Residual effectiveness of boron fertilizer for oilseed rape in intensively cropped rice-based rotations

Long-term field experiments (3–4 years) were conducted to evaluate the residual effect of boron (B) fertilizer for oilseed rape (Brassica napus L.) in an intensive crop rotation including two rice (Oryza sativa) crops per year. Experiments were conducted on four sites where the soil types were sandy, silty and clayey Inceptisols, and an Ultisol, located in the Zhejiang Province, Southeast China. Application of B fertilizer at rates of 1.1, 1.65 and 3.3 kg B/ha in the first year showed a different residual effect on oilseed yield in successive years, but had only small positive effects on the rice grain yield at two sites. The residual effect of 1.1 kg B/ha remained fully effective in correcting B deficiency in oilseed rape for 2 years in the Inceptisols, whereas the residual effect of 1.65 kg B/ha continued to correct B deficiency for at least 3 years in both the Inceptisols and the Ultisol. Foliar application of B fertilizer generally corrected B deficiency for oilseed rape but showed limited residual effect in the following years after application. The decline in residual values of B from a single fertilizer addition was closely related to the soil and leaf B concentration. Soil available B also decreased dramatically with the advance of rotation, but a larger decrease was found at a depth of 20–40-cm for the Inceptisols and the Ultisol. Thus, a more detailed understanding of the B cycling in the system is now needed to optimize management of B fertilizer.