THE EFFECT OF GREEN MANURING AND FERTILIZER N APPLICATION ON ENHANCING CROP PRODUCTIVITY IN MUSTARD-RICE ROTATION IN SEMIARID SUBTROPICAL REGIONS

Mustard (Brassica napus) is an important oilseed crop grown in semi-ar id subtropical soils of South Asia. Information on its fertilizer N an d S requirements, nitrogen supplying value of green manure, and compli mentary and synergistic effects of green manure and fertilizer N is ne eded 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 wit h or without cowpea green manuring on yield, nutrient uptake, protein and oil content of mustard. The residual effects of green manuring wer e assessed on the succeeding rice crop. The seed yield of mustard incr eased significantly up to 100 kg N ha(-1), but decreased thereafter wi th further increase in fertilizer N rate (150 kg N ha(-1)). Cowpea gro wn as green manure crop in situ for 45-50 days during September-Octobe r, prior to seeding of mustard, accumlated 62-86 kg N ha(-1). Green ma nuring, in the absence of fertilizer N, significantly increased the mu stard yield. All through the 4 years, the combined application of gree n manure with 100 kg N ha(-1) (otherwise optimum rate) further improve d the yield potential of mustard, illustrating the benefit that any am ount of fertilizer N cannot achieve. Green manuring to mustard substan tially improved the yield (920 kg ha(-1)) of the succeeding crop of ri ce. Protein concentration in mustard seed increased greatly from 14.0% in the control to 21.6% under fertilizer N and S treatments. Applicat ion of 100 kg N ha(-1) + 20 kg S ha(-1) + green manure increased the o il 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 t he yields, uptake of N and S, protein, percentage of oil and oil produ ction in mustard and its residual effect on succeeding rice crop. (C) 1998 Elsevier Science B.V.