Soil sampling depth effect on critical soil test values of phosphorus for conservation agriculture

The critical soil test value (CSTV) of phosphorus (P) is the threshold where it is possible to obtain 95–100% of the maximum crop yield. Although the P buildup in the topsoil of conservation tillage may affect this threshold, the effect of soil sampling depth on CSTV value has not been determined for conservation tillage in the southern United States. The objective of this study was to evaluate CSTV of P using different soil layers from two long-term experiments managed under varying P rates, planted to corn (Zea mays L.)/soybean [Glycine max (L.) Merr.] rotation under minimum tillage (Tidewater) or no-tillage (Piedmont) in North Carolina. Soil samples were taken from depths of 0–2, 2–4, 4–8, and 8–12 inches. The CSTV for different soil layers was calculated using a quadratic-plateau model with Mehlich-3 P and relative yield of soybean and corn (2021–2022). The CSTV decreased as the soil sampling depth increased. The CSTV of P at the Tidewater site was 128, 111, 86, and 74 lb ac⁻¹, and at the Piedmont site was 28, 20, 16, and 15 lb ac⁻¹ for the 0–2, 0–4, 0–8, and 0–12 inches soil layers, respectively. Using multiple sampling layers or deeper layers did not improve the quality of CSTV measurement. The current sampling depths used in North Carolina are appropriate for minimum tillage (0–8 inches) and no tillage (0–4 inches), as similar quality models were obtained using either soil layers in both sites. However, it is important to be cautious when changing the sampling depth, as this affects the CSTV value.