Low risks of toxicity from boron fertiliser in oilseed rape-rice rotationsin southeast China

Yields in intensive rice (Oryza sativa L.) -based rotations in Asia are sta gnating or declining because of decreasing nutrient availability and deplet ion of soil nutrient reserves. In the rape (Brassica napus L.)-rice-rice cr opping rotation of southeast China, where boron (B) deficiency is widesprea d and B fertiliser is needed to correct it, our objective was to evaluate t he risks of fertiliser-induced B toxicity in oilseed rape and in rice. Resp onse of oilseed rape to B fertiliser application at rates up to 6.6 kg B ha (-1) was studied in seven field experiments on three contrasting soils of Z hejiang province, alluvial, red and blue-purple soils. The effects of up to 3.3 kg B ha(-1) in 1 year, 6.6 kg B ha(-1) in 2 years or 9.9 kg B ha(-1) i n 3 years were studied on oilseed rape and the one or two rice crops grown immediately after rape in each annual crop rotation. Soils varied in initia l hot CaCl2-extractable B in the 0-15-cm layer from 0.24 to 0.99 mg kg(-1). At the stem elongation stage of oilseed rape, 3.3 kg B ha(-1) depressed sh oot dry matter on soils with low clay and organic matter content. However, the subsequent effects of high fertiliser B on seed yield were minimal even at 6.6 kg B ha(-1). Moreover, the application of a total of 6.6 or 9.9 kg B ha-1 as successive annual applications of 3.3 kg B ha(-1) to oilseed rape generally had no negative effect on seed yield of oilseed rape. The single exception was on a sandy alluvial soil where a total of 6.6 kg B ha(-1) in 2 years slightly depressed seed yield of oilseed rape. Grain yields of ric e crops grown in rotation after oilseed rape were unaffected by B applicati ons up to 6.6 kg ha(-1). The minimal effects of a total of 6.6 kg B ha(-1) applied over 2 years on seed yield were consistent with the modest increase in hot CaCl2-extractable B levels. It is concluded that there is limited r isk of B toxicity from the use of borax fertiliser at up to 4-8 times recom mended rates in rape-rice cropping rotations in southeast China. The low ri sk of B toxicity can be attributed to the relatively high B removal in harv ested seed, grain and stubble, the redistribution of fertiliser B by leachi ng in the 0-60 cm layer and to B sorption.