The Distribution of Phosphorus Pools in Alabama Soil Regions and their Variation with Soil Depth

Abstract

The poultry industry in Alabama is the state's second-largest agriculture industry generating approximately 1.25 million tons of poultry litter (PL) annually and containing 19,000 tons of phosphorus (P). The PL has a typical fertilizer grade of 3-3-2 and is applied locally to the agricultural farmlands as a source of nutrient for plants. Long-term use of PL on agricultural lands causes accumulation of P, leading to elevated soil test P levels. The excess amount of P accumulated in soil is termed as legacy P. This legacy P makes its way to the water bodies via surface runoff, leaching and erosion, and therefore is an environmental concern. Strategies for the mitigation of P pollution requires a fundamental understanding of the P dynamics and distribution of P pools in soil profile. Limited studies have documented the distribution of P pools in weathered Alabama soil regions namely Appalachian Plateau (AP), Coastal Plains (CP), Limestone Valley (LV), Piedmont Plateau (PP), and Blackbelt prairie (BP). The objectives of this study were 1) to quantify the distribution of P pools within 0 to 45 cm soil depth in five Alabama soil regions and, 2) to determine how P fractions change with change in soil test P fertility rating. Soil samples were collected from Alabama farmlands from four depths: 0-5 cm, 5-15 cm, 15-30 cm, and 30-45 cm. The modified Hedley fractionation procedure, which included extraction with deionized water, 0.5 M NaHCO3, 0.1 N NaOH, and 1 M HCl and residue P, was used to characterize P into H2O-Pt, NaHCO3-Pt, NaOH-Pt, HCl-Pt, and residue-Pt fractions. The soil test phosphorus (STP) was determined using Mehlich-1 (M1) extraction for non-calcareous soils (AP, CP, LV, PP) and Lancaster (La) extraction for calcareous soils (BP). The samples were categorized into six Alabama STP ratings namely extremely high (EH), very high (VH), high (H), medium (M), low (L), and very low (VL) based on STP concentrations. The results from this study indicated that P fractions were highly stratified across depths in all five soil regions. The H2O-Pt and HCl-Pt pool were found to be the least dominant P pools and NaOH-Pt and residue-Pt were the most dominant P pools in non-calcareous soil regions. In case of calcareous soils, HCl-Pt and residue-Pt were found to be the two dominant pools in the whole soil profile. In addition, with the transition of P fertility rating from VL to EH, the proportion of residue-Pt pool decreased significantly with the corresponding increase in labile P fractions (e.g., H2O-Pt, NaHCO3-Pt). Greater amounts of P in labile and moderately labile P pool (e.g., NaOH-Pt, HCl-Pt) in EH, VH, and H soils pose environmental concern. Best management practices should be targeted to manage EH, VH and H soils.