International Journal of Plant & Soil Science

Cobalt (Co) is a transition metal of increasing environmental and biological significance because of its essential and toxic roles across soil-plant-animal-human systems. This review aims to synthesise the literature on cobalt across the soil-plant-animal-human continuum. Naturally occurring in minerals such as cobaltite and erythrite, its concentration in soils varies widely depending on parent material, climate, and anthropogenic activities, including mining, fertiliser use and industrial waste disposal. Cobalt predominantly exists as Co²⁺ and Co³⁺ in soils, with its solubility, mobility and plant availability strongly influenced by soil pH, redox status and interactions with organic matter. Acidic soils enhance its presence in the soil solution, whereas alkaline conditions reduce availability through precipitation and adsorption.

In plants, cobalt is not universally classified as an essential nutrient; however, it performs important biochemical functions, particularly in nitrogen-fixing legumes. Co supports Rhizobium activity, leghaemoglobin formation and nitrogenase protection, thereby enhancing biological nitrogen fixation. Low concentrations improve chlorophyll content, urease activity and seed yield, whereas excessive accumulation disrupts iron uptake, impairs chloroplast structure, reduces photosynthesis, and leads to growth inhibition, chlorosis and necrosis.

Animal nutrition is strongly linked to soil and forage cobalt levels. Cobalt is required for ruminal synthesis of vitamin B₁₂, which supports metabolic pathways involving carbohydrate, fatty acid and amino acid metabolism. Deficiency in livestock results in reduced weight gain, anaemia, poor feed utilisation and reproductive impairment, while excess intake may disrupt enzymatic activity and mineral metabolism.

In humans, cobalt functions through vitamin B₁₂, which is essential for DNA synthesis, nervous system health, red blood cell formation and energy metabolism. Deficiency, which is more prevalent among vegetarians and elderly individuals, leads to anaemia and neurological impairment. Conversely, excessive cobalt exposure through contaminated food, water or occupational sources can induce gastrointestinal distress, immune dysfunction, cardiotoxicity and DNA damage.

Overall, this review presents the importance and behaviour of cobalt in soil, plants, animals and humans. Understanding cobalt behaviour in soils and its movement through the food chain is vital for managing deficiency-related disorders while preventing toxicity within interconnected agricultural and ecological systems.