Evaluation of integrated nitrogen and phosphorous management using the TT biplot method in soybean

To investigate the effects of integrated nutrient management on oil, protein, grain yield and some traits of soybean, we conducted a factorial experiment involving 4 bio-fertilizer (no inoculation, inoculation with Barvar-2, inoculation with Biosoy and dual inoculation with Biosoy and Barvar-2) and 3 chemical fertilizer levels (no chemical fertilizer, 66 kg ha-1 diammonium phosphates + 50 kg ha-1 urea, 132 kg ha-1 diammonium phosphates + 100 kg ha-1 urea) with 100% chemical fertilizer (200 kg ha-1 diammonium phosphates + 150 kg ha-1 urea) as control base in a randomized complete block with four replications at the research farm of the University of Mohaghegh Ardabili, Iran. Analysis of variance showed that treatment combinations affected most of the traits (P>0.01). We used the biplot analysis as the treatment ? trait (TT) biplot to determine the best treatment combinations and traits. The first two principal components (PC1 and PC2) explained 94 and 96% of the total variant of the standardized data in 2017 and 2018, respectively. Accordingly, application of Biosoy and 150 kg ha-1 urea + 200 kg ha-1 diammonium phosphate significantly increased stem height at harvest, number of grains per plant, biomass, hundred-grain weight, oil and protein yield, protein percent and grain yield compared to the other treatment combinations. The results showed that there were positive correlations between these traits. Also, non-inoculated plants and no chemical fertilizer treatments significantly increased oil percent. The results indicate that higher-yielding treatment combinations had lower oil percent. The biplot was used for ranking of treatment combinations based on a single trait. These study results suggest that bio-fertilizers had a positive influence on soybean and that they could diminish the use of chemical fertilizers. The study reveals that the TT biplot was able to graphically show the interrelationships between traits and support visual comparison of treatments.