Re-evaluating cobalt in agricultural soils: plant requirement and phytotoxic risk

Abstract Background and aims Cobalt (Co) is frequently presented as a beneficial or even essential plant micronutrient, yet the evidence supporting its broader use in agricultural systems remains uncertain. Although its role in symbiotic nitrogen fixation in legumes is well established, claims of wider necessity across higher plants often rely on limited evidence, repeated citation patterns, and insufficient distinction between true physiological requirement and plant exposure under varying environmental conditions. This review critically reassesses the addition of cobalt to agricultural soils by examining whether its routine inclusion in fertilisation practice is scientifically justified. Methods Available evidence, including recent reviews and primary studies on cobalt in the soil–plant system, is critically reviewed, focusing on agricultural soils, cobalt inputs and plant responses. Results The review identifies a narrow and context-dependent range between reported beneficial and adverse cobalt responses, highlights the role of soil properties in controlling cobalt availability and uptake, and summarises the implications of cobalt loading for plant physiology and soil health. Conclusion By integrating current knowledge on cobalt inputs in agricultural soils, plant responses, phytotoxicity and soil-health implications, this review argues that cobalt should not be treated uncritically as a routine fertiliser constituent. Instead, its use should be preceded by soil-specific assessment and considered within a more precautionary framework.