Information about availability of boron in the Brazilian semi-arid soils is scarce. A greenhouse assay was conducted with 29 superficial samples (0-20 cm) obtained from representative mapped soils of the State of Ceará, Brazil, using sunflower as an indicator. The experiment was performed in 2.5 kg pots containing soil, fertilized with two boron levels (0 and 1.5 mg kg-1) with three replicates per treatment. The boron availability in the soil was determined by hot water, 0.05 mol L-1 hydrochloric acid and 0.05 mol L-1 mannitol + 0.01 mol L-1 CaCl2 extractors. The canopies of the sunflower plants were harvested from two successive cultivations, each having a duration of 40 days. Total dry matter and boron in the plant tissues were analyzed and dry matter yield and boron extracted by the sunflower plants were calculated. The results showed the hot water to be the best extractor of available boron in soils, followed by hydrochloric acid and mannitol. Available soil boron critical levels for relative dry weight matter above 70 and 90% were, respectively, 0.24 and 0.45 mg kg-1 for hot water; 0.28 and 0.56 mg kg-1 for 0.05 mol L-1 HCl; 0.13 and 0.24 mg kg-1 for 0.05 mol L-1 mannitol + 0.01 mol L-1 CaCl2.
soil boron; availability; boron extrators; sunflowers