Laws and guidelines limiting P applications to cropland based on soil P exist in the Mid‐Atlantic USA because of water quality concerns. We evaluated Mehlich 3 (M3) as an environmental soil P test using 465 soils typical to the Mid‐Atlantic region and found M3‐P accurately predicted water soluble P (WSP), desorbable P (Fe oxide strip P [FeO‐P]), and total sorbed P (oxalate P). The M3‐P saturation ratio (M3 [P/(Al+Fe)]) was linearly related to the well‐established oxalate P saturation method (DPSox) and a M3 [P/(Al+Fe)] range of 0.10 to 0.15 corresponded to reported environmental limits for DPSox (25–40%). Rainfall simulation and column leaching studies showed M3 [P/(Al+Fe)] predicted runoff and leachate P concentrations better than M3‐P. We suggest consideration of the following approach now used in Delaware for agri‐environmental interpretation of M3‐P and M3 [P/(Al+Fe)]: (i) Below optimum (crop response likely; M3‐P ≤ 50 mg kg−1; M3 [P/(Al+Fe)] < 0.06); (ii) Optimum (economic response to P unlikely, recommendations for P rarely made; M3‐P = 51–100 mg kg−1; M3 [P/(Al+Fe)] = 0.06–0.11); (iii) Above Optimum (soil P will not limit crop yields, no P recommended; M3‐P > 100 mg kg−1; M3 [P/(Al+Fe)] > 0.11); (iv) Environmental (implement improved P management to reduce potential for nonpoint P pollution—in Delaware M3‐P > 150 mg kg−1; M3 [P/(Al+Fe)] > 0.15 is now used). (v) Natural Resource Conservation (no P applied even if the potential water quality impact is low to conserve P, a finite natural resource).

Subsurface pathways can play an important role in agricultural phosphorus (P) losses that can decrease surface water quality. This study evaluated agronomic and environmental soil tests for predicting P losses in water leaching from undisturbed soils. Intact soil columns were collected for five soil types that a wide range in soil test P. The columns were leached with deionized water, the leachate analyzed for dissolved reactive phosphorus (DRP), and the soils analyzed for water-soluble phosphorus (WSP), 0.01 M CaCl2 P (CaCl2-P), iron-strip phosphorus (FeO-P), and Mehlich-1 and Mehlich-3 extractable P, Al, and Fe. The Mehlich-3 P saturation ratio (M3-PSR) was calculated as the molar ratio of Mehlich-3 extractable P/[Al + Fe]. Leachate DRP was frequently above concentrations associated with eutrophication. For the relationship between DRP in leachate and all of the soil tests used, a change point was determined, below which leachate DRP increased slowly per unit increase in soil test P, and above which leachate DRP increased rapidly. Environmental soil tests (WSP, CaCl2-P, and FeO-P) were slightly better at predicting leachate DRP than agronomic soil tests (Mehlich-1 P, Mehlich-3 P, and the M3-PSR), although the M3-PSR was as good as the environmental soil tests if two outliers were omitted. Our results support the development of Mehlich-3 P and M3-PSR categories for profitable agriculture and environmental protection; however, to most accurately characterize the risk of P loss from soil to water by leaching, soil P testing must be fully integrated with other site properties and P management practices.

The role that soil testing can play in identifying agricultural soils with an increased potential for P loss is an important topic. Our research compared the Mehlich 3 P saturation ratio (M3‐PSR) with the ammonium oxalate degree of P saturation (DPSox), and the M3‐PSR was then evaluated for predicting agronomic and environmental soil P saturation thresholds. Intact soil columns (15‐cm diam, 20 cm deep) and soil samples were collected from five soil series that ranged in soil texture, chemical properties, and Mehlich 3 P. The soils were analyzed for pH, organic matter (OM) and oxalate and Mehlich 3 extractable P, Al, and Fe. Each intact column was leached with the equivalent of 5 mm of rainfall and resulting leachate analyzed for P. Mehlich 3 extractable Al, Fe, and P were closely related to oxalate extractable Al, Fe, and P, although Mehlich 3 extracted only a small amount of Fe compared with oxalate. The M3‐PSRs, calculated as the molar ratios of Mehlich 3 extractable P/[Al + Fe] (ratio I) and P/Al (ratio II), were well correlated to each other and to DPSox All three P saturation measurements showed a threshold or change point above which the concentration of P in column leachate increased rapidly. Both the agronomic optimum M3‐PSRs and the environmental limit suggested in the Netherlands for DPSox (25%) were below the observed change point. The M3‐PSR measured in a single Mehlich 3 extraction shows excellent promise for identifying soils that represent an increased risk for P leaching losses.

