Abstract Phosphorus (P) management is pivotal for soybean Glycine max (L.) Merr. productivity, yet overapplication can increase environmental risk. We conducted 61 site‐year field trials from 2020 to 2023 across Louisiana to evaluate soybean yield response to fertilizer‐P (triple superphosphate), correlate yield with Mehlich‐3 soil‐test P (STP) at commonly used depths of 0–10, 0–15, 0–20, and 0–30 cm, and calibrate fertilizer‐P rate. Trials were designed as randomized complete blocks with four to five replications and three to five fertilizer‐P rates (0–78 kg ha −1 ). Mehlich‐3 STP decreased with depth due to P stratification, and strong linear relationships ( r 2 = 0.96–0.98) among depths allowed conversion of STP measured at nonstandard depths to the recommended depth. Across depths, STP at 0–15 cm best explained relative yield variation and P responsiveness and was therefore used for final calibration. Yield responses to fertilizer‐P were frequent and often large when STP was ≤20 mg kg −1 , less common at 21–30 mg kg −1 , and rare above 30 mg kg −1 . Regression models identified a critical STP range of 22–28 mg kg −1 , used as a basis to refine STP categories to very low (≤10), low (11–20), medium (21–30), high (31–50), and very high (≥51 mg kg −1 ) with sufficiency‐based fertilizer‐P recommendations of 44, 34, 24, 0, and 0 kg ha −1 , respectively. The companion build‐and‐maintain rates were 59, 49, 39, 24, and 0 kg ha −1 , derived using sufficiency rates plus estimated soybean P removal. Results highlight the importance of appropriate sampling depth, provide a field‐based update to Louisiana soybean P recommendations, and offer a framework for regions with stratified P.
Parvej et al. (2026) studied this question.