Though iodine was among the first nutrients recognized as vital to humans today, deficiency affects 780 million people worldwide. Inadequate intake of iodine can result in a number of disorders including: miscarriage, stillbirths, cretinism (permanent, severe mental retardation, deaf-mutism and motor spasticity disorders), goiters, impaired mental function, retarded physical development and hypothyroidism. Iodine deficiency disorder is the leading cause of preventable brain damage in children worldwide and deficiency alone can lead to IQ levels 10 to 15 points lower than those with adequate levels of iodine consumption. In 1990, it was found that nearly 30% of the world population was iodine deficient and 11 million people were affected by cretinism. When taking a closer look at Iodine Deficiency Disorder (IDD), it is found to characterize areas with low bioavailability of iodine in soils.

Regardless of food preference, lifestyle and consumption patterns, micronutrients come from the same source: the Earth. Minerals are naturally occurring substances in the Earth’s crust and vitamins are chemical substances found in food that is grown in the Earth or raised on plants. It is well understood that plant growth and crop yield depends on the nutrients present in the soil and as such, these nutrients can be modified to optimize both growth and yield. While some food plants and meat are richer sources of micronutrients, the soil is the predetermining factor of micronutrient levels in any plant. Crops grown in low-quality or deficient soils have lower nutrient content in both shoot and seed. Furthermore, soil type is an identifier of micronutrient deficient populations. For example, soils with low organic matter and highly weathered parent materials are prone to zinc deficiencies. Sedimentary rock, especially shale, is rich in iodine while igneous rock has very low levels of iodine.

While other nutrients, like zinc, are derived from the parent materials of the soil (minerals and organic material transported by water, ice, wind, etc. that make up soil) the majority of iodine in soils is derived from the atmosphere and ultimately, the oceans. But the organic content, texture, oxidation and pH condition of the soil (which parent material does have a role in) determines retention and concentration of iodine. Sandy soils and low clay soils report very low levels of iodine while clay-rich soils, organic rich soils and alkaline soils report much higher levels of iodine. Despite the high concentrations of iodine in organic-rich soils, the iodine is strongly fixed and in the form of iodate, which is insoluble to plants or the food supply. So, iodine deficient soils and iodine rich soils alike present problems for raising healthy, nutritionally robust citizens.

Soil solutions to iodine deficiency

Iodine Deficiency Disorder (IDD) is found to characterize areas with low bioavailability of iodine in soils, presenting a case for soils to be utilized as an avenue for combating and preventing IDD. Despite this, most intervention programs focus on fortification initiatives, specifically in the form of iodized salt. Universal salt iodization was recommended by the World Health Organization and UNICEF in 1994 to combat iodine deficiency. USI is the process of iodizing human and livestock salt, including salt utilized in the food industry for processing. Alongside seafood, eggs, and dairy, iodized salt is one of the richest sources of dietary iodine. Despite iodized salt being widely available, many populations still suffer from iodine deficiency disorders all across the world. There have even been cases of endemic goiter found in areas with iodized salt programs, which brings to light the need for alternative solutions to combating iodine deficiencies.

The population of Xinjiang Province, China suffers from high goiter prevalence and a high infant mortality rate, making them prime recipients for iodized salt. Despite iodized salt programs introduced in the area, local preference for rock salt has prevented uptake of the fortified variety. To improve the population’s iodine intake, a study in 1992 introduced a drip of potassium iodate to the irrigation water used on the locally consumed crops. Use of the iodized irrigation water led to soil iodine levels that were three times higher than before and plant and animal iodine levels that were two times higher. The population also reported higher iodine levels with a significant increase in children’s iodine levels as measured by median urinary iodine excretion. The following season reported a four-fold increase in soil iodine and a nearly 2-fold increase in crops. Another study in the same area found that the addition of potassium iodate to irrigation water was responsible for increasing iodine concentrations in women of child-bearing age and reducing neonatal and infant mortality.

A study using iodized fertilizer found increases in the iodine concentrations in food plants. A kelp-based iodized fertilizer was created that would also prevent volatilization, or losses, of iodine from soil. After application of the fertilizer to three different crops, iodine concentrations in the leaves, fruit and roots increased. A specific quantity of the fertilizer also left an increasing residual concentration of iodine in the soil that could be transported into surface and ground water, resulting in a broad spectrum of iodine in the local area.

HarvestZinc which focuses on use of zinc fertilizers to improve zinc grain concentrations, will be introducing iodine fertilizer in a new project phase. This phase will focus on learning more about the role of iodine fertilizer in improving iodine concentrations in cereal crops. The project will explore and test the impact of iodine fertilizers on iodine concentrations in wheat and rice varieties grown in several countries.

These studies, along with many others, show the success of soil interventions in remediating iodine deficiency and relevance for additional research. Especially where the population has access to iodized salt but is not utilizing it, iodine fertilization or iodized irrigation water can be easily utilized and integrated to improve iodine intake levels. Because soils are varied across the world, it is important to note that there are no one-size-fits all solution. Improving the soils capacity to retain iodine should go hand-in-hand with introductions of iodine through fertilizer or irrigation water, which will vary from region to region and even crop-to-crop. Additional research could provide more context-specific insight on the use of soil interventions to improve human nutrition outcomes.

Impaired immune function, disease, increased mortality and morbidity rates, lower worker productivity, decreased intellectual performance and a lower standard of living are the consequences of micronutrient deficiencies. The base of nutrient availability is tied to agriculture and the local food systems so it is paramount that efforts be focused at this level when formulating interventions. While further research is still needed, soil interventions are a worthy investment to decrease micronutrient malnutrition.