Evaluation of phosphorus (P) management strategies to protect water quality has largely relied on research using simulated rainfall to generate runoff from either field plots or shallow boxes packed with soil. Runoff from unmanured, grassed field plots (1 m wide x 2 m long, 3-8% slope) and bare soil boxes (0.2 m wide and 1 m long, 3% slope) was compared using rainfall simulation (75 mm h(-1)) standardized by 30-min runoff duration (rainfall averaged 55 mm for field plots and 41 mm for packed boxes). Packed boxes had lower infiltration (1.2 cm) and greater runoff (2.9 cm) and erosion (542 kg ha(-1)) than field plots (3.7 cm infiltration; 1.8 cm runoff; 149 kg ha(-1) erosion), yielding greater total phosphorus (TP) losses in runoff. Despite these differences, regressions of dissolved reactive phosphorus (DRP) in runoff and Mehlich-3 soil P were consistent between field plots and packed boxes reflecting similar buffering by soils and sediments. A second experiment compared manured boxes of 5- and 25-cm depths to determine if variable hydrology based on box depth influenced P transport. Runoff properties did not differ significantly between box depths before or after broadcasting dairy, poultry, or swine manure (100 kg TP ha(-1)). Water-extractable phosphorus (WEP) from manures dominated runoff P, and translocation of manure P into soil was consistent between box types. This study reveals the practical, but limited, comparability of field plot and soil box data, highlighting soil and sediment buffering in unamended soils and manure WEP in amended soils as dominant controls of DRP transport.

Laws mandating phosphorus (P)-based nutrient management plans have been passed in several U.S. Mid-Atlantic states. Biosolids (sewage sludge) are frequently applied to agricultural land and in this study we evaluated how biosolids treatment processes and biosolids P tests were related to P behavior in biosolids-amended soils. Eight biosolids generated by different treatment processes, with respect to digestion and iron (Fe), aluminum (Al), and lime addition, and a poultry litter (PL), were incubated with an Elkton silt loam (fine-silty, mixed, active, mesic Typic Endoaquult) and a Suffolk sandy loam (fine-loamy, siliceous, semiactive, thermic Typic Hapludult) for 51 d. The amended soils were analyzed at 1 and 51 d for water-soluble phosphorus (WSP), iron-oxide strip--extractable phosphorus (FeO-P), Mehlich-1 P and pH. The biosolids and PL were analyzed for P, Fe, and Al by USEPA 3050 acid-peroxide digestion and acid ammonium oxalate, Mehlich-1, and Mehlich-3 extractions. Biosolids and PL amendments increased extractable P in the Suffolk sandy loam to a greater extent than in the Elkton silt loam throughout the 51 d of the incubation. The trend of extractable WSP, FeO-P, and Mehlich-1 P generally followed the pattern: [soils amended with biosolids produced without the use of Fe or Al] > [PL and biosolids produced using Fe or Al and lime] > [biosolids produced using only Fe and Al salts]. Mehlich-3 P and the molar ratio of P to [Al + Fe] by either the USEPA 3050 digestion or oxalate extraction of the biosolids were good predictors of changes in soil-extractable P following biosolids but not PL amendment. Therefore, the testing of biosolids for P availability, rather than total P, is a more appropriate tool for predicting extractable P from the biosolids-amended soils used in this study.

