Avaliação da disponibilidade de boro em solos cultivados com algodoeiro através do método biológico do girassol

The determination of boron by the sunflower biological method in a medium-textured red-yellow latosol soil cultivated with cotton (Gossypium hirsutum L.) for many years, was studied in a greenhouse experiment using soils from Leme and Santa Cruz da Conceicao counties, State of Sao Paulo, Brazil. The soil samples were drawn at depths of 0 to 20 and of 20 to 40 cm and compared with a collection of standards in water-rinsed sand with the objective of standardizing the results obtained. The amount of boron added to the collection of standards and to the pots containing soil and subsoil was 0.0, 1.0, 2.0, 4.0, 5.0, 6.0 and 10.0 mg of B dm -3 in split applications of 10 mL of nutrient solution at thinning time, and at 5, 10 and 15 days later. The best plant response to boron applied in the collection of standards occurred at the 6.9 mg of B dm -3 . In the soil from Santa Cruz da Conceicao, with less boron, the best boron rates were between 4.8 and 4.6 mg of B dm -3 in accordance with the profundity, while in the soil from Leme, the best rates occurred between 3.0 and 3.4 mg of B dm -3 . In the case of dry matter, the best results obtained in sand culture were for doses up to 4.0 mg of B dm -3 and, in samples soil of Santa Cruz da Conceicao, for doses of up to, approximately, 3.0 mg of B dm -3 ; the results obtained from pots of Leme soil samples were not significant. Thus, the sunflower biological method was effective for the evaluation of boron availability in the samples studied.