Effect of Phosphorus Fertilizer Placement on Corn and Soybean Yield in Phosphorus Stratified Soils of Alabama

Abstract

Alabama has highly weathered soils making them prone to erosion during intense precipitation events. Adopting conservation tillage practices is encouraged to minimize soil erosion and improve soil health. Currently, Alabama ranks eighteen in terms of adopting no-till practices, with 27.2% of cropland area under no-till (NT) system. The NT system involves minimal mixing of soil that leads to the accumulation of non-mobile nutrients like phosphorus (P) in the upper soil profile, and a sharp decline in P concentrations at lower depths, leading to P stratification. Consequently, stratified subsoil has a lower P concentration, which can potentially limit P uptake by the crop and impact crop yield. Phosphorus stratification can affect crops typically during water stress conditions as plant roots grow deep in search of water to meet crop water requirements. Therefore, the objective of this study was to evaluate the effect of deep placement of P fertilizer on corn and soybean yield. The treatments comprised of three fertilizer placement methods: injection (15 cm deep directly under crop row), banding (5 cm away from crop row), broadcast, and a control with no P fertilizer application. In 2021, field trials were conducted at E.V. Smith Research Centre (EVS) and a farmer field located in Blackbelt region of Alabama. The P fertilizer was applied based on the soil test P fertility rating for Alabama soils. In 2021, banding resulted in greater yields than the other treatments but was not statistically different. We hypothesize that the P injected below at a 15 cm depth got fixed by aluminum (Al), iron (Fe), and calcium (Ca) minerals and was not available for crop uptake. Therefore, in 2022, P fertilizer application rate was determined based on the Phosphorus Saturation Index (PSI, a single-point isotherm approach to estimate the P sorption capacity) to overcome P fixation by subsurface soil, and field trials were conducted at EVS, Gulf Coast Research and Extension Center (GCREC), Tennessee Valley Research and Extension Centre (TVREC), and a farmer field located in Blackbelt region of Alabama. Corn and soybean crops were grown in these field trials. Initial soil samples were collected before planting from three soil depths (0-5, 5-15, and 15-30 cm), and the final soil sampling was done near crop harvest. Soil samples were analyzed for water and Mehlich-1/Lancaster extractable P to understand the environmental and agronomic P availability, respectively. Biomass samples were collected at mid-season and at physiological maturity and were analyzed for P uptake. Initial soil sample analysis confirmed P stratification at all four locations. Injecting P fertilizer significantly increased M1-P/Lancaster-P below 15 cm compared to the other two P application treatments. No significant difference in corn yield was observed between treatments and the control at GCREC, potentially due to high precipitation leading to no water stress condition, whereas a significant difference in corn yield was observed between treatments and the control at EVS. Banding and injection methods resulted in significantly higher corn yields at the farmer’s fields. In the case of soybean, injecting P fertilizers resulted in significantly greater yields than the control at TVREC. However, multiyear data needs to be collected to address the P stratification issue in Alabama.