An experiment was conducted in six locations in Nigeria to determine the optimum sampling depth and optimum number of core samples required for diagnosing soil nutrient status accurately for crop production. The optimum sample size varies significantly with soil type, sampling depth, and nutrient type. However, 25 to 30 and 30 to 40 core samples are recommended for newly opened land and intensively cultivated land, respectively. Samples taken within the 0–20 cm depth gave the highest significant relationship between soil test level and nutrient uptake.