Clearly, the greatest direct benefit of earth materials to public health with respect to what people eat is that surface soils provide a medium for food production, either directly consumed by humans or indirectly consumed via food animals. In either case, plant nutrition is the result of soil characteristics that ultimately affect human health and welfare. Many soil microorganisms aid plant growth and food production. These include free-living microbes in the rhizosphere and symbiotic associations involving rhizobia and mycorhizal fungi. Both of these symbiotic associations improve the nutritional content of plants, contributing nitrogen, phosphorus, and trace elements. In addition, some natural soil microbes suppress plant pathogens (Press et al., 2001; Zehnder et al., 2001), and other soil microbes can remove or transform organic toxicants in soil (see Box 5.1).
The simplest examples of geologically influenced direct microbial threats to human health in food are soilborne human pathogens. Many of the major enteric pathogens are transmitted via the fecal-oral route. Pathogenic organisms in soils can infect and damage either food crops or the animals and humans that ingest them (Tate, 2000). Antibiotic uptake by plants has also been demonstrated, with clear potential to adversely impact human health (Kumar et al., 2005). Some human pathogens are naturally present in soils but do not commonly infect plants as an intermediate host (food spoilage and fermentation are excluded as examples here). The most common mechanism for transmission to humans is from soil adhering to unwashed agricultural products or by transfer of waterborne pathogens introduced during irrigation or food processing (Heinke, 1996; Maier et al., 2000; Tate, 2000). Agricultural uses of treated sewage sludge (biosolids), sewage effluent, or human waste (night soil) are all potential sources of human pathogens in agricultural products. Human pathogens found in these materials include viruses (e.g., coxsackie or poliovirus), bacteria (including Salmonella and E. coli), and protozoan parasites (e.g., Giardia or Cryptosporidium) (NRC, 2002a).
Another important group of introduced pathogens commonly present in soils are helminthes (worms—roundworms, flatworms, and tape-worms). Roundworms (nematodes) are the most common helminthes in soils, and these are frequently ingested by people in developing countries and in the southeastern United States. Surveys have demonstrated that 75% or more of the populations in rural areas in Latin America and Africa are infested with intestinal roundworms such as Ascaris lumbricoides. In many cases, their presence is asymptomatic, but in other cases, heavy worm burdens can cause anemia, vitamin deficiencies, and blockages of the intestine and common bile duct.
The other direct threat to public health is from plant pathogens that