Application of Nutrient Index and Multivariate Analysis for Soil Fertility Assessment of Rice Farms in Okigwe, Southeastern Nigeria

Abstract

Soil fertility decline remains a major constraint to sustainable rice production in southeastern Nigeria, largely due to continuous cultivation and poor nutrient management practices. This study evaluated the nutrient status of selected rice farm soils in Okigwe area of Imo State using the nutrient index (NI) approach, alongside multivariate statistical techniques. Surface soil samples (0–15 cm) were collected from ten rice farms and analyzed for physical and chemical properties using standard procedures. Results showed that the soils were predominantly sandy, with sand content ranging from 45.2% to 68.4% (mean = 54.5%), while clay and silt contents varied widely between 5.6–41.6% (mean = 22.2%) and 6.0–48.0% (mean = 23.3%), respectively, indicating considerable spatial variability. Soil pH ranged from 4.60 to 5.96 (mean = 5.46), classifying the soils as slightly to moderately acidic. Available phosphorus ranged from 2.18 to 16.25 mg kg⁻¹ (mean = 6.41 mg kg⁻¹), while total nitrogen varied from 0.60 to 3.10 g kg⁻¹ (mean = 1.21 g kg⁻¹), both indicating low to moderate nutrient status. Organic carbon content ranged from 7.9 to 36.3 g kg⁻¹ (mean = 14.31 g kg⁻¹). Exchangeable Ca, Mg, K, and Na ranged from 2.0–5.6, 1.2–2.8, 0.11–0.29, and 0.05–0.21 cmol kg⁻¹, respectively. Cation exchange capacity (CEC) varied from 4.83 to 10.54 cmol kg⁻¹ (mean = 6.28 cmol kg⁻¹), while base saturation ranged from 71.3% to 86.9% (mean = 79.77%). Nutrient index values indicated low fertility status for pH (1.0), total nitrogen (1.6), available phosphorus (1.1), and CEC (1.1), while organic carbon (2.1), calcium (2.1), potassium (1.7), and sodium (1.8) were in the medium category, and magnesium (3.0) and base saturation (2.5) were high. Principal component analysis extracted five components explaining 90.19% of total variance, with soil fertility (37.27%) and soil acidity (19.85%) as dominant factors. Strong positive correlations were observed between organic matter and total nitrogen (r = 0.99), available phosphorus (r = 0.93), calcium (r = 0.90), and CEC (r = 0.83), highlighting the central role of organic matter in nutrient dynamics. The study concludes that although the soils are moderately fertile, acidity, low nitrogen and phosphorus, and low CEC are major constraints to rice productivity. Liming, integrated nutrient management, and site-specific fertilizer application are recommended to enhance soil fertility and sustain rice production in the area.