The loss of P in runoff from agricultural land to which manure has been applied is related to water extractable P (WEP) in manure. However, a standard method to routinely measure WEP in manures has not been established and variables impacting the measurement have not been widely studied. In this investigation, the impact of manure holding times (1–22 d), WEP extract holding times (0–17 d with and without acidification), and method of P measurement on WEP were evaluated. In addition, four manure samples (one dairy, one swine, two poultry) and proposed WEP method were distributed to seven public and private laboratories to assess inter‐ and intralaboratory variability of WEP test results. The proposed method entailed extracting manures with water at a 1:200 ratio (manure solids/water), shaking for 60 min on a reciprocating shaker, and either filtering or centrifuging before P measurement by inductively coupled plasma atomic emission spectroscopy (ICP). Results show refrigerated (4°C) manure samples can be held up to 22 d and acidified extracts 18 d before analysis. Separation procedures (filtering vs. centrifuging) did not impact WEP measurements. While the method of P measurement (ICP vs. colorimetric) did have a significant impact on test results, the two methods were highly correlated and results within 5 to 10%. The proposed manure WEP method shows a high level of precision (relative standard deviations [RSDs] < 6.5) within laboratories, although greater variability (RSDs of 13.3–28.8) exists among laboratories when compared with other standard manure analyses, such as total P.

Water‐extractable P (WEP) in manure is increasingly used as an environmental indicator as it is correlated with P in runoff from soils recently amended with manure. Little information exists on WEP variability across livestock manures. A survey of 140 livestock manures was conducted to assess trends in WEP (dry weight equivalent) related to livestock types and manure storage. Manure WEP ranged widely (0.2–16.8 g kg−1), with swine (Sus scrofa domestica L.) having the highest average concentrations (9.2 g kg−1), followed by turkey (Melleagris gallopavo) (6.3 g kg−1), layer chickens (Gallus gallus domesticus L.) (4.9 g kg−1), dairy cattle (Bos taurus) (4.0 g kg−1), broiler chickens (Gallus gallus domesticus L.) (3.2 g kg−1), and beef cattle (Bos taurus) (2.3 g kg−1). Manure WEP also differed by general storage system; dry manures contained significantly lower WEP concentrations (3.9 g kg−1) than manure from liquid storage systems (5.4 g kg−1). Within liquid storages, no significant differences in WEP were observed between covered and uncovered storages or between bottom‐loaded and top‐loaded storages. Dry‐matter (DM) content of manure was weakly correlated to WEP across all manures (r = −0.44), but strongly correlated with WEP in liquid swine manure (r = −0.87) and dairy manure (r = −0.72), suggesting dissolution of phosphate compounds as manure solids are diluted in storage. Varying positive correlations were observed between WEP in manure and water‐extractable Ca, Mg, and Fe, or total P, depending on livestock category. Results of this study show that livestock manure can be categorized by WEP, a key step toward differential weighting of agricultural P sources in P site assessment indices.

Concern over nonpoint source P losses from agricultural lands to surface waters in frigid climates has focused attention on the role of freezing and thawing on P loss from catch crops (cover crops). This study evaluated the effect of freezing and thawing on the fate of P in bare soils, soils mixed with dairy manure, and soils with an established catch crop of annual ryegrass (Lolium multiflorum L.). Experiments were conducted to evaluate changes in P runoff from packed soil boxes (100 by 20 by 5 cm) and P leaching from intact soil columns (30 cm deep). Before freezing and thawing, total P (TP) in runoff from catch-cropped soils was lower than from manured and bare soils due to lower erosion. Repeated freezing and thawing significantly increased water-extractable P (WEP) from catch crop biomass and resulted in significantly elevated concentrations of dissolved P in runoff (9.7 mg L(-1)) compared with manured (0.18 mg L(-1)) and bare soils (0.14 mg L(-1)). Catch crop WEP was strongly correlated with the number of freeze-thaw cycles. Freezing and thawing did not change the WEP of soils mixed with manures, nor were differences observed in subsurface losses of P between catch-cropped and bare soils before or after manure application. This study illustrates the trade-offs of establishing catch crops in frigid climates, which can enhance P uptake by biomass and reduce erosion potential but increase dissolved P runoff.

