Plant Science 17: 478–486. DOI: 10.1016/j.tplants.2012.04.001.