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The aim of this study was to evaluate the effects of different application rates of nitrogen and phosphorus on seed-cotton yield and total biomass of cotton (G. hirsutum L.), grown during the period 2012-2014 in Chirpan, Bulgaria, in crop-rotation with durum wheat under non-irrigated conditions. The soil type was Leached vertisols. The experimental design was a randomized complete block with four replications. Single and combined nitrogen as NH4NO3 in rates 0; 80; 120 and 160 kg.ha-1 and phosphorus in rates 0; 80 and 120 kg.ha-1 were tested. The year conditions had greatest share in the total variation of the factors – 67.4 %. The N influence on seed-cotton yield was 13.0 % and of phosphorus - 0.45 % of total variation. No significant differences in the total seed-cotton yield occurred as a function of the NP interaction. The nitrogen and phosphorus fertilizers used in cotton production remain important, as N has a decisive influence, while phosphorus has less effect. Under the influence of N fertilization, the total seed-cotton yield significantly increased by 26.6-32.9 % compared to the check (1.32 t.ha-1), and under fertilization P120– by 2.9 %. An alone phosphorous fertilization was not cost-effective agronomic activity. Application of increasing NP rates in different rates showed good economic results regarding the yield. The maximum effective yield and net return from cotton cultiar Darmi can be secured by application of N120-160P80-120 to cotton crop at Central South Bulgaria - 26.6-37.8 % more than the unfertilized. Total average dry biomass at maturity was 5.51 t.ha-1. The total dry matter was more than the unfertilized by 24.0; 40.8 and 62.8 % at N80, N120 and N160 respectively. The phosphorous fertilization increased the yield of biomass by 7.6 % at P80 and by 3.4 % at P120. The year conditions had strong effect on seed-cotton yield and total biomass. The main reason for the high yields in 2014 for all tested rates, as well as the control, was the very good combination of temperature and rainfall during the vegetation period.


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