In addition to calories, people and animals need a range of nutrients, including amino acids, lipids, vitamins and minerals, to stay healthy. Most of these come from our diets (Vitamin D from sunshine is one exception). While the amino acids, lipids and vitamins in our diets are produced by plants or animals, all the required minerals originate in soils. Many of the minerals that animals need are also necessary for plants to grow and develop. Plants access these minerals from soil and we, in turn, meet our needs for minerals either through eating plants or other animals that meet their need for minerals by eating plants. Our staple grains, including wheat, soybean, corn and rice, are our primary source of calories for energy, but they also are a source of important mineral nutrients like phosphorous (P), potassium (K), magnesium (Mg), calcium (Ca), iron (Fe), zinc (Zn), copper (Cu), manganese (Mn) and selenium (Se).
Even with sufficient calories, a diet that does not provide enough of these nutrients creates a deficiency often referred to as “hidden hunger.” Hidden hunger currently affects more than 3 billion people around the world, most stemming from micronutrient deficiencies. There are well-established links between mineral deficiencies and slow growth and learning in children.
The green revolution has been able to increase crop yields through development of high-yielding varieties and use of nitrogen and phosphorus fertilizers. However, in many cases the plants that are grown do not contain high enough concentrations of micronutrients to keep people healthy. Introduction of improved and better yielding varieties have resulted in lower mineral content in grain crops (Garvin et al., 2006) as well as in vegetables and fruits (Davis, 2009). Research is under way to develop crop varietals that are both more efficient at taking up these nutrients from soils and more easily absorbed by people.
One other tool to increase plant concentrations of micronutrients and thereby reduce hidden hunger is to increase micronutrient content in soils. During the early 1930s, manures and compost were the main sources of soil nutrients whereas use of synthetic fertilizers and other chemicals was rare. After World War II, the agricultural practices changed drastically and use of synthetic fertilizers and insecticides/pesticides became common practice. This increased reliance on NPK fertilizers and synthetic pesticides occurred without consideration of soil health or micronutrient status.
One consequence of the production of high-yielding annual crops through use of synthetic fertilizers has been reduction in the micronutrient content of soils. The capacity of soils to supply micronutrients has been exhausted. As a result, micronutrient deficiency is on the rise in the United States and worldwide. Figure 1 clearly shows widespread Zn deficiency in our soils.
Soil deficient in micronutrients translates directly to deficiencies in the food supply. The quality of food — especially in terms of macro and micronutrients — has kept declining (Garvin et al., 2006). Historical data show that between 1919 and 2000, grain concentration of Fe, Zn and Se declined at a rate of 0.23 percent/year to 0.36 percent/year. This means the wheat being consumed now has about 18 to 30 percent less micronutrients than the grains produced in 1919. Milling and processing grains further decrease the Zn and Fe content by as much as 70 percent in comparison to unprocessed crops (Welch and Graham, 1999). There is evidence that certain fruits and vegetables have lost 5 to 40 percent of certain micronutrients and minerals over the last 50 to 70 years (Davis 2009).

Integrated Nutrient Management

Steps to improve nutrient density should not only be focused on plant breeding but also enhancing micronutrient availability in soils. Integrated nutrient management offers potentially the best approach to doing this. With integrated nutrient management, fields are fertilized with a combination of inorganic fertilizers and organic amendments like biosolids, manures and composts. These amendments all originate from plant material and thus contain all of the micronutrients necessary for plant growth. Although they have traditionally been used primarily for their nitrogen, phosphorus and soil conditioning value, they also can be a source of micronutrients. Each person in the United States is responsible for production of 48.5 lbs of dry biosolids, 194 lbs of yard trimmings, 174 lbs of food scraps and 800 lbs of manure annually. Approximately 50 percent of the biosolids, 97 percent of the food scraps, and 45 percent of the yard trimmings are currently disposed — a tremendous loss of nutrients that could replace a lot of inorganic fertilizers and replenish micronutrients and minerals in agricultural soils if they were to be land applied (Brown et al., 2011).
Much of the research on land application of these materials has been done with the focus of protecting soil and ecosystem health from excess concentrations of many of these elements.  Regulations and source control have reduced metal concentrations in biosolids to levels well within the safety limits set by the US EPA. It may now be appropriate to reexamine these materials as a micronturient fertilizer for plants. The Metropolitan Wastewater Reclamation District of Greater Chicago monitored soil and plant concentrations of select metals on a long-term site both during and after biosolids applications. Applying biosolids increased corn kernel Zn concentrations but did not change kernel copper concentrations.
Many studies have shown that the soils’ net primary productivity is increased by improving soil organic matter whether it is attained by conservation tillage or using organic amendments like manures, composts or biosolids. This could be achieved by integrated use of organic amendments and synthetic fertilizers. In addition to providing necessary nutrients for growing crops, these organic amendments act as a soil conditioner, sequester carbon in soil and play a critical role in improving soil physical properties such as water retention, porosity, structure and bulk density, thus making the soil more productive and a better medium for plant growth (Brown et al., 2011).
Kuldip Kumar, Lakhwinder S. Hundal and Albert E. Cox are with the Metropolitan Water Reclamation District of Greater Chicago. Sally Brown is with the University of Washington.

References

Brown, S., K. Kurtz, A. Bary, and C. Cogger. 2011. Quantifying benefits associated with land application of organic residuals in Washington State. Environ. Sci. Technol., 45: 7451-7458.
Davis, D.R. 2009. Declining fruit and vegetable nutrient composition: What is the evidence? HortSci., 44: 15-19.
Garvin, D.F., R.M. Welch, and J.W. Finley. 2006. Historical shifts in the mineral micronutrient concentration of US hard red winter wheat germplasm. J. Sci. Food Agr., 86:2213-2220.
Welch, R.M. and R.D. Graham. 1999. A new paradigm for world agriculture: meeting human needs productive, sustainable, nutritious. Fld Crops Res., 60: 1-10.