Field experiments were conducted at the Experimental Farm of the National Horticultural Research Institute (NIHORT), Ibadan, Nigeria (7° 30' N, 3° 50' E), at altitude 168 m above the sea level, during the 2010 and 2011 cropping seasons, to assess the yield responses of two grain amaranth varieties (TE81/28 and CEN 18/97) to planting densities (100,000, 60,000 and 40,000 plants ha-1) and soil amendments (0, 25, 50, 75 and 100 kg N ha-1 organic fertilizer and 100 kg N ha-1 inorganic fertilizer). The experiment, designed as 2 x 3 x 6 factorial and fitted to randomized complete block design (RCBD) was laid out in split-split-plots and replicated three times. Measurements were taken on yield components (dry matter, biological yield, unthreshed seed weight and harvest index) and grain yield. All data were analyzed using the analysis of variance (ANOVA) with the split-split-plots model and significant means separated by the least significant difference at five percent probability level (LSD0.05). The results revealed that grain amaranth gave optimum grain yield responses when grown at a planting density of 60,000 plants ha-1 and with the application of soil amendment at 100 kg N ha-1 inorganic fertilizer, while the grain yield was significantly (p < 0.01) higher with the CEN18/97 amaranth variety than in the TE81/28 across soil amendment and planting density treatments in both years of assessment. However, significant second order interaction effects of variety x planting density x soil amendment observed in the study revealed that grain yield was best with variety TE81/28 planted at 60,000 plants/ha density with the application of 100 kg N/ha of inorganic soil amendment. In conclusion, results of this study revealed that the application of inorganic fertilizer was the best soil amendment treatment for the grain amaranth production. Nevertheless, the results suggested that the use of organic fertilizer at the rate higher than 100 kg N ha-1 holds a great potential as an alternative, where the use of inorganic fertilizer has an issue, especially for environmental and health reasons.
Key words: Grain amaranth (Amaranthus cruentus L), responses to, organic and inorganic fertilizers, planting density.
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