Illinois Fertilizer
Conference Proceedings
January 25-27, 1999
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Regis Voss1
Phosphorus (P) is an essential nutrient for terrestrial and aquatic plants. The beneficial effects of P on the growth and yield of crops has been recognized. There is increasing concern and attention being given to P losses from agricultural soils. Substantial amounts of P entering surface waters (lakes, other surface impoundments, and streams) contribute to accelerated eutrophication of lakes and reservoirs. Eutrophication is a process by which a water body becomes rich in dissolved nutrients and, often, seasonably deficient in oxygen. Eutrophication due to excessive algal and other plant growth and their ultimate decomposition, which consumes oxygen, limits the use of surface waters for aesthetics, fisheries, recreation, industry, and drinking.
In recent years there has been a change to more intensive agricultural production
systems, especially the localization and intensification of animal production
systems. With this intensification has come a buildup of soil P levels in site-specific
areas to levels rarely encountered in past decades. As a result, there is increased
potential for P losses from these site-specific areas and environmental risk
to affected surface waters. Many of these high-P soil-test areas are located
near sensitive water bodies. When adsorption sites for P in the soil become
saturated, P is potentially more available for runoff and leaching losses.
Traditional soil test extractants for P were developed by research to provide
indexes of P availability to plants. There currently is no standardized P testing
procedure to identify critical soil P levels associated with environmental
risks. There is a need to develop field and soil measurements to help identify
P problem areas and to target these areas with acceptable management practices
to achieve satisfactory economic and environmental solutions.
There is a general conclusion that aquatic growth in inland surface waters is P limited (i.e., as P concentration in surface water increases, aquatic growth increases). According to the National Research Council (1993), overall trends indicate about equal numbers of U.S. rivers with increasing and decreasing P loads. In general, decreases are linked to point-source reductions and increases are linked to nonpoint source increases that are associated with increased sediment loads and agricultural land use.
The critical concentration of P associated with accelerated aquatic growth
is very low, 0.01 parts per million (ppm), but a range from 0.01 to 0.03 ppm
seems to be accepted (NCR 1993). These values are roughly one-tenth of the
soil solution concentration critical for plant growth. The U.S. Environmental
Protection Agency (US EPA) has not yet developed P water-quality criteria for
freshwater bodies but has established 0.001 ppm elemental P as a criterion
for marine and estuarine water (Parry 1998). Daniel et al. (1998) stated, however,
that water quality criteria have been established to control eutrophication
(US EPA 1986). For example, total P should not exceed 0.05 ppm in streams entering
lakes or reservoirs or exceed 0.025 ppm within lakes or reservoirs. For the
prevention of plant nuisances in streams or other flowing waters not discharging
to lakes or impoundments, the concentration of total P should not exceed 0.10
ppm. A dissolved P concentration of 1 ppm is the limit required of sewage-treatment
output and one advocated by some as a critical flow-weighted-mean-annual concentration
for agricultural runoff.
Phosphorus can reach surface waters as P dissolved in runoff water, P attached to soil particles contained in soil erosion, and P contained in tile effluent. Not all agricultural land, nor all that contained in a watershed, contributes to any or all of these processes.
Phosphorus potentially available to algal uptake is termed bioavailable phosphorus (BAP), which is comprised of dissolved phosphorus (DP) and particulate forms of phosphorus (PP). Dissolved P is mostly available for algal uptake, but PP, associated with eroded sediment and organic matter, contributes a variable but long-term source of BAP (Sonzogni 1982, Sharpley and Smith 1991).
If runoff containing DP and PP from agricultural fields enters a surface stream, the DP may be adsorbed (concentration decreases) by stream sediments or PP may be desorbed (DP concentration increases), depending on the P sorption saturation of the stream sediments. Thus, the concentration and amount of BAP entering a lake or reservoir may be different from that leaving an agricultural field. Sediments with high P concentrations entering a lake or reservoir can contribute BAP by desorption for a prolonged period of time.
The effect of BAP entering lakes or surface impoundments on eutrophic growth depends greatly on their characteristics. Turbidity, depth of water, flushing rate, stratification, and background P level of lakes or reservoirs affect growth of algae and other aquatic vegetation. In general, P control strategies have greatest benefit on deeper, stratified lakes with a low flushing rate (fewer than six times per year) and low background levels of P.
Studies have found higher concentrations of DP in surface water runoff from no-till fields with surface crop residue than in runoff from conventional-till fields (Romkens et al. 1973). Also, studies have identified higher runoff concentrations from fields covered with frozen crop residue, such as alfalfa, than from tilled fields (Wendt and Corey 1980), and from fields with surfaceapplied nonincorporated fertