Mead Hardwick

Louisiana Farmer|Hardwick Planting Company

2026 Abstract - SOYBEAN SESSION - SOIL SAMPLING TIMING AND POSITION SHAPE PHOSPHORUS AND POTASSIUM GUIDANCE FOR SOYBEAN: Land-grant phosphorus (P) and potassium (K) recommendations rely on pre-plant soil samples, but most producers sample after harvest in fall. Because soil tests vary in space and time, this shift may alter soybean fertilizer rates. We sampled soils and crop residues every 15–20 days from harvest to planting from 2022-2024 for corn–soybean, rice–soybean, and cotton-soybean rotations. Soil-test K at 0-6-inch depth was ~31 ppm greater in mid-March than mid-September after corn and rice harvest; increases from mid-October (post-soybean) to mid-March ~12 ppm, and from mid-November (post-cotton) to mid-March ~17 ppm. Soil-test P differences for these same periods after harvesting corn, rice, cotton, and soybean were around 7-11 ppm. Rain leaches K from residues, raising spring K and reducing K needs. Soil P changes modestly but may also justify lower spring P rates. 2026 Abstract - CORN SESSION - UPDATING SOIL-TEST-BASED PHOSPHORUS AND POTASSIUM RECOMMENDATIONS FOR CORN: We re-evaluated soil-test phosphorus (P) and potassium (K) recommendations for Louisiana corn using 49 P trials and 53 K trials (2021–2024) with fertilizer rates of 0, 40, 80, 120, and 160 lb P2O5 or K2O acre-1. Fertilizer-P increased yield at 23 of 49 sites. Soil-test P at 0–6-inch depth explained 59% of relative yield and identified P-responsive sites with 95% accuracy using a 25–35 ppm critical range. Fertilizer-K increased yield at 25 of 53 sites. Soil-test K at 0–6-inch depth explained 92% of yield variability and identified K-deficient sites with 100% accuracy using a 90–120 ppm critical range. These results support updated, depth-standardized P and K thresholds for corn.