INFLUENCE OF TILLAGE TREATMENTS ON N-2 FIXATION OF SOYBEAN

With the potential of conservation tillage practices to increase soybe an grain yields and soil erosion protection, 2 field experiments were established to determine its effect on N-2 fixation by soybean. The ma jor tillage practices evaluated were the conservation practices of no- tillage (standing stubble), mulch (stubble slashed) and incorporated l ate (stubble ploughed once immediately before sowing). These were comp ared to the conventional tillage practices of repeated stubble incorpo ration (incorporated early) over a fallow and stubble removal by burni ng and incorporated (stubble burnt then multiple ploughing over the fa llow) and no-tillage burn (stubble burnt). Two types of cropping syste m were studied: (1) double cropping (opportunity cropping) and (2) cro pping after a fallow. N-2 fixation was measured by the N-15 dilution t echnique and the ureide method. Conservation tillage (no-tillage, mulc h and incorporated late) increased N-2 fixation to over 85% of grain N compared to the conventional tillage practices (early incorporation a nd burn followed by incorporation), where N-2 fixation was <75%. N-2 f ixation was increased in tillage treatments that maintained high amoun ts of previous crop residues on the soil surface and not by pre-sowing tillage. Conservation tillage increased soybean Nr fixation when sown as a double crop or after a fallow.