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Determining environmentally sound soil phosphorus levels.

Bioavailable phosphorus (BAP) in dissolved (DP) and particulate (PP) forms in agricultural runoff can promote freshwater eutrophication (Sharpley et al. 1994b). While DP is immediately available for uptake by aquatic biota, a variable portion of PP represents a secondary and long-term source of BAP in lakes (Sharpley 1993). Dissolved P in runoff originates from the release of P from a thin zone of surface soil and vegetative material (Sharpley 1985) [ILLUSTRATION FOR FIGURE 1 OMITTED]. Particulate or sediment-bound P is associated with soil and vegetative material eroded during runoff. Bioavailable P includes DP and a portion of PP that is in equilibrium with DP and available for algal uptake (Sharpley and Smith 1993). The loss of P in agricultural runoff is of increasing concern in several areas of the United States, primarily where the production of P in manure from confined animal operations exceeds local crop requirements of P. Some crop production systems are forced to continually use manures as fertilizers because of the lack of economically viable alternatives for manure disposal. The systems almost always build soil P levels well beyond the ranges considered optimum for most agronomic crops, because manures are normally applied at rates designed to meet crop N requirements while avoiding groundwater quality problems created by leaching of excess manure N. The unfavorable N:P ratio in most manures then results in over-application of manure P relative to crop P needs. Consequently, soil test P has accumulated to levels that are of environmental rather than agronomic concern in many areas, although high soil P can create an imbalance of micronutrients [ILLUSTRATION FOR FIGURE 2 OMITTED!. Much of this increase has occurred in the last decade (Combs and Burlington 1992; Sims 1993).

Environmental concern has forced many states to consider the development of recommendations for manure applications based on the potential for P loss in runoff, as well as crop N and P requirements. A major difficulty in the development of these recommendations has been the identification of soil test P levels that are high enough to raise concerns about the potential for unacceptable levels of P loss in runoff (Table 1). Establishing these levels is often a highly controversial process for two reasons. First, the data base relating soil test P to runoff P is limited to a few soils and crops and there is often a reluctance to rely upon data of this type generated in other states or regions. Second, the economic implications of establishing soil test P levels which limit manure applications are significant. In many areas dominated by animal-based agriculture, there simply is no economically viable alternative to land application. Because of these factors, those most affected by these soil test P limits are vigorously challenging their scientific basis. Clearly, there is a need to assess the validity of the use of soil test P values as indicators of P loss in runoff.

The current lack of P management recommendations relating P in runoff to soil stem from two areas of limited information. First, there is a lack of standards or guidelines on the concentration of P in agricultural runoff that is considered eutrophic. While not the objective of this paper, it is worth noting that several recommendations have been advanced as critical levels expected to promote objectionable aquatic plant growth in receiving water bodies. Sawyer and Vollenweider suggested the critical level of DP in lake systems is 10 [[micro]gram] [L.sup.-1]. The U.S. EPA has established guidelines of 100 and 50 [[micro]gram] total P [L.sup.-1] for streams and lakes, respectively. Only recently, Florida identified 50 [[micro]gram] [L.sup.-1] as the concentration of DP allowable in drainage water entering the Everglades (USA vs South Florida Water Management District 1994) and by the year 2000 hopes to reduce this concentration to 10 [[micro]gram] [L.sup.-1]. In other areas of the country, a flow-weighted-annual DP runoff concentration limit of 1,000 [[micro]gram] [L.sup.-1] for agricultural runoff, similar to that required of sewage treatment plants, has been proposed (US EPA 1986).

The issue is further complicated by the fact that we have an inadequate understanding of the changes that can occur in DP concentrations between "edge-of-field" and receiving waters sensitive to eutrophication. Sediments in streams and drainageways connecting fields with lakes may be sinks or sources for DP depending upon their physical and chemical properties. Hence, even if DP concentrations at the original source (an agricultural field) are well in excess of critical levels for the ultimate sink (a lake), sufficient reduction in DP may occur during the transport process to reduce the environmental risk considerably; conversely, DP enrichment during transport could also occur and increase the potential for eutrophication.

