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Organic manure plays an important role in improving the fertility of Ultisols. A pot experiment to investigate the effect of dried algae and N fertilizer on the chemical properties and the yield of Amaranthus on Ultisol was conducted from March to June, 2017 in the Green House of Michael Okpara University of Agriculture, Umudike, Abia State of Nigeria, located at latitude 5°29’N and longitude 7°35E and on elevation of about 122 m above sea level with mean annual rainfall of about 2163 mm, mean daily temperature range of 35°C to 20°C and a relative humidity of about 72%. The experiment was arranged as Complete Randomized Design (CRD) with factorial pattern, consisted of two factors and provided with three replications. The first factor was the application of dried algae (0, 1, 2, 3 and 4 t/ha) and the second factor was the application of N fertilizer (0, 40, 80 and 120 kg/ha). The observed responds were chemical soils properties (pH, organic carbon, available phosphorus, total nitrogen, exchangeable bases, exchangeable acidity and effective cation exchange capacity) and the growth and the crop yield (plant height, numbers of leaves, stem girth and fresh yield). The experimental results revealed that the addition of dried algae as organic ameliorant and N-fertilizer improved most of the soil fertility properties as lone and combined treatments except in available P, total N were interaction effect were not significant. Application of 1-4 ton of algae without N fertilizers has increased the fresh yield about 73.85 to 91.65%, while the application of N fertilizers without dried algae application increased only about 25.20 to 29 87% of the fresh crop yield. The application 4 ton ha-1 dried algae without N-urea had the highest fresh yield of 89.87 g plant-1, while the highest fresh yield of about 30.13 g plant-1 without application of dried algae was obtained by the application of 120 kg ha-1 N-urea. Generally, the highest fresh yield of 97.13 g plant-1 was obtained by application of 4 ton ha-1 dried algae and 80 kg ha-1 N-urea. Consequently, the application of 4 ton ha-1 dried algae and 80 kg ha-1 N-urea has a great prospect to improve the properties of Ultisols, to enhance the fertilizers efficiency and to increase the yield of Amaranthus.