Effect of Nitrogen and Phosphorus Application Rate on Growth and Yield of Groundnut (Arachis hypogaea) on the Coastal Sandy Soil

Groundnut is a vital oilseed and cash crop in Vietnam, particularly in Nghe An province, where it is extensively cultivated on nutrient-poor coastal sandy soils. However, the productivity of groundnut in these areas is limited due to unbalanced fertilization practices. To address nutrient limitations in this challenging environment, this study evaluated the effects of six fertilization treatments formed by combining three nitrogen (N) application rates (20, 40 and 60 kg. ha-1) with two phosphorus (P2O5) rates (60 and 90 kg. ha-1). Accordingly, six treatment combinations were established in a factorial arrangement: T1 (N20P60), T2 (N20P90), T3 (N40P60), T4 (N40P90), T5 (N60P60) and T6 (N60P90), with T1 serving as the control treatment based on local farmer practices. All experimental plots received the same base fertilization: 10 tons ha-1 of manure, 60 kg. ha-1 of K2O and 800 kg. ha-1 of lime. The results showed that N and P fertilization significantly influenced growth, reproductive development, and yield components. The treatment 40 kg N ha-1 + 90 kg P2O5 ha-1 produced the highest plant height (37.2 cm), leaf number (32.7 leaves plant-1), flower production (74 flowers plant-1) and biomass accumulation. It also led to the highest pod number (22.4), 100-pod weight (114.4 g), 100-seed weight (51.3 g), shelling percentage (64%), seed protein content (31.10%), and actual yield (3.31 tons ha-1). In terms of economic performance, the treatment 40 kg N ha-1 + 90 kg P2O5 ha-1 recorded the highest net return (1702.75 USD ha-1) and value-cost ratio (VCR = 2.58), outperforming both lower and higher fertilizer rates. These findings highlight the agronomic and economic advantage of balanced nutrient management, particularly the combination of 40 kg N ha-1 + 90 kg P2O5 ha-1, as an optimal strategy for sustainable groundnut production on coastal sandy soils in central Vietnam.