The accelerated eutrophication of most freshwaters is limited by P inputs. Nonpoint sources of P in agricultural runoff now contribute a greater portion of freshwater inputs, due to easier identification and recent control of point sources. Although P management is an integral part of profitable agrisystems, continued inputs of fertilizer and manure P in excess of crop requirements have led to a build‐up of soil P levels, which are of environmental rather than agronomic concern, particularly in areas of intensive crop and livestock production. Thus, the main issues facing the establishment of economically and environmentally sound P management systems are the identification of soil P levels that are of environmental concern; targeting specific controls for different water quality objectives within watersheds; and balancing economic with environmental values. In developing effective options, we have brought together agricultural and limnological expertise to prioritize watershed management practices and remedial strategies to mitigate nonpoint‐source impacts of agricultural P. Options include runoff and erosion control and P‐source management, based on eutrophic rather than agronomic considerations. Current soil test P methods may screen soils on which the aquatic bioavailability of P should be estimated. Landowner options to more efficiently utilize manure P include basing application rates on soil vulnerability to P loss in runoff, manure analysis, and programs encouraging manure movement to a greater hectareage. Targeting source areas may be achieved by use of indices to rank soil vulnerability to P loss in runoff and lake sensitivity to P inputs.

The importance of P originating from agricultural sources to the nonpoint source pollution of surface waters has been an environmental issue for decades because of the well-known role of P in eutrophication. Most previous research and nonpoint source control efforts have emphasized P losses by surface erosion and runoff because of the relative immobility of P in soils. Consequently, P leaching and losses of P via subsurface runoff have rarely been considered important pathways for the movement of agricultural P to surface waters. However, there are situations where environmentally significant export of P in agricultural drainage has occurred (e.g., deep sandy soils, high organic matter soils, or soils with high soil P concentrations from long-term overfertilization and/or excessive use of organic wastes). In this paper we review research on P leaching and export in subsurface runoff and present overviews of ongoing research in the Atlantic Coastal Plain of the USA (Delaware), the midwestem USA (Indiana), and eastern Canada (Quebec). Our objectives are to illustrate the importance of agricultural drainage to nonpoint source pollution of surface waters and to emphasize the need for soil and water conservation practices that can minimize P losses in subsurface runoff.

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).

Soil testing has been an accepted agricultural management practice for decades. Interpretations and fertility recommendations based on soil analyses and the information obtained with soil samples on cropping systems, tillage practices, soil types, manure use, and other parameters have contributed to the increased efficiency of agricultural production. Recently, however, analyses of long‐term trends in soil test P values have shown that soil P in many areas of the world is now excessive, relative to crop P requirements. The role of P in the eutrophication of surface waters and emerging concerns about the human health impacts of toxic algal/dinoflagellate blooms have heightened public awareness of nonpoint source pollution by agricultural P. The greatest concerns are with animal‐based agriculture, where farm and watershed‐scale P surpluses and over‐application of P to soils are common. The need for nutrient‐management plans based on N and P is now an issue of intense debate in the U.S. and Canada. This paper addresses three issues: Should the applications of organic wastes and fertilizers be based on soil P and, if so, what is the most appropriate testing method to assess environmental risk? How can our knowledge of soil P chemistry be integrated with the expertise of hydrologists, agronomists, aquatic ecologists, and others to assess the risks that P in agricultural soils poses to surface waters? And, finally, how can we use soil P testing to evaluate new best management practices (BMPs) now being developed to reduce P transport from soil to water?

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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.

Crop response to micronutrient fertilization on the poorly buffered coastal plain soils of Delaware has received renewed interest in recent years. Specifically, given the susceptibility of these soils to overliming and the widespread use of manures, questions have been raised concerning the influence of soil pH and micronutrient source on the distribution and plant availability of Mn, Cu, and Zn, Four soils that varied widely in organic matter content (16‐100 g/kg), texture, and cation exchange capacity (CEC) were studied. A fractionation scheme was utilized that partitioned Mn, Cu, and Zn into the exchangeable, organic, Mn‐oxide bound, amorphous Fe‐oxide bound, and crystalline Fe‐oxide bound forms. Micronutrient distribution among these fractions was studied over a pH range of 4.0 to 7.7 in the soils alone, and in the same soils amended with poultry manure (PM) or (MnSO4 + CuSO4 + ZnSO4). The various fractions of each element were then correlated with Mn, Cu, and Zn uptake by wheat (Triticum aestivum L.) in a greenhouse experiment. Soil pH markedly altered the distribution of Mn and Zn but had little effect on Cu. Although soil type did have some influence, exchangeable Mn and Zn were generally the dominant species of the elements below pH 5.2, while at higher pH values organically complexed and Fe‐oxide bound forms were dominant. The majority of soil Cu was in the organic fraction, but considerable percentages were found in the amorphous Fe‐oxide fraction. Significant source by pH interactions were observed only for exchangeable Mn and Zn. Organic ligands in the manure retained more Zn in a nonexchangeable form under acidic conditions, but had only minor effects on exchangeable Mn. Plant uptake of Mn and Zn was primarily related to the exchangeable fractions; however, predictive models were significantly enhanced by the inclusion of soil pH. Inconsistent correlations and low r2 values for all models involving Cu illustrate the difficulty in predicting crop response to native or added Cu.