Total and soluble boron contents were evailable by means of three extractors, used as plant availability indexes, in order to study their relationships with soil properties in representative mapping units of the state of Ceará, Brazil. Composite soil samples from the surface soil layers (0-20 cm) of Quartz Sands (AQd), Alluvium (Ae), Planosol (PL), Bruno non Calcic (NC), Cambisol (Ce), Red-Yellow Podzol (PV), Red-Yellow Latosol (LV) and Littosol (Re) were used. Total boron was determined after fusion with sodium carbonate and soluble boron by using the following extractors: hot water, HCl 0.05 mol L-1 and Mehlich-1. Total boron content ranged from 8.3 to 52.2 mg kg-1, with on average of 16.0 mg kg-1. The lowest averages were observed for NC, Re and AQd, and the highest for LV, PL and PV. Available boron, extracted with hot water, ranged from 0.22 to 1.22 mg kg-1, an average of 0.60 mg kg-1; for boron extracted by HCl 0.05 mol L-1, the content ranged from 0.21 to 1.24 mg kg-1, an average of 0.53 mg kg-1; for Mehlich-1, the boron content ranged from 0.50 to 1.81 mg kg-1, an average of 1.07 mg kg-1. The soils with the lowest averages were AQd and LV and the soil with the highest was Ce. Available boron correlated positively in the three chemical extractors. For hot water extraction, total B and organic matter were the main sources of available boron (28.8% and 38.3%) with minor influence of the clay content, Mn oxides, sesquioxides of Fe and Al.
micronutrients; soil boron; chemical extractors; availability