Phosphorus (P) runoff from agricultural soils is a concern due to eutrophication. The simultaneous corn and cover crop system was developed by U.S. Department of Agriculture's Natural Resources Conservation Service (USDA-NRCS) for dairy farms in the northeastern United States, where short growing seasons have limited fall seeding of cover crops. The simultaneous corn and cover crop system uses post-emergence imidazolinone herbicides to allow for simultaneous seeding of cover crops with silage corn. Trials were established at two locations in the Cannonsvitte Reservoir watershed, New York, part of New York City's drinking water supply system, to assess the effects of this cover cropping system on water quality. Rain simulations (60 mm hr(-1)) were conducted to evaluate the initial 30 minutes of runoff from small (1 X 2 m; 3 ft x 7 ft) plots before and after surface application of dairy manure (50 or 100 kg total P ha(-1); 45 or 89 lb P ac(-1)). Corn yields from plots interseeded with red clover compared most favorably with the conventionally cropped controls, with no significant differences in yields noted between the two treatments at either location. Prior to dairy manure application, losses of P in runoff were primarily a function of erosion. Because at[ cover crops increased ground cover (up to 81 percent greater than the control), total P loads in runoff were significantly lower from cover cropped plots (averaging 10 mg per plot) than from conventionally cropped controls (averaging 39 mg per plot). At the same time, suspended solids loads averaged 25.3 g (1 oz) from the control plots and 5.9 g (0.2 oz) from the cover crop plots. Despite concern that release of soluble P from the cover crops could enrich dissolved reactive P in runoff, dissolved reactive P losses from the simultaneous corn and cover crop system were generally not different from conventionally-cropped silage corn losses. Application of manure obscured cover crop/conventional silage corn treatment differences with regard to P runoff, with dissolved reactive P becoming the dominant form of P in runoff due to contributions of readily soluble P in manure. Because runoff P tosses were already high from unmanured conventional silage corn plots, application of manure did not significantly increase P losses from some of the conventional silage corn treatments. Results highlight the agronomic and water quality benefits of the simultaneous corn and cover crop system, particularly when implemented with red clover.

Continually land‐applying manure at rates exceeding crop removal can change soil P chemistry and increase soil P to levels that are of environmental concern. To assess the effect of long‐term manure application on soil P forms and solubilities, we determined water‐extractable P, Mehlich‐3 P, Hedley‐P fractions, and crystalline Ca‐P minerals in surface soil (0–5 cm) from 20 locations in New York (n = 6), Oklahoma (n = 8), and Pennsylvania (n = 6), which received dairy, poultry, or swine manure (40–200 kg ha−1 yr−1) for 10 to 25 yr. For all untreated and manured soils, the pH averaged 5.9 and 6.6; exchangeable Ca, 0.9 and 6.2 g kg−1; organic C, 15.7 and 32.6 g kg−1; and total P, 407 and 2480 mg kg−1, respectively. As Mehlich‐3 P increased (64–2822 mg kg−1), the proportion that was water extractable (14–3%) declined as exchangeable soil Ca increased (R2 = 0.81). Results suggest that addition of manure to soils shifts P from Al‐ and Fe‐ to Ca‐P reaction products, accounting for the relatively greater Mehlich‐3 but lower water extractability of soil P. This shift has implications to environmental soil P testing. For instance, the fact that Mehlich‐3 P has been shown to overestimate potential losses of P in overland flow from heavily manured soils may be explained by dissolution of Ca‐P minerals not soluble in water.

The targeting of critical surface runoff‐producing zones should account for the influence of subsurface soil characteristics. In this study we assessed the runoff response of contrasting colluvial and residual soils. The study was conducted along two hillslopes within a 39.5‐ha mixed land use watershed in Pennsylvania. Six sites (four colluvial, two residual) were monitored for runoff, hydraulic head, water table depth, and soil water content. A total of 111 rainfall events were monitored during the periods of July to December 2000, April to December 2001, and April to December 2002. Two high‐intensity (5‐min peak > 8 cm h−1) events had return periods of 2.5 and 4 yr. The colluvial soils are somewhat poorly and moderately well drained with fragipans and high clay content (37–44%) argillic horizons (fine, mixed, semiactive, mesic Aquic Fragiudalfs); the residual soils are well drained with moderate clay content (24%) argillic horizons (fine‐loamy, mixed, semiactive, mesic Typic Hapludults). Across all events, overall runoff yields averaged 2.4% from the four colluvial sites and 0.01% from the two residual sites. The two colluvial sites with the greatest runoff production were located at the base of a primarily colluvial hillslope. The largest events at these sites occurred during periods of surface saturation (soil surface to a depth of at least 30 cm). These results suggest that nonwinter P management for these residual soils should focus on rare, large events. Nutrient management planning could be improved if runoff estimation methods were to better integrate information on subsurface and upslope soil hydrologic properties.