Secondly, and most importantly, there is limited information on the concentration of P in runoff supported by a given soil P level and how the properties of the zone of maximum interaction between runoff and soil (upper 5 cm) affect P concentrations [ILLUSTRATION FOR FIGURE 1 OMITTED]. Specifically, how will differences in soil organic matter, pH, and texture affect the concentration of P in runoff from soils with similar soil test P values?

The aim of this paper is to address this second issue by presenting information on the relationship between surface soil P and P transport in runoff from several published and unpublished field studies. The advantages and disadvantages of using environmentally based soil test P recommendations are presented.

Soil test methodology

The literature and experience relating the level of P in soil to that in runoff is limited. Routine soil tests have been used for years to estimate plant available P, but they have not been evaluated for their ability to predict runoff P concentrations. A relationship between the level of P in soil and that in runoff was documented by Romkens and Nelson using simulated rainfall and later confirmed by Sharpley et al. (1978; 1986) using small plots and watersheds, respectively. These early efforts and those that followed (Daniel et al. 1993; Schreiber 1988; Vaithiyanathan and Correll 1992) related runoff concentration of DP to levels of soil P as determined by traditional soil test methods.

While these results showed promise in describing the relationship between the level of soil and runoff P, they are limited for several reasons. First, while DP is an important water quality parameter, it only represents the dissolved portion of runoff P readily available for aquatic plant growth. It does not represent absorbed soil P that can become available through desorption. To overcome this limitation, Sharpley (1993) developed an approach using iron-oxide impregnated paper, which provides an accurate, theoretically sound estimate of algal available P (BAP).

A second limitation of all approaches developed to date is that they rely on soil tests traditionally used to assess the fertility status of the soil and not runoff water quality. These methods (Bray I, Olsen, and Mehlich I and III) were developed with crop production in mind and utilize chemical extractants to estimate the amount of soil P available for plant growth. The extractants usually contain chemicals which remove some mineral and sorbed P (Sharpley et al. 1994a). For an environmental soil P test, an extractant such as distilled water, or a dilute salt solution similar in ionic strength to the soil solution (e.g., 0.01M Ca[Cl.sub.2]), may be more appropriate. However, these procedures need further evaluation.

Another soil testing approach which is gaining acceptance estimates soil P sorption saturation, which is defined as

P sorption saturation = (Extractable soil P/P sorption capacity) x 100 [1]

where the units of extractable soil P and P sorption capacity are unit mass of DP for a given mass of soil (mg [kg.sup.-1]). This approach is based on the fact that more P is released from soil to runoff or leaching water as P saturation or amount of P sorbed increases with P additions.

Soil P saturation has been used in the Netherlands, where national management farm recommendations are designed to limit the loss of P in surface and ground waters (Breeuwsma and Silva 1992). These recommendations are based on identification of a critical soil P sorption saturation level above which P applications should not exceed crop removal rates (Breeuwsma and Silva 1992; Van der Zee et al. 1987). For Dutch soils, a critical P saturation of 25% has been established as the threshold value above which the potential for P movement in surface and ground waters becomes unacceptable (Van der Zee et al. 1987).

Although the P saturation approach requires measurement of soil P sorption capacity, in addition to soil test P, it may describe the effect of soil type on soil P release to runoff by accounting for soil properties affecting P sorption and desorption. As the P saturation approach will be more tedious than soil test P measurement, its application may be limited to soils already identified as being vulnerable to P loss.

Field observations

Dissolved phosphorus. Soil test P content (mg [kg.sup.-1]) of surface soil (x variable) is related to the concentration ([[micro]gram] [L.sup.-1]) of DP in runoff (y variable) from watersheds of varying management and diverse agricultural land use (Table 2). In fact, soil test P accounted for 58 to 98% of the variation in DP concentration of runoff. In a more detailed study, Pote et al. (1996) found the DP concentration of runoff from a Captina silt loam under fescue in Arkansas was related (P [less than! 0.001) to the Mehlich 3 extractable P content of surface soil (0-2 cm) [ILLUSTRATION FOR FIGURE 3 OMITTED]. For this study, a wide range in soil test P was obtained by previous applications of poultry litter at varying rates. According to Figure 3, approximately 200 mg [kg.sup.-1] Mehlich-3 P in the surface 2 cm of soil would produce a DP concentration of 1,000 [[micro]gram] [L.sup.-1] in runoff.

