Supriatin Supriatina, Liping Wenga, Wim Bussinkb, Rob N.J. Comansa
a Department of Soil Quality, Wageningen University; b Nutrient Management Institute NMI Agro, Wageningen
Have you ever heard an element called selenium (Se)? This element is apparently one of the essential nutrients required by humans and animals (cattle or livestock). Studying the amount and availability of selenium in agricultural soils in the Netherlands is as important as finding options to have adequate intake of selenium in humans and cattle. In this article we will bring you to the results of current research about the consequences of low selenium content in agricultural soils, especially in the Netherlands, on the above ground food chain, including crops, animals (cattle or livestock) and humans, and possible options to alleviate the effects.
It is common for Dutch people to drink milk every day, either during breakfast or lunch. Up to now, they probably do not realize that their milk does not only contain protein and vitamins, but also essential nutrients, such as iron, zinc, etc., including selenium. Any other foods such as cereals, meat, seafood, nuts, and other dairy products also contain selenium.
Table 1. Selenium rich foods. Source: Vineetha. October 27, 2014. Twenty selenium rich foods you should include in your diet. http://www.healthbeckon.com/selenium-rich-foods/
(µg Se per 100 g serving)
|% Daily Value (DV)a|
|Eggs, 1 medium size||13.9||20|
|Whole grain rye||35.3||50|
aDaily Value of selenium for human intake is 70 µg per day.
What is the primary source of selenium in our foods? The answer is soil. Selenium in soil comes to our foods through a food chain (soil → crops → humans or soil → crops → cattle → humans). The amount of selenium ends up in our foods depending on the amount of selenium in soils, especially the available ones.
Humans and cattle require selenium in a small amount, but it is essential, we call it as essential micronutrient. On the other hand, selenium is not an essential micronutrient for plant growth, although plants can take up selenium from soil. In human body, selenium helps to protect cells from damage (i.e. antioxidants), enhance immune system, maintain thyroid health, and reduce cardiovascular disease and cancer risks as suggested in previous studies. For cattle, sufficient selenium intake gives benefits to increase growth rate, immune system, and fertility.
As we may know that dairy farming is one of the major agricultural sectors in the Netherlands. Therefore, research on selenium in agricultural soils in the Netherlands is being an interest because selenium relates to the health status of cattle and the nutrition value of agricultural and dairy products for human intake. In order to have sufficient intake of selenium in cattle, in practice selenium is commonly added in fodder. Another option to increase selenium intake in cattle (through grass), such as selenium fertilization on grasslands, has not been considered so far.
Our recent study showed that around 75% of 83 soil samples taken from grasslands and arable lands in the Netherlands are mainly in the low range of total selenium content (i.e. selenium deficient). The low total selenium content in the soils has led to the low selenium content in crops, such as in wheat and grass, although it is not always true, depending on the amount of available selenium in soils.
Further finding indicated that without selenium fertilization on grasslands, selenium content in grass samples taken in the Netherlands is mainly too low for dairy cattle intake. Consequently, their dairy products may also contain low amount of selenium for human intake. The wheat grown on arable land soils in the Netherlands contains low selenium for human intake also.
The amount of selenium in crops depends not only on the total selenium content in soils, but particularly on the available amount of selenium in the water phase of the soils. Only a few percentage of the total selenium content in soils are available in the water phase and able to be taken up directly by crops. This available selenium can be as soluble inorganic selenium ions or soluble small organic selenium. Other selenium form in the solid phase and water phase of the soils, especially in Dutch soils, is predominantly organic in colloidal size or bigger, which is not readily available for crop uptake.
Actually, what controls the (potentially) available selenium in Dutch agricultural soils for crop uptake? Our recent study has revealed that soil properties, such as pH, soil carbon to nitrogen ratio (C:N ratio), clay content and soil organic matter content are relevant. In general, (potentially) available selenium in soils increases towards high pH, high clay content, low soil C:N ratio and low soil organic matter content. Agricultural management practices, such as application of nitrogen (N), phosphorus (P) and potassium (K) together on soils influences selenium uptake by crops as well. For example, selenium content in grass grown on grassland peat soils decreased up to 50% due to NPK fertilization. This decrease is likely associated with the increase of soil organic matter content. Other factors, such as soil water content (and thus oxygen content) and soil temperature may also influence mineralization of unavailable organic selenium into available selenium in the water phase for crop uptake. From all of these factors, soil pH, NPK fertilization and soil water content can be managed in a certain way to increase the availability of selenium in the water phase for crop uptake. However, the increase of selenium availability upon management practices may not be as significant as application of selenium fertilizer.
In summary, agricultural soils in the Netherlands are too low to supply sufficient amount of selenium in crops for cattle and human intake due to low amount of available selenium in the soils. Similar condition is also faced by agricultural soils and crops in other parts of Europe, such as Sweden, United Kingdom and Belgium as shown by other studies. An input from selenium fertilizer can be an option to have an adequate amount of selenium in crops for cattle and human intake. However, the scarcity of mineral selenium resources is a challenge for the application of selenium fertilizer in agriculture. Therefore, in order to increase the availability of selenium to crops and to use selenium fertilizer efficiently, appropriate management practices on soils should be integrated with application of selenium fertilizer. Further research is necessary in this field. Moreover, taking various foods containing selenium is another option to have sufficient intake of selenium in humans.
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