Cobalt is a trace element in plants (it has not been proven to be essential for plant life). It is a component of a number of enzymes and increases the drought resistance of seeds. In legumes, cobalt is important for nitrogen fixation by the bacteria that associate with legumes. It is also a component of vitamin B12 so is important for human consumption. Some research suggests that cobalt plays a role in the production of ethylene by plants. Interestingly, high levels of cobalt can reduce the amount of cadmium that is uptaken by plants. On average, 1 kg of soil contains about 8 mg of cobalt though this amount varies widely around the Earth from 0.1‑70 mg/kg. Ideally, for healthy and productive soil, the concentration of cobalt should at least 1‑2 mg/kg. Anything less than that will likely result in cobalt deficient plants even if the low level of soil cobalt is natural. If the concentration of cobalt in the soil is greater than 100 mg/kg, it would be a good idea to have some plant tissue analysed and then seek expert guidance on any action you should take.
Not a lot is known about cobalt deficiency currently, though research produces new information all the time. Cobalt deficiency in plants is more likely to affect the animals (or people) eating the plants than it is to affect the plants themselves.
Symptoms of Cobalt Deficiency
Cobalt deficiency may result in reduced seed germination in dry conditions and reduced plant growth. In legumes, cobalt deficiency may result in symptoms of nitrogen deficiency.
Treating Cobalt Deficiency
Owing to the vague symptoms of cobalt deficiency, a problem is usually only diagnosed by soil test. If a soil test reveals a cobalt deficiency first ensure that the soil pH is around 7 so that the affected plants will be able to to uptake the cobalt that is in the soil more easily. You should also add some organic matter to the soil. You may then apply a cobalt containing fertiliser such as cobalt sulfate or a general trace element fertiliser that contains cobalt.
Cobalt toxicity seems to be more common than cobalt deficiency although prevalence does depend on the area (in some areas, cobalt deficiency is more common).
Symptoms of Cobalt Toxicity
High levels of cobalt can result in iron deficiency in plants so symptoms are often those of iron deficiency. Cobalt can also produce its own toxicity symptoms and these include loss of leaves from a plant, pale coloured leaves and discoloured veins.
Treating Cobalt Toxicity
As always, ensure the soil has a neutral pH (around 7) and contains plenty of organic matter. Other than that, there's not a lot that you can do if your soil naturally contains a lot of cobalt although it is unlikely that it will naturally contain so much cobalt as to cause toxicity symptoms in plants. As such, you should try to determine whether something is boosting the levels. Check your irrigation water and any run off that enters your property for instance. It would also be a good idea to check how much nickel is in your soil. If the soil has a low level of nickel, adding nickel to the soil may help reduce symptoms of cobalt toxicity.