Phosphorus Characterization of Alabama Soils and Evaluation of Extractants for Environmental Phosphorus Loss Risk Assessment

Abstract

Poultry industry is the second largest agricultural industry in Alabama and generates 1.5 million tons of poultry litter, which contains approximately 19,000 tons of phosphorus (P). Long term application of fertilizers and manure in excess of crop requirement leads to buildup of P in soil. Increase in soil test P level increases risk of P loss from soil to surface water which can accelerate eutrophication and reduce water quality. As the P transport from agricultural land to water systems and looming concerns on water quality increases, it becomes necessary to evaluate the risk of P loss from an environmental standpoint. Soil test phosphorus (STP) originally developed for agronomic purposes is also used as the first line of defense to evaluate P loss risk. For example, STP methods have been integrated into P index (PI) which is considered the most comprehensive tool for environmental P loss risk assessment. Underestimation or overestimation in STP may misguide nutrient recommendations for fertilizer or manure management. Therefore, it is important to choose an appropriate soil test method that adequately captures environmental P loss risk as well as agronomic P needs. The objectives of this study were (i) to survey the soil P levels of major soils areas of Alabama at different soil depths and land use management (ii) to evaluate and compare the extraction potential of Mehlich-1/Lancaster and Mehlich-3 for P, aluminum (Al), iron (Fe), calcium (Ca), and magnesium (Mg) for major Alabama soil areas (iii) determine the conversion equations between Mehlich-1/Lancaster and M3 STP values and (iv) to determine if observable distinct relationship exist between WSP and STP for different soil types and depths in Alabama soils. Soil samples were collected form five major soil areas of Alabama, namely, Appalachian Plateau (AP), Coastal Plain (CP), Limestone Valleys (LV), Piedmont Plateau (PP) and Blackland Prairie (BP). Based on calcium content, Alabama soils from AP, CP, LV, and PP are typically categorized as non-calcareous soils whereas soils from BP are categorized as calcareous soil. The soil samples were collected from four soil depths (0-5, 5-15, 15-30 and 30-45 cm). Non-calcareous soil samples were extracted for water-soluble phosphorus (WSP), M1, and M3 extractable P, Fe, Al, Ca, and Mg using the standard protocols and relationship were studied. Lancaster was used for calcareous soils from Blackland Prairie instead of M1 and the above stated relationship for calcareous were studied. The critical soil test level for Alabama soils are 25 mg kg-1 M1-P (non-calcareous) and 36 mg kg-1 Lancaster-P (La-P) (Blackland Prairie) soil. The mean M1-P for AP, CP, LV, and PP soils at 0-5 cm depth were 143 mg kg-1, 76 mg kg-1, 73 mg kg-1, and 91 mg kg-1 soil, respectively. The mean La-P for BP soil at 0-5 cm depth was 60 mg kg-1 soil. This indicated that STP at surface 0-5 cm are above the critical soil test level for Alabama soils. The percent distribution of major soil areas above soil test critical level for AP, LV, PP, BP, and CP for 0-5 cm depth were 93%, 84%, 77%, 67%, and 54%, respectively. Similarly, the distribution of soils from major soil areas above soil test critical level for 5-15 cm depth followed the order: AP (87%) > CP (69%) > LV (42%) > PP (41%) > BP (10%). Mehlich-3 showed better extraction efficiency than M1 for P, Al, Fe, Ca, and Mg for wide range of Alabama soils. A linear and strong correlation was observed between M1-P and M3-P for all soil from Alabama. For Blackland Prairie soils, Lancaster method showed better extraction for P, Ca and Mg compared to M3. However, correlation between La-P and M3-P showed, both the extractant have similar P extraction potential in BP soils. For the combined depths, the relationship between WSP and M3-P was better correlated (based on r2 value) than the relationship between WSP and M1-P for all soil types except CP. The correlation between WSP and M3-P was better than WSP and La-P which suggested the suitability of use of M3 for BP soils. This research provides solid background for introducing M3 as an alternative soil test method for environmental purpose for wide range of Alabama soils.