Summary The test for the degree of phosphorus (P) saturation (DPS) of soils is used in northwest Europe to estimate the potential of P loss from soil to water. It expresses the historic sorption of P by soil as a percentage of the soil's P sorption capacity (PSC), which is taken to be α (Alox + Feox), where Alox and Feox are the amounts of aluminium and iron extracted by a single extraction of oxalate. All quantities are measured as mmol kg soil−1, and a value of 0.5 is commonly used for the scaling factor α in this equation. Historic or previously sorbed P is taken to be the quantity of P extracted by oxalate (Pox) so that DPS = Pox/PSC. The relation between PSC and Alox, Feox and Pox was determined for 37 soil samples from Northern Ireland with relatively large clay and organic matter contents. Sorption of P, measured over 252 days, was strongly correlated with the amounts of Alox and Feox extracted, but there was also a negative correlation with Pox. When PSC was calculated as the sum of the measured sorption after 252 days and Pox, the multiple regression of PSC on Alox and Feox gave the equation PSC = 36.6 + 0.61 Alox+ 0.31 Feox with a coefficient of determination (R2) of 0.92. The regression intercept of 36.6 was significantly greater than zero. The 95% confidence limits for the regression coefficients of Alox and Feox did not overlap, indicating a significantly larger regression coefficient of P sorption on Alox than on Feox. When loss on ignition was employed as an additional variable in the multiple regression of PSC on Alox and Feox, it was positively correlated with PSC. Although the regression coefficient for loss on ignition was statistically significant (P < 0.001), the impact of this variable was small as its inclusion in the multiple regression increased R2 by only 0.028. Values of P sorption measured over 252 days were on average 2.75 (range 2.0–3.8) times greater than an overnight index of P sorption. Measures of DPS were less well correlated with water‐soluble P than either the Olsen or Morgan tests for P in soil.

Managing manure in reduced tillage and forage systems presents challenges, as incorporation by tillage is not compatible. Surface-applied manure that is not quickly incorporated into soil provides inefficient delivery of manure nutrients to crops due to environmental losses through ammonia (NH3) volatilization and nutrient losses in runoff, and serves as a major source of nuisance odors. An array of technologies now exist to facilitate the incorporation of liquid manures into soil with restricted or minor soil disturbance, some of which are new: shallow disk injection; chisel injection; aeration infiltration; pressure injection. Surface banding of manure inforages decreases NH3 emissions relative to surface broadcasting, as the canopy can decrease wind speed over the manure, but greater reductions can be achieved with manure injection. Soilaeration is intended to hasten manure infiltration, but its benefits are not consistent and may be related to factors such as soildrainage characteristics. Work remains to be done on refining its method of use and timing relative to manure application, which may improve its effectiveness. Placing manure under the soil surface efficiency by injection offers much promise to improve N use efficiency through less NH3 volatilization, reduced odors and decreased nutrient losses in runoff, relative to surface application. We identified significant gaps in our knowledge as manyof these technologies are relatively new, and this should help target future research efforts including environmental, agronomic, and economic assessments.

Diet modification to decrease phosphorus (P) concentration in animal feeds and manures can reduce surpluses of manure P in areas of intensive animal production. We generated turkey and broiler litters from two and three flock trials, respectively, using diets that ranged from "high" to "low" in non-phytate phosphorus (NPP) and some of which contained feed additives such as phytase. Phosphorus forms in selected litters were analyzed by sequential chemical fractionation and solution (31)P nuclear magnetic resonance (NMR) spectroscopy. Selected litters were also incubated with four contrasting soils. Reducing dietary NPP and using phytase decreased total P in litters by up to 38%. Water-soluble phosphorus (WSP) in litters was decreased 21 to 44% by feeding NPP closer to animal requirement, but was not affected by phytase addition. Solution (31)P NMR spectroscopy showed that feeding NPP closer to requirement decreased orthophosphate in litters by an average of 38% and that adding phytase to feed did not increase the concentration of orthophosphate in litters. Phytase also decreased phytate P in litters by 25 to 38%, demonstrating that it increases phytate P hydrolysis. Incorporation of litters with soils at the same total P rate increased WSP in soils relative to the control; this increase was correlated to soluble P added with litters at 5 d, but not by 29 d. Changes in soil Mehlich-3 phosphorus (M3-P) were related to total P added in litter, rather than soluble P. We conclude that feeding NPP closer to requirement and using feed additives such as phytase decrease total P concentrations in litters, while having little effect on P solubility in litters and amended soils.