EFFECT OF FARMYARD MANURE ON NITROGEN-FIXATION IN SOYBEAN (GLYCINE-MAX) AND ITS NET POTENTIAL CONTRIBUTION TO N BALANCE AS MEASURED BY N-15-TRACER METHODOLOGY

A micro-plot field experiment was conducted during rainy season 1993 o n a vertic Haplustert soil, to study the effect of farmyard manure on yield, N-2-fixation and N partitioning in 'Punjab I' soybean [Glycine max (L.) Merr.] by N-15-dilution technique using 'CHS 5' sorghum [Sorg hum bicolor (L.) Moench] as a non-nodulating reference crop. Manuring the soil with farmyard manure @ 4, 8 and 16 tonnes/ha increased seed y ield by 39.6, 69.5 and 85.4% compared with no farmyard manure (1.44 to nnes/ha seed). Out of tile total plant N, 75.6-76.1% was translocated to seed irrespective of farmyard manure treatment. Application of 4 to nnes/ha farmyard manure increased nitrogen (%) derived from atmosphere from 54.1 (in no-farmyard manure treatment) to 69.2, and Further incr ease in the level of farmyard manure to 8 and 16 tonnes/ha decreased t he nitrogen (%) derived from atmosphere to 61.8 and 59.2 respectively. In the no-farmyard. manure treatment, total quantity of N fixed from atmosphere was 65.8 kg/ha, which increased to 117.4, 126.1 and 131.3 k g/ha owing to application of 4, 8 and 16 tonnes/ha farmyard manure res pectively. The potential contribution of N-2 fixation to the N balance in soil was found negative, being 55.9-90.5 kg N/ha. Calculations sho wed that such negative balance could be reduced by 11.6-36.4 kg N/ha o n returning the whole soybean trash to the soil.