Influence of Barium On the Availability of Sulfur in Soil and Uptake by Plants.

Barium (Ba) can be toxic to plants and affect their nutrition. Among other effects, free barium (Ba²⁺) reacts with sulphate (SO₄²^-) and forms an insoluble precipitate (BaSO₄) that is unavailable for plant uptake. The objective of this study was evaluating the effect of Ba on the availability of SO₄² in soil and its uptake by pea (Pisum sativum), sorghum (Sorghum bicolor), maize (Zea mays) and soybean (Glycine max).

A pot experiment was carried out under greenhouse conditions with two typical Brazilian soils (Oxisol and Entisol) collected under forest and amended with lime to correct soil acidity. Four rates of Ba-nitrate were added to the soils: 75; 150; 300 e 600 mg kg^-1 of Ba, and a control treatment was also included. Plants were grown until flowering stage, and then harvested, dried, ground and acid digested. SO₄²^- concentration in the digests was determined turbidimetrically after reaction with BaCl₂. Available SO₄²^- in soils was also determined turbidimetrically after extraction with monocalcium phosphate.   

Ba rates reduced the availability of SO₄²^- in both soils, the reduction being higher in the Entisol. SO₄²^- mean concentration in the Oxisol was 98 mg kg^-¹ at the control treatment and was reduced to 25 mg kg^-¹ when 600 mg kg^-1 of Ba were added. Conversely, available SO₄²^- concentration was 130 mg kg^-¹ the Entisol at the control treatment and was reduced to 12 mg kg^-¹ at the highest added Ba rate.

The uptake of SO₄²^- decreased in sorghum in the Entisol, and in pea and maize in both soils. Pea plants grown in the Entisol were severely affected. In this soil SO₄² accumulation decreased more than 90% when compared to the control. SO₄²^- uptake by soybean was not reduced in both soils regardless of Ba rates.