Soybean plants require phosphorus (P) primarily during pod formation and throughout their life cycle for synthesis of biomolecules such as ATP and ADP. These biomolecules drive the plant’s photosynthetic factory, growth and reproduction. Soybean growers apply P based fertilizer or animal waste such as poultry litter to maintain adequate levels of P in the soil to meet the plant P needs. Many Alabama soils are rich in iron (Fe) and aluminum (Al) that binds P and renders it unavailable for crop use. Code 590 of the Natural Resources Conservation Service (NRCS) mandates soil testing and site assessment (P-index) to determine the suitability of a cropland to receive manure/litter. If the average soil test P and P-index for the field falls under “very high” range, manure application is not allowed. Soil test procedures such as Mehlich-1 extraction provide information on potential plant-available P but they do not provide any information on whether a given soil is capable of retaining additional P (P "sink") as opposed to functioning as a P source that would pose an environmental risk. Additionally little or no data exists on validation of the P-index for Alabama soils. A decision support tool is required that can help growers determine the amount of P that can be safely applied to a soil before the soil becomes an environmental risk.

The University of Florida has developed and used the concept of soil phosphorus storage capacity (SPSC) to determine whether a soil is a P source (capacity of the soil to release P) or sink (capacity of soil to retain P) for Florida sandy soils. We are proposing to evaluate this tool for Alabama soils. The data from this project might enable NRCS to modify their guidelines or loosen restrictions on manure/litter application on soils that have high P storage (sink) capacity.

The objectives of this study are:
1. To quantify the P storage capacity of soybean fields that receive P fertilizer or poultry litter on a regular basis.
2. To develop a soil test procedure that can assist soybean growers in making decision on P application in fields that have “high” or “very high” soil test P (STP) rating.

Soils samples (0-6, 6-12, 12-24 inch depth) will be collected from soybean fields across the state of Alabama that have a history of litter or P based fertilizer application. These soil samples will be brought to the laboratory, dried and extracted using water, oxalate, Mehlich-1 and Mehlich-3 methods. The extract will be analyzed for extractable P, Fe and Al. Soil phosphorus storage capacity (SPSC) will be determined based on the methods developed by Nair and Harris (2004). The relationship between different extractants and SPSC will be evaluated for different Alabama soils. The best extractant for SPSC calculation will be determined and growers will be informed of the phosphorus storage capacity of their soil.