Cobalt fertilizers

Cobalt content in soil

Under natural conditions associated with the geochemical cycles of iron and manganese, cobalt occurs in ionic forms Co²⁺ and Co³⁺. In an acidic environment, cobalt is mobile but does not migrate in solutions because of sorption by iron and manganese oxides and clay minerals. At low pH, exchange of Co²⁺ and Mn²⁺ occurs with the formation of Co(OH)₂, which precipitates on the surface of oxides. As pH increases, sorption by manganese oxides sharply increases.

In soil solutions, cobalt concentration ranges from 0.3 to 87.0 µg/L. The distribution of cobalt along the soil profile is influenced by soil organic matter and the content of clay particles. Montmorillonite and illite clays sorb this element well. Organic chelates of cobalt are mobile, migrate in soil, and are available to plants.

The poorest soils are sod-podzolic light sandy soils. When soils are limed, the need for cobalt increases. A positive effect is manifested on soils supplied with major nutrients and with a soil solution reaction close to neutral: chernozems, sierozems, cultivated sod-podzolic and chestnut soils. In neutral soils, cobalt is in a poorly mobile form and unavailable to plants.

Cobalt fertilizers

Cobalt fertilizers include cobalt sulfate CoSO₄⋅7H₂O (20–21%), cobalt chloride CoCl₂⋅6H₂O (24.8%), and cobalt nitrate Co(NO₃)₂⋅6H₂O (20.3%).

Use of cobalt fertilizers in agriculture

With cobalt deficiency, the conditions for physiological and biochemical processes and plant growth worsen; productivity and crop quality decrease. Enrichment of plant products with cobalt is very important. From 5 to 50 g/ha of cobalt is removed with crop harvests.

In experiments on sod-podzolic soils, the yield of sugar beet roots with application of cobalt fertilizers, according to data from 44 experiments, increased on average by 3.5 t/ha, sugar content by 0.8%, and as a result sugar yield increased by 1 t/ha. The yield increase of lupine on sod-podzolic soils was 0.12 t/ha of seeds and 6.5 t/ha of green mass (control yield — 32.5 t/ha).

Cobalt-containing fertilizers are effective when the content in soils of the Non-Chernozem zone is 1.0–1.1 mg/kg soil, in the Chernozem zone — 0.6–2.0 mg/kg soil, in the zone of sierozems and chestnut soils — 1.0–1.5 mg/kg soil, and in flooded soils of rice fields of the Kuban — 0.8–1.2 mg/kg soil. However, for growing complete feed and food products, cobalt fertilizers must be applied when the content is 2.0–2.5 mg/kg soil.

They are applied at 200–400 g/ha<1> (0.5–2 kg/ha<2>) calculated as the element. For seed treatment, 0.5% aqueous solutions of cobalt salts are used; for foliar feeding, 0.01–0.1% aqueous solutions are used. Foliar feeding is carried out before the flowering phase at a working solution consumption rate of 300 L/ha; seed treatment of grain crops is performed by the semi-dry method (10 L/t).

Legumes, sugar beet, wheat, rice, and grapes are most sensitive to cobalt deficiency.

Plants are equally sensitive to deficiency and excess of this element in soil. At high soil content, cobalt toxicity may develop, which for rice, for example, manifests at soil content above 25 mg/kg.

The use of cobalt-containing fertilizers is promising on chernozems under grain legumes and grapes, and on cultivated sod-podzolic soils.

References

1. Yagodin B. A., Zhukov Yu. P., Kobzarenko V. I. (2002). Agrochemistry. Ed. by B. A. Yagodin. Moscow: Kolos.

2. Mineev V. G., Sychev V. G., Gamzikov G. P. et al. (2017). Agrochemistry. Textbook. Ed. by V. G. Mineev. Moscow: VNIIA Publishing House named after D. N. Pryanishnikov.