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.

Concern over eutrophication has directed attention to manure management effects on phosphorus (P) loss in runoff. This study evaluates the effects of manure application rate and type on runoff P concentrations from two, acidic agricultural soils over successive runoff events. Soils were packed into 100- x 20- x 5-cm runoff boxes and broadcast with three manures (dairy, Bos taurus, layer poultry, Gallus gallus; swine, Sus scrofa) at six rates, from 0 to 150 kg total phosphorus (TP) ha(-1). Simulated rainfall (70 mm h(-1)) was applied until 30 min of runoff was collected 3, 10, and 24 d after manure application. Application rate was related to runoff P (r2 = 0.50-0.98), due to increased concentrations of dissolved reactive phosphorus (DRP) in runoff; as application rate increased, so did the contribution of DRP to runoff TP. Varied concentrations of water-extractable phosphorus (WEP) in manures (2-8 g WEP kg(-1)) resulted in significantly lower DRP concentrations in runoff from dairy manure treatments (0.4-2.2 mg DRP L(-1)) than from poultry (0.3-32.5 mg DRP L(-1)) and swine manure treatments (0.3-22.7 mg DRP L(-1)). Differences in runoff DRP concentrations related to manure type and application rate were diminished by repeated rainfall events, probably as a result of manure P translocation into the soil and removal of applied P by runoff. Differential erosion of broadcast manure caused significant differences in runoff TP concentrations between soils. Results highlight the important, but transient, role of soluble P in manure on runoff P, and point to the interactive effects of management and soils on runoff P losses.

Phosphorus (P), an essential nutrient for crop and livestock production, can accelerate freshwater eutrophication, now one of the leading water quality impairments in the United States. In response, the U.S. Department of Agriculture and the Environmental Protection Agency proposed a new nutrient management policy, now addressing P as well as nitrogen (N), which each state must enact by 2008. There are three approaches that address P-agronomic soil test P recommendations, environmental soil test P thresholds, and a P index to rank fields according to their vulnerability to potential P toss. There are many versions of the P index now in use, demonstrating the robustness and flexibility of the indexing framework to better target remedial measures. Of the three P-based approaches, the P indexing approach has been most widely adopted with 47 states using this approach to target P management. This paper charts the development of the indexing approach, which ranks site vulnerability to P loss by accounting for source (soil test P, fertilizer, and manure management) and transport factors (erosion, runoff, teaching, and connectivity to a stream channel) and outlines modifications made among states that reflect local conditions and policy. Additional factors include flooding frequency, STP modifiers (texture, pH, P sorption, reactive aluminum [Al]), conservation practices, and priority of receiving waters. While computation of the final index value is additive in 20 states, 17 multiply source and transport factors to define critical source areas. Most states (47) have maintained the original indexing approach of assessing site vulnerability to P loss, with indices in three states quantifying P toss. We demonstrate using three management scenarios (changing the time of applied manure, riparian buffer establishment, and reduced feed P ration) that overall P index ratings can be decreased, giving farmers more options for manure management than by simply reducing application rates.

Transport of P by subsurface flow pathways can be an important mechanism of P transfer from land to water, particularly in manured soils that are artificially drained. This study was conducted to determine whether detailed description and interpretation of soil P profile data provide adequate insight into P leaching potential. Evidence of P translocation within soil profiles of a tile-drained Buchanan (fine-loamy, mixed, semiactive, mesic Aquic Fragiudult)-Hartleton (loamy-skeletal, mixed, active, mesic Typic Hapludults) catena was assessed by measuring oxalate-extractable P, P sorption saturation, Mehlich-3 P, water-extractable P in bulk and clay film samples obtained from individual horizons. Tile-drain monitoring and column leaching experiments were conducted to evaluate interpretations derived from soil P profile data. Soil P fractions were not correlated with P losses in lysimeter studies, indicating the limited potential of using soil profile P data for quantitative prediction of leaching losses. Application of manure to the soil surface resulted in significant increases in leachate P concentrations from the lysimeters. Soil profile P data did, however, provide some evidence of long-term P leaching. While bulk horizon samples did not indicate significant long-term P translocation to soil depths corresponding with artificial drainage, some clay film samples had significantly elevated oxalate P, P sorption saturation and Mehlich-3 P at lower depths. Elevated P concentrations in clay films may be associated with preferential transport of P along soil macropores, although, not all clay films sampled in this study were necessarily associated with active macropores. Thus, soil P profile data appear to provide limited insight into P leaching potential.

