Based on field observations it is often impossible to detect differences in soil B contents after the application of B fertilizers, even when increased leaf B contents are detected. This has raised doubts as to the efficacy of the B extractor used in routine soil B determinations. An experiment was conducted to study B extraction and availability to soybean in an Oxisol in the state of Mato Grosso. Boron (as boric acid) doses ranging from 0.0 to 10.0 kg ha-1 were combined with lime rates from 1.5 to 9.0 t ha-1. Soybean was grown for three years. Soil samples were taken from the 0-15 cm (arable) layer and leaf samples were collected at soybean flowering each year. Boron was extracted using CaCl2 and BaCl2 solutions and the traditional hot water. Soybean responded to lime and B from the second year onwards, but the response was not correlated with foliar B concentrations. In the three growing seasons leaf boron was best related with soil B contents extracted with CaCl2. However, no extractor differentiated the possible soybean response to B and it was not possible to establish soil B levels as low, medium or high as related to soybean response since the soybean yields were not correlated with the soil B levels.
extractors; fertilization; liming; micronutrients; soil analysis; soil fertility
