promoting the sustainable and responsible use of cobalt in all forms

Cobalt in soils

Here, the importance of cobalt in soils is explained, concentrating on European Soils for which more information is currently available.

How much cobalt is in European soils?

Cobalt in soils throughout the world result from a combination of natural and man’s activities. Cobalt soil concentrations depend on a number of factors including local geology, atmospheric deposition of cobalt-containing dust, land use and associated amendments, mineral particle distribution, soil age, and climactic and transport factors affecting localised concentrations (Wendling et al. 2009). The average cobalt concentration in European soils is between 1-20mg/kg dry weight, although this can become much higher in areas which are geologically rich in cobalt such as North Wales. For example, Paveley (1998) found natural levels of cobalt at over 2,500mg/kg dry weight in soil. The study noted that the area had a healthy eco-system that had adapted to these naturally high cobalt concentrations.

A very large amount of cobalt would have to be introduced into a volume of soil before local wildlife could be adversely affected.

A study aimed at the determination of geochemical baseline concentrations for soils, stream waters, and sediments across Europe was recently published by the Forum of European Geological Surveys (FOREGS; Salminen et al 2005). The survey was designed to demonstrate the current geochemistry of the surface environment; data were based on samples of stream water, stream sediment, and soils collected from all over the European continent. Results from this effort (i.e., statistical data) and maps of cobalt concentrations and other metals in European soils can be found on the FOREGS website (http://weppi.gtk.fi/publ/foregsatlas/). An additional study, the GEMAS (Geochemical Mapping of Agricultural and Grazing Land Soils) project, was carried out by the EuroGeoSurveys (EGS) Geochemistry Expert Group. This study provides high quality data for metal concentrations and soil properties known to influence metal bioavailability (pH, organic matter content, clay content and effective CEC) in agricultural and grazing land soil in Europe (Reimann et al . 2009, 2011). The median and 90th percentile total cobalt concentrations from FOREGS data are respectively 7.0 and 17.0mg/kg dry weight. In the same range GEMAS database for grazing land present a median of 7.2 and a 75 percentile of 12mg/kg dry weight (Albanese et al 2015).

Similar geochemical and mineralogical maps have recently been published for the United States (Smith et al. 2014). Mean cobalt concentrations in this study was 7.8 mg/kg in topsoils (0-5 cm depth).

The majority of cobalt in the soil is not biologically available, i.e., cobalt forms stable carbonate and hydroxide minerals that cannot be absorbed by animal or plant life (Perez-Espinosa et al. 2004). Consequently, a very large amount of cobalt would have to be introduced into a volume of soil before local wildlife could be adversely affected.


How does cobalt get into the soil?

Cobalt occurs naturally in soils through two major pathways: the breakdown of organic matter, and the weathering of the local minerals into soil particles. Soil mobility is inversely related to the strength of adsorption by soil constituents. Adsorption of cobalt to soils is rapid (Kim et al. 2006). Man’s contribution to cobalt concentrations in the terrestrial environment has arisen mainly from mining, smelting and industrial activities (Collins and Kinsela 2010). Mankind also adds cobalt to the soil, primarily through three mechanisms. The major mechanism is use of cobalt salts, e.g. cobalt sulphate, as a feed additive to keep cattle and crops healthy in areas where there is insufficient natural bioavailable cobalt. Smaller amounts of cobalt also enter the soil from the airborne transport of particulate emissions and application of sewage sludge onto fields.

Moose had inadequate levels of cobalt
Moose in Eastern North America were observed to have inadequate levels of bioavailable cobalt in their diet


Why is cobalt added to some soils?

Due to problems associated with cobalt deficiency in agricultural soils, the behaviour of cobalt entering, and within, soils has been studied for a number of years. A lack of cobalt in a form which plants or earth dwelling organisms are able to absorb can have major effects on the health of the wildlife in an area. A classic example of this is the “Nova Scotia Moose Mystery” (Frank et al. 2004), where moose in Eastern North America were observed to have a wasting debilitating disease. It was found to be related to inadequate levels of bioavailable cobalt in their diet. The authors concluded that cobalt salt licks should be introduced in limited areas of Nova Scotia to balance the moose’s diet and restore them to health.

Bioavailable cobalt in soil is also necessary for the healthy functioning of some plants. This is especially true for leguminous plants, with cobalt being an essential nutrient for the micro-organisms which fix atmospheric nitrogen in the plants’ root nodules (Gad 2002).


References and further reading


DISCLAIMER

This summary is intended to provide general information about the topic under consideration. It does not constitute a complete or comprehensive analysis, and reflects the state of knowledge and information at the time of its preparation. This summary should not be relied upon to treat or address health, environmental, or other conditions.