Concern over nonpoint-source phosphorus (P) losses from agricultural lands to surface waters has resulted in scrutiny of factors affecting P loss potential. A rainfall simulation study was conducted to quantify the effects of alternative P sources (dairy manure, poultry manure, swine slurry, and diammonium phosphate), application methods, and initial soil P concentrations on runoff P losses from three acidic soils (Buchanan-Hartleton, Hagerstown, and Lewbeach). Low P (12 to 26 mg kg(-1) Mehlich-3 P) and high P (396 to 415 mg kg(-1) Mehlich-3 P) members of each soil were amended with 100 kg total P ha(-1) from each of the four P sources either by surface application or mixing, and subjected to simulated rainfall (70 mm h(-1) to produce 30 min runoff). Phosphorus losses from fertilizer and manure applied to the soil surface differed significantly by source, with dissolved reactive phosphorus (DRP) accounting for 64% of total phosphorus (TP) (versus 9% for the unamended soils). For manure amended soils, these losses were linearly related to water-soluble P concentration of manure (r2 = 0.86 for DRP, r2 = 0.78 for TP). Mixing the P sources into the soil significantly decreased P losses relative to surface P application, such that DRP losses from amended, mixed soils were not significantly different from the unamended soil. Results of this study can be applied to site assessment indices to quantify the potential for P loss from recently manured soils.

Modeling phosphorus (P) loss from agricultural watersheds is key to quantifying the long term water quality benefits of alternative best management practices. Scientists engaged in this endeavor struggle to represent processes controlling P transport at scales and time frames that are meaningful to farmers, resource managers, and policy makers. To help overcome these challenges, we reviewed salient issues facing scientists that model P transport, providing a conceptual framework from which process-based P transport models might be evaluated. Recent advances in quantifying the release of soil P to overland and subsurface flow show that extraction coefficients relating soil and flow P are variable but can be represented as a function of land cover or erosion. Existing information on best management effects on P export should be linked to watershed models to better represent changes in P transport. The main needs of P transport models are inclusion of flexible coefficients relating soil and overland flow P, fertilizer and manure management and P loss, stream channel effects on edge-of-field P losses prior to water body input, and linkage of watershed and water-body response models. However, it is essential that the most appropriate model be carefully selected, according to a user's needs in terms of available input data, level of predictive accuracy, and scale of simulation being considered.

Concern over the contribution of agricultural phosphorus (P) to fresh water eutrophication has focused attention on practices that decrease P losses from agricultural soils. At present, there are few management options for soils with P levels in excess of crop P requirements other than restricting P applications. This study assesses various readily available materials as possible P Sorbing Soil Amendments (PSSAs) by comparing their near- and long-term effects on soil P solubility and potential availability to runoff with their impact on plant available P. Specifically, anthracite refuse ash, bituminous refuse ash, by-product gypsum (CaSO4), siderite (FeCO3), steel processing sludge, water treatment residual, and wollastonite (CaSiO3) were incubated in three acidic and one neutral loam soils for 21 and 120 days to compare starting and ending water extractable soil P (WEP) and Mehlich-3 P concentrations. Across all soils, bituminous refuse ash, by-product gypsum, and water treatment residual decreased WEP consistently at the lowest rates of application without lowering Mehlich-3 P to less than crop requirements. In addition, no significant difference in WEP and Mehlich-3 P was observed for by-product gypsum between the 21-day and 120-day incubation periods, indicating that sorption reactions induced by this material are both rapid and stable with time. Anthracite refuse ash, siderite, steel processing sludge, and wollastonite were ineffective at decreasing WEP in soil at practical application rates. Results support the use of by-product gypsum and water treatment residual on acidic and neutral soils representing areas of high P loss potential.

