Phosphorus and Potassium Management for Soybean Production with No-till and Strip-till Tillage under Rain-fed Conditions.

Tillage and rate and placement of phosphorus (P) and potassium (K) can impact bioavailability of these nutrients for rain-fed soybean. Our objectives were to determine the impact of these variables on seed yield and composition, and shoot and root growth. The experiment was designed as a split-split-block design with three replications with tillage/placement— NT broadcast (NTBC), NT deep placement at 15 cm (NTDP), and ST deep placement (STDP)— as the main plot, P rate (0, 28, 56, 84 kg P₂O₅ ha^-1) as the subplot, and K rate (0, 50, 101, 202 kg K₂O ha^-1) as the sub-subplot in a corn-soybean rotation. Soil samples were collected from the 0-0.05, 0.05-0.1, 0.1-0.2, and 0.2-0.4 m depth increments from the in-row and between row positions pre-plant and post-harvest from all treatments for P and K analysis. Similar soil depths were sampled at R1 development stage from plots receiving P-K rates of 0-0, 84-0, 0-202, 84-202, and 56-101 kg P₂O₅-K₂O ha^-1 to measure P, K, and root parameters, and weekly gravimetric soil water content. Tillage, placement, and K levels had no affect on grain yield. Since soil P levels were below the critical level, grain yield was maximized by applying 58 kg P₂O₅ ha^-1. Fiber content in harvested seed was increased significantly in the STDB treatments compared to NTBC and NTDB treatments but was not influenced by P and K treatments. Protein and oil content of the seed was not affected by treatment variables. Soil water content increased with increasing soil depth across all treatments throughout the growing season. Greater root length density and soil water content at the in-row position suggest that nutrient placement in the row may be advantageous for soybean production. Shoot biomass data will be presented.