Results & Discussion Phosphorus Mobility from Organic and Inorganic Soil Amendments: Rainfall Simulation Studies T.J. Rew, D.A. Graetz, M.S. Josan*, V.D. Nair, W.G. Harris Introduction A protocol for site-specific assessment of phosphorus (P) assimilation or release potential from soils requires that variations in P release potential and extractability from different P-bearing amendments be known. For example, the P in some amendments may be more stable than for other amendments, but yet have comparable extractability by various standard chemical methods used for assessing P status of soils. The rising cost of inorganic fertilizer has encouraged many producers to land apply manures and biosolids. Organic fertilizers are often applied based on the nitrogen (N) content of the source resulting in excessive P application. However, watersheds in Florida and across the nation are receiving attention as a result of their contribution of excessive P to surficial water bodies. The amendment with more stable P (e.g., some biosolids) would likely release P at a slower rate with less environmental risk than amendments with less stable P (e.g., inorganic fertilizers and some biosolids) when applied at equivalent P loading rates. Hence, simply knowing the total P (TP) applied is not sufficient to evaluate the risk of P movement offsite. Also, it is necessary to take into account variable effects of P sources on thresholds (e.g., change points in ratios of extractable P to extractable metals ratios beyond which P release increases precipitously) used in the protocol for risk assessment. Methods and Materials Six P sources: Dairy Manure, Black Kow Compost, two biosolids (Milorganite and Orlando Biosolids), Triple Super Phosphate (TSP), Poultry Manure, and Dairy Manure. All sources, except TSP were air-dried and ground. Amendments were analyzed for pH, electrical conductivity (EC), TP, WSP, Mehlich-1 P, Ca, Al, Mg, and Fe. A-horizon of an Immokalee fine sand was collected in bulk from an unimpacted site (University of Florida, Immokalee Research Center—Fig. 1; Photos 1-3). Soil was mixed and sieved for uniformity before packing into boxes designed to measure runoff, subsurface flow, and leachate (Fig. 1; Photo 4). Phosphorus amendments were surface applied at a rate of 300 kg hectare -1 (9 g of TP per box—Fig. 1; Photo 5). Rainfall (RF) was applied according to the National P Protocol every week for 7 rainfall simulation events. Leachate plus any runoff that occurred was collected for 30 minutes during 7 simulations (Fig. 1; Photos 6-7). Soil cores were taken prior to initial simulation and after simulation 7. Leachates were analyzed for pH, EC, soluble P, NH 4 +, NO 3 -, Fe, Al, Ca, Mg. Soil cores were analyzed in 5 cm increments for WSP, Mehlich-1 P (MP), Ca (MCa), Al (MAl), (MMg), and Fe (MFe). P Saturation Ratio (PSR) was calculated as MP/(MFe+MAl) and SPSC was calculated as (0.15 – PSR)*(MFe+MAl) (Nair V.D. and Harris W.G NZ J. Agric. Res. 47:491–497). Conclusions P source characteristics including P solubility must be accounted for when evaluating P loss risk during land application of organic amendments. The amount of P leached correlated significantly (p<0.05) with WSP concentration but not with TP concentration. The remaining P storage capacity of a soil is a function of P solubility of the P source rather than TP concentration. Objectives Characterize P sources to confirm diversity with respect to TP content, Mehlich-1 P, and water soluble P (WSP). Estimate P source effects on: P release during rainfall simulation. P storage within the soil after 7 successive rainfall events. Table 1. Chemical characterization data for phosphorus sources Figure 3. Release of P with successive rainfall (RF) events from different sources applied to sandy soil Figure 4. Relation between average P leached in the laboratory rainfall simulation study and TP content, Mehlich-1 P, and WSP of the source materials. Source Characterization The relation between Mehlich-1 (Fe+Al)-based PSR and WSP was very weak (Fig. 2). However, the relation between a PSR based on Mehlich-1 (Ca+Mg) or total Mehlich-1 metals is strong, likely owing to the compositional dominance of Ca and Mg over Al and Fe (Fig. 2; Table 1). In effect, the higher the molar ratio of P to metals, as extracted by the Mehlich-1 procedure, the higher the WSP (R 2 = 0.97, but inflated by one extreme value). Ca and Mg must be accounted for in many heavily- amended soils, and not just Al and Fe which often are predominant in acidic soils of humid regions. Total P ranged from mg/kg for the Orlando Biosolids. (Table 1). Figure 2. Plots of water soluble P versus molar ratios of Mehlich-1 P with Mehlich-1 Fe+Al, Ca+Mg, or the sum of these four metals. Figure Soil site at the University of Florida Immokalee Research Center; 2. Bulk soil collection; 3. Profile of Immokalee fine sand; 4. Rainfall simulation box diagram; 5. Applying amendments; 6. Rainfall simulator; 7. Boxes during a rainfall event P Leached during Rainfall Simulation TSP and Orlando Biosolid leached the highest amount of P in the beginning as compared to other P-sources, and P concentrations declined with successive RF leaching events (Fig. 3). Amount of P leached increased with successive leaching events for dairy manure, Black Kow Compost, and Milorganite and decreased for TSP & Orlando Biosolid. Leached P correlated significantly (p<0.05) with WSP of the P-sources (Fig. 4). Leached P was not correlated with TP. Figure 5. P-source effects on the Soil Phosphorus Storage Capacity (SPSC) expressed as mg of P per box Acknowledgements Funding Source - Florida Department of Agriculture and Consumer Services Dawn Lucas, Brandon Downing, Steve Robinson, Kenny Woodard, Erik Finlay, Ryan Bryant, Solomon Haile P Sources and Soil P Storage Capacity The remaining capacity of the soil to store P increased in the order: TSP, Dairy Manure, Poultry Manure, Orlando Biosolid, Milorganite Biosolid, Black Kow Compost, Control (Fig. 5). Soils with TSP as amendment had no remaining SPSC, some remaining P storage was available for the Dairy and Poultry Manure, and Orlando Biosolid treatment while the Milorganite Biosolid, Black Kow Compost and the Control had remaining P storage capacity at all depths within the 20 cm soil profile (Fig. 5).