The widespread use of soil phosphorus (P) data, either in the context of agronomic or environmental management, requires an explicit understanding of potential errors related to soil P testing. This study compares a variety of soil P extraction methods, each performed by 9 separate laboratories on 24 soils from across the United States. Soil clay content ranged from 0 to 47%, pH from 4.2 to 8.6, and Mehlich-3 P concentration from 2 to 205 mg kg−1. Average interlaboratory coefficients of variation (CVs) ranged from 0.11 to 0.22 for solution extracts (Bray-1 P, Fe-strip P, Mehlich-3 P, and Olsen P) and from 0.11 to 0.70 for saturated paste extracts (resin capsules and resin membranes, incubated for 2, 4, and 7 days). For soil tests based upon solution extracts, Olsen P exhibited the greatest variability among laboratories (CV = 0.22), despite its reputed suitability for a wider range of soils than Bray-1 and Mehlich-3. Soil test data were highly correlated, with the lowest correlations occurring between Olsen and Bray-1 P or Olsen and Mehlich-3 P (r = 0.77 and 0.84, respectively) and the highest correlations occurring between Olsen P and Fe-strip P or Mehlich-3 and Bray-1 P (r = 0.94 for both correlations). Results indicate that some common soil test P protocols, when carefully conducted, yield data that may be reliably compared, such as in the compilation of regional and national soil databases.

Concern over the transport of phosphorus from agricultural soils to surface waters has focused attention on the role of soil phosphorus in environmental risk assessment. This study explores the existence of natural soil phosphorus thresholds as expressed by Quantity/Intensity relationships. Fifty-nine samples, collected from agricultural soils in New York's Delaware River Watershed, were analyzed for Morgan, Mehlich III, and 0.01 M CaCl2 extractable P. Soil P sorption saturation was calculated as a function of oxalate extractable P, Fe, and Al. In addition, P sorption isotherms were determined for all soils. Thresholds in the relationships between CaCl2 P and Morgan P, Mehlich III P, and P sorption saturation were identified by segmented linear regression (change point analysis), Thresholds in the relationship between CaCl2 P and Morgan P, Mehlich III P, and P sorption saturation occurred at CaCl2 P concentrations of 0.9 mg kg(-1), suggesting a threshold for soil P that may have use in environmental risk assessment. A P sorption threshold was also identified by segmented, quadratic-linear regression of the sorption isotherms, Results described a fundamental property of soils: a nonlinear sorption of P in soils that exhibits a threshold, above which the potential for P release from soil to water increases, This threshold describes a critical point in the release of P and, therefore, may be of environmental importance in estimating the potential for soluble P loss from soil by runoff and leaching.

Soil P saturation affects the risk of P loss to surface and ground water and is therefore a critical environmental indicator in regions where eutrophication is a concern. In the USA, most soil testing laboratories do not include environmental indicators such as son P saturation as standard soil test options. Development of pedotransfer functions that relate soil test data to soil P saturation, however, would enable soil testing laboratories to estimate soil P saturation as part of soil test results without significant additional expenditures. This study examines associations between readily-available soil test data (pH, soil organic matter, and extractable P, Al, Fe, and Ca) and soil P saturation as estimated by add ammonium oxalate extraction. Fifty-nine soil samples were collected from the Delaware River Watershed in New York State (42 degrees 21'N, 74 degrees 52'W) and subjected to standard soil test analyses as well as to acid ammonium oxalate extraction. Some soil test variables were well correlated with soil P saturation. As a single predictor, soil test P was most highly correlated to soil P saturation (r = 0.88). This association supports the use of soil test P as an environmental indicator. Soil test Al also was well: correlated with soil P saturation following logarithmic transformation (r = 0.73). Multivariate pedotransfer functions containing soil test P, Al, Fe, soil organic matter, and pH did not significantly improve estimation of soil P saturation (R = 0.91) above soil test P alone.

The objective of this study was to assess the near- and long-term impact of traditional slash-and-burn agriculture on soil fertility in the village of Kembera, West Kalimantan, Indonesia. Soil fertility is widely viewed as one of the key limits to sustainable production in slash-and-burn agroecosystems. A chronosequence of sites ranging from currently cultivated swiddens to a 28-yr fallow was surveyed. Organic C, cation exchange capacity (CEC), pH, nitrate, extractable basic cations, available and total P, and extractable Al were measured as indicators of soil fertility Inferred short-term trends point to significantly improved soil fertility conditions after burning in currently cropped swiddens relative to falloffs. During the fallow, soil organic C, CEC, nitrate, total P, and extractable basic cations all manifest positive associations with fallow length after 3 to 11 yr of fallow; No declines in soil parameters were detected between plots based on the frequency of past slash-and-burn activity Therefore, there is no evidence that slash-and-burn agriculture in Kembera has degraded these soil resources. While the results of this study suggest that slash-and-burn agriculture in Kembera has been ecologically sustainable, recent changes in socioeconomic factors influencing farmer's land-use decisions cast doubt upon the future sustainability of this agroecosystem.