Phosphorus Gradient of on Two Contrasting Ohio Soils.

Phosphorus (P) speciation (0-24 cm) analysis was conducted on soils collected from seven sites (low to extremely high Bray P_1, grass, and forest) and on two contrasting Ohio soils (Epiagualfs and Agriaquolls). Soil samples were separated by <50PPM, 50-100, 100-150, 150-250, and >250PPM Bray P₁ for low to extremely high cropland P. Samples were collected in the Grand Lake St. Marys Watershed where phosphorus runoff is a major problem. Samples were split by depth (0-2, 2-6, 6-12, 12-18, and 18-24 cm). Tests were conducted for pH, soluble reactive P (SRP), exchangeable P (ExP), iron/aluminum P (FeP), residual organic P (ResP), particulate organic matter P (POM-P), and total phosphorus (TP). Statistical analysis showed that the P level distribution was different than predicted (P <0.0001) for AC, pH, SRP, POM-P, ExP, FeP, ResP, and TP.  For Soil type, the means were significantly different for pH (P<.0355), POM-P (P<.0019), FeP (.0062) ResP and TP (P<.0001 for both).  SRP, POM-P, FeP, Res-P and TP mean distributions was different than predicted (P<.0001) for interactions between P level *Depth and Soil type * Depth. POM-P, SRP, FeP, TP, and ResP had R²   (.78, .82, .89, .74, and .82, respectively and means (Root MSE) of 3.5(2.4), 26.5 (6.4), 174.3 (51.5), 512.4 (118.0) and 718.6 (140.8) respectively. Forested and grass had low to medium levels of soil test  Bray P₁ values but much higher percentage of AC, POM-P and Res-P values and the lowest FeP fraction. FeP is relatively unstable under saturated and anaerobic soil conditions and release SRP.  The FeP mean values were the highest on the cropland sites and lowest on the grass and forested sites. Future studies will focus on the impact of tillage and cover crops on tying up and storing P in the stable organic fraction rather than in the unstable FeP fraction.