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Mustard (Brassica napus) is an important oilseed crop grown in semi-arid subtropical soils of South Asia. Information on its fertilizer N and S requirements, nitrogen supplying value of green manure, and complimentary and synergistic effects of green manure and fertilizer N is needed for the development of a sustainable mustard-rice cropping system. A field experiment was conducted for 4 years (1990-1991 to 1993-1994) to evaluate the effects of different rates of fertilizer N and S with or without cowpea green manuring on yield, nutrient uptake, protein and oil content of mustard. The residual effects of green manuring were assessed on the succeeding rice crop.

The seed yield of mustard increased significantly up to 100 kg N ha(-1), but decreased thereafter with further increase in fertilizer N rate (150 kg N ha(-1)). Cowpea grown as green manure crop in situ for 45-50 days during September-October, prior to seeding of mustard, accumlated 62-86 kg N ha(-1). Green manuring, in the absence of fertilizer N, significantly increased the mustard yield. All through the 4 years, the combined application of green manure with 100 kg N ha(-1) (otherwise optimum rate) further improved the yield potential of mustard, illustrating the benefit that any amount of fertilizer N cannot achieve. Green manuring to mustard substantially improved the yield (920 kg ha(-1)) of the succeeding crop of rice. Protein concentration in mustard seed increased greatly from 14.0% in the control to 21.6% under fertilizer N and S treatments. Application of 100 kg N ha(-1) + 20 kg S ha(-1) + green manure increased the oil yield to 992 kg compared with 518 kg in the control. These results illustrated the complementary effects of green manuring in conjunction with the optimum rates of 100 kg N and 20 kg S ha(-1) in maximizing the yields, uptake of N and S, protein, percentage of oil and oil production in mustard and its residual effect on succeeding rice crop. (C) 1998 Elsevier Science B.V.