INFLUENCE OF NITROGEN FERTILIZATION ON BULGARIAN COTTON CULTIVARS

The field experiment was conducted from 2011 through 2014, at the Field Crops Institute, Chirpan, to evaluate the influence of nitrogen rates of 0, 60, 120 and 180 kg. ha(-1). The response of four Bulgarian cultivars - Chirpan 539 and Boyana (G. hirsutum L.), Darmi (G. hirsutum x G. barbadense) and Helius (developed by experimental mutagenesis) was studied. Various indices were determined: the total seedcotton yield (t. ha(-1)), number of bolls per plant, boll weight (g), 1000 seeds weight (g), plant height in maturity (cm), fibre length (mm). The soil type was Pellic Vertisols, with neutral soil reaction, with poor to middle nitrogen supply, low content of mobile phosphates and well provided with available potassium. The results demonstrated that the N rates affected greatly all the studied indices except fibre length, and caused largest differences to plant height, boll weight, and 1000 seeds weight. The cultivar led to the greatest significant differences in fibre length, lint percentage, 1000 seeds weight, September and lint yield, while they were not significantly different in boll weight and plant height. The total seedcotton yield ranged from 1.18 t. ha(-1) in 2011 to 2.50 t. ha(-1) in 2013. The N rates increased the yield with 21.7-28.8% in comparison with the unfertilized. The optimal effective N rate for yield was 120 kg. ha(-1). The agronomic efficiency of N was 3.87 kg seedcotton at 80 kg N. ha(-1) and decreased to 3.33 and 2.56 kg at 120 and 160 kg N. ha(-1), respectively. The studied cultivars had similar demands to the levels of N treatment. N fertilization at different rates had a positive impact on the cotton yield components. After 120 kg N. ha(-1), boll weight increased with 3.5% compared to the unfertilized, harvested bolls per plant-with 15.0%, plant height-with 32.5%. Seeds per boll and fibre length were variable and we could observe no consistent trends.