Abstract
Soil application of borax is recommended but not widely adopted to correct boron (B) deficiency of oilseed rape (Brassica napus L.) in southeast China. In order to estimate the residual value of applied B fertilizers field experiments in three important soils of southeast China were conducted to study the effect of B fertilizer on the depthwise distribution of soil extractable B over a 3 year period in rape-rice (Oryza sativa L.) rotations. A single application of up to 30 kg borax ha-1 increased soil extractable B concentrations significantly not only in the 0-20 cm layer but also in the 20-40 cm layer. Repeated B fertilizer application increased soil extractable B concentrations with each successive application and to greater soil depths. Applications of 15 and 30 kg borax ha-1, while increasing B levels in the profile in the year of addition, had no residual effect on soil extractable B in the next year in the blue purple paddy soil, but did in the 0-20 cm layer of alluvial and red soils in the second year. There was limited B leaching from borax applications up to 30 kg ha-1 except in the red soil. At 15 kg borax ha-1 , soil B levels were maintained over 3 successive years under the rape-rice rotation at levels greater than control soils.