Clearly, there is a greater potential for enrichment of the DP concentration of runoff as soil test P increases. However, the relationship between soil test P and DP concentration of runoff varied between studies as a function of management and soil type (Table 2). For example, the slope of the soil test P-DP concentration regression for runoff from grass (average of 5.89) was lower than from cultivated land (average of 10.47), except for the low slope value for the data of Andraski et al. (1985).

For those studies where runoff from both grass and cultivated land was related to soil test P on the same soil type (Olness et al. 1975; Sharpley et al. 1981), slopes were lower for grass (6.09 and 4.09) than cultivated land (12.47 and 9.62) (Table 2). This difference may result from less interaction of runoff with surface soil, due to a greater vegetative cover and surface soil protection by grass than crops (Sharpley 1985; Sharpley et al. 1981). Thus, if we assume surface soil (0 to 5 cm) to be the main source of P in runoff, a given soil test P level should maintain a lower DP concentration of runoff from grass than from cultivated land. However, recent broadcast applications of fertilizer or manure may reverse this trend, particularly if the application is unincorporated on grass but incorporated during a crop tillage operation.

Even though the slope of the regression between DP concentration (y variable) and soil test P (x variable) was lower for runoff from grass than cultivated land, values varied within each management category (Table 2). For grass, slopes varied from 4.09 to 7.00 and for cultivated land from 8.30 to 12.47. Thus, for a given land management, soil type as well as other edaphic and agronomic factors will influence the relationship between soil test P and DP concentration of runoff. In other words, for a given soil test P level, the concentration of P maintained in runoff will be influenced by soil type, because of differences in P buffering capacity between soils caused by yawing levels of clay, Fe and Al oxides, carbonates, and organic matter content.

Soil test P is an important and quantifiable parameter that influences the DP concentration of runoff. However, soil test P was not significantly related to the loss of DP (kg [ha.sup.-1]) in runoff for any of the studies presented in Table 2 or Figure 3. In fact, regression coefficients ranged from only 0.04 (Andraski et al. 1985) to 0.26 (Sharpley et al. 1978). It is evident that variability in runoff volume as a result of climatic, edaphic, and agronomic factors plays a larger role in determining P loss than the soil test P content of surface soil. Although soil test P is related to the DP concentration of runoff, it is not a reliable indicator of DP loss. Thus, care should be taken in using soil test P as the sole criteria to determine the potential for P enrichment of runoff and subsequent fertilizer or manure application, particularly without an estimate of runoff or erosion potential.



Bioavailable phosphorus. Less information is available on the relationship between soil P and BAP transport in runoff. Recently, Pore et al. (1996) found the Fe-oxide strip P (strip P) content of the surface soil (0 to 2 cm) was related (P [less than] 0.001) to the BAP concentration of runoff from a Captina silt loam under fescue in Arkansas [ILLUSTRATION FOR FIGURE 4 OMITTED!. The concentration of BAP in runoff from native grass in Oklahoma was also related to the strip P content of surface soil [ILLUSTRATION FOR FIGURE 5 OMITTED!. However, the BAP concentration of runoff from the same Kirkland silt loam soil cultivated with wheat was curvilinearly related to surface soil strip P [ILLUSTRATION FOR FIGURE 6 OMITTED].

The regression coefficient between strip P and BAP concentration of runoff was lower from wheat ([r.sup.2] = 0.64) than native grass ([r.sup.2] = 0.80) [ILLUSTRATION FOR FIGURE 5 OMITTED]. This difference may result from the fact that overall, bioavailable PP (BPP) comprised a greater portion of BAP in runoff from wheat (56%) than native grass (19%). Unlike DP, BPP transport in runoff is dependent on erosion potential as well as surface soil P content. The variation in BAP concentration from the regression line for runoff from the wheat watersheds at El Reno was greater at high than low soil P [ILLUSTRATION FOR FIGURE 5 OMITTED].

Clearly, surface soil strip P content by itself is not a reliable estimate of the potential for BAP enrichment of runoff from cropped land. In such cases, an estimate of erosion potential (e.g., USLE value) is also needed to provide a more accurate indicator of BAP transport.

As noted earlier for DP, the loss of BAP in runoff was not related (P [greater than] 0.05) to surface soil strip P, for either the Arkansas ([r.sup.2] = 0.14) or Oklahoma studies ([r.sup.2] = 0.24 for native grass and 0.32 for wheat).

Phosphorus sorption saturation. The relationship between the P sorption saturation of surface soil and DP concentration of runoff from 56 plots of a Captina silt loam under fescue in Arkansas was investigated. Phosphorus sorption saturation was calculated from Equation [1], where extractable soil P was represented by surface soil water soluble P content of each plot (1 g soil shaken with 25 mL water for 1 hr). Water extractable soil P was used to more closely reflect the release of soil P to rainfall/runoff. Phosphorus sorption maximum was calculated from a Langmiur sorption isotherm for an unfertilized Captina silt loam (Nair et al. 1984).

Phosphorus sorption saturation was related (P [less than] 0.001) to the DP concentration of runoff from fescue [ILLUSTRATION FOR FIGURE 6 OMITTED!. With an increase in the amount of P sorbed, the DP concentration of runoff increased. This results from a greater release of P from Captina silt loam to runoff as the degree of saturation of P sorption sites increased. Using the relationship between soil P saturation and runoff DP shown in Figure 6, we can determine either a P saturation that will support an "acceptable" DP concentration of runoff or vice-versa, a DP concentration that could be expected in runoff from a soil of given P saturation.

Using the Dutch critical P saturation value of 25% (Breeuwsma and Silva 1992; Van der Zee et al. 1990), this would support a DP concentration of runoff of 1,000 [[micro]gram] [L.sup.-1] [ILLUSTRATION FOR FIGURE 6 OMITTED!. As mentioned earlier, a DP concentration of 1,000 [[micro]gram] [L.sup.-1] is the limit required of sewage treatment output and one advocated by some as a critical flow-weighted-mean-annual concentration for agricultural runoff.


Soils with extremely high soil test P levels logically are presumed to be more at risk to P loss in runoff and thus to require more intensive management. However, while soil test P may be related to DP concentrations of runoff (Table 2), it is clear that soil test P alone cannot accurately predict the mass of P lost from an individual field or within a watershed because of the highly variable influences of climate, topography, and vegetation on runoff volume. A more comprehensive approach, one that integrates predicted P concentrations in runoff with estimates of runoff volume to estimate the mass of P lost, will be required to accomplish this. Recent innovations in soil testing and field rating systems for P can facilitate improved soil P management and help to more clearly define the relationship between runoff P and soil test P.

Use of soil testing to predict P loss in runoff. Soil testing procedures for P were developed to estimate the amount of plant-available P in the soil, not the amount of bioavailable P that might be transported in runoff waters. It is not illogical, however, to think that the two types of P measurements would be reasonably well correlated because the forms of soil P of importance to terrestrial plants are those that will be soluble, readily desorbable, or "labile" (available to plants in the short-term, e.g., a growing season). Thus, if a soil is very high in soil test P it is also likely to contain a high amount of soluble or readily desorbed P. To date, however, most soil testing laboratories have not adopted methods to identify soils with a high potential to lose DP and BPP in runoff, i.e., an "upper critical level" for soil P.

Since the zone of surface soil-runoff interaction is usually less than 5-cm deep [ILLUSTRATION FOR FIGURE 1 OMITTED], traditional methods of agronomic soil sampling (0-15 cm) also require evaluation for environmental applications. This would be of particular importance on pasture, where applications of P are usually broadcast. A lack of incorporation of P in the plow layer can result in dramatic accumulations of P in the surface 5 cm of soil (Sharpley et al. 1994b). In such cases, a 0-15 cm soil sample could underestimate the amount of surface soil P potentially available to release to runoff.

Traditionally, critical values for soil test P have been defined as the break-point between a medium soil test value, where crop response to fertilizer P is likely, and a high soil test value, where adding fertilizer will usually not produce an economic yield increase. Critical values for soil test P vary with test and geographic location (Soil and Plant Analysis Council 1992). Typical values for the Mehlich-1, Mehlich-3, and Bray-1 P soil tests in the eastern United States would be 30, 50, and 15 mg [kg.sup.-1], respectively; for the Olsen P soil test, commonly used in the western U.S., critical values usually range from 10 to 15 mg [kg.sup.-1]. These agronomic critical values are three- (for Mehlich-3 P) to ten-fold lower than "environmental" critical values now being considered in several states (Table 1). The rationale used by these states in establishing upper limits for soil test P has generally been that these values are so far in excess of crop P needs that it will be many years before additional fertilizer or manure P will be needed. Further applications, then, enhance the risk for P losses in runoff. This is supported by the data in Figure 3, which shows that at a Mehlich 3 soil test P level of 200 mg [kg.sup.-1] the DP concentration in runoff approaches 1,000 [[micro]gram] [L.sup.-1], a suggested upper limit for DP.

It is, however, extremely time-consuming and costly to obtain data on DP concentrations in runoff and unrealistic to expect that a large data base relating soil test P of most soils to DP will be available in the near future. Because of this, one role soil testing laboratories could play in improved soil P management would be to provide predictions of "readily desorbed P," based on soil test P and other routinely measured soil properties (e.g., pH or buffer pH, organic matter content). This data could then be integrated with other information specifically related to runoff volume, to rate the potential of individual fields to be significant sources of P to nearby surface waters, which is essentially the goal of the Phosphorus Index System described in more detail below.

Some recent data obtained from two watersheds in Delaware illustrate this approach (Sims, unpublished data; [ILLUSTRATION FOR FIGURE 7 OMITTED!). Readily desorbed P (that extracted by 0.01 M Ca[Cl.sub.2], soil:solution ratio of 1:20) and soil test P (Mehlich 1:0.05 M HCl + 0.0125 M [H.sub.2]S[O.sub.4]) were measured in 111 soil samples collected from the upper 0-5 cm of agricultural fields used for the production of corn, soybeans, and small grains. While the two parameters were reasonably well correlated ([ILLUSTRATION FOR FIGURE 7a OMITTED]; [r.sup.2] = 0.66), particularly at higher soil test P values where P loss in runoff is of greatest concern, a better prediction ([ILLUSTRATION FOR FIGURE 7b OMITTED!; [r.sup.2] = 0.77) of readily desorbed P could be made by a multiple regression equation that included soil test P, organic matter content and buffer pH. Since many soil testing laboratories now include organic matter, usually by a loss-on-ignition technique, and buffer pH as part of the routine soil test, predictive equations such as those illustrated in Figure 7 could be developed for different physiographic regions and used to identify soils with the greatest likelihood of desorbing P into runoff waters.

Soil testing laboratories can contribute further to more environmentally efficient P management by offering special tests for P that would be conducted on samples from areas with high potential for P losses in runoff. Fields for more intensive sampling and testing could be identified based on data available in routine soil tests (e.g., regression approaches in Figure 7) and supplemental information related to runoff volume such as USLE erosion values or even field estimates of slope degree and length. Once high-risk areas are known, conservation agencies (e.g., USDA-NRCS, ASCS) could conduct more intensive sampling of the upper 0-5 cm, focusing on the most erosion or runoff prone areas. Together this data would not only identify fields where additional P should not be applied, but also specific sites where more intensive soil conservation practices would be needed because of topographical and hydrologic considerations.

Examples of special tests include direct measurements of readily desorbable P, Fe-oxide strip P, biologically available P, and P sorption saturation. While most soil testing laboratories could not conduct these tests on every sample received because of the time, labor, and specialized equipment/glassware required, these tests are in many ways little different from other special tests already offered by state and private testing labs, such as cation exchange capacity, particle size, total nitrogen, and some tests now used for heavy metals. Incorporating them into a soil testing program, therefore, is primarily a matter of realizing the importance these tests can play in improving the environmental efficiency of modern production agriculture.

Finally, soil testing laboratories must provide interpretive and educational information to those involved in assessing the impact of P losses in runoff, including programs to explain the meaning and use of current and newly developed soil P tests. Since it is highly unlikely that dozens of multi-year, multi-site studies relating runoff P to soil test P and the factors controlling runoff volume (climate, topography, etc.) will be conducted in the next five to ten years, interim steps are needed to identify soils that now contribute significant amounts of runoff P to surface waters. There is a definite need to continue efforts to identify soils that are excessive in P and to develop a logical approach to establish environmental upper limits for soil test P. Just as important is the need to prevent soils that are near this state from getting worse by developing fertilizer and manure recommendations that maintain agronomic profitability and minimize environmental risk from high P soils.

Assessing site vulnerability to phosphorus loss in runoff. A comprehensive approach to rank site vulnerability to P loss in runoff was developed by Lemunyon and Gilbert. The approach integrates soil test P, fertilizer and manure management, and the potential for P transport by erosion and runoff into an overall index. This index ranks sites within a given watershed in terms of their potential to excessively enrich P in surface waters. Some of the principles of this P index system were recently incorporated in the development of revised Waste Utilization Standard and Specifications by NRCS in Oklahoma. In an effort to address the potential for soil P buildup with continual manure applications, specifications were based on soil test P level and the potential for off-site loss, summarized by land slope (Table 1). These specifications attempt to incorporate site characteristics such as P source and transport processes. Thus, they allow more flexibility in P management by considering site vulnerability to P loss.


The concentration of P in runoff is related to the amount of P in the surface layer of soil (about 0 to 5 cm), which reacts with rainfall-runoff. This relationship can be used as the theoretical basis to establish critical soil test P levels, above which P enrichment of runoff becomes unacceptable. The first step in formulating P-based management recommendations is establishment of an acceptable P concentration in runoff for a given physiographic region and presence or proximity of P-sensitive waters to identified P sources. Following this, critical soil test P values which have the potential to support this runoff P concentration, can then be determined. Soil test P or P saturation values in excess of critical values may indicate a need to change the management of fertilizer or manure P inputs. This change may include conservation measures to reduce erosion and runoff, removal of harvested crop P from the management system, and a possible reduction in fertilizer and manure application rates. We have shown that a 1,000 [[micro]gram] [L.sup.-1] DP concentration in runoff is supported by a soil test P (Mehlich 3) of about 200 mg [kg.sup.-1] or a P saturation of 25% for a Captina silt loam in fescue.

Both of these critical soil P values have been used in P-management recommendations. Soil test P values proposed by several states (75 to 200 mg [kg.sup.-1]; Table 1) are similar to or below 200 mg [kg.sup.-1] and thus, not unrealistic; Dutch farm advisors have established a critical soil P saturation of 25%. If DP concentrations lower than 1,000 [[micro]gram] [L.sup.-1] are determined to be unacceptable, then a lower critical soil test P or P saturation would be necessary. We are not promoting a DP concentration of 1,000 [[micro]gram] [L.sup.-1] in runoff as an unacceptable value at this time; it is used merely as an example of how critical soil P values may be justified.

While soil test P is related to P concentrations of runoff, different amounts of P can be lost from sites with similar surface soil test P contents. Clearly, variability in runoff volume and erosion as a result of climatic, topographic, and agronomic factors plays a larger role than soil test P in determining P loss. Thus, a more comprehensive approach is needed for reliable yet flexible recommendations of fertilizer and manure P management. Such an approach must integrate soil test P with estimates of potential runoff and erosion losses.

Research scientists, extension agents, and soil testing personnel will play a vital, interactive role in developing reliable yet flexible fertilizer and manure management recommendations based on soil and runoff P levels. They must also be involved in educating farmers and consumers to the importance of environmentally-sound production agriculture.


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Andrew Sharpley is a soil scientist with USDAARS, Pasture Systems and Watershed Management Research Laboratory, Curtin Road, University Park, PA 16802-3702; T.C. Daniel is professor of Agronomy; J.T. Sims is a professor of Soil Sciences in the Department of Plant Science, University of Delaware, Newark, DE 19717-1303; and D.H. Pote is a graduate assistant in the Department of Agronomy, 115 Plant Sciences Building, University of Arkansas, Fayetteville, AR 72701.
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Author:Sharpley, Andrew; Daniel, T.C.; Sims, J.T.; Pote, D.H.
Publication:Journal of Soil and Water Conservation
Date:Mar 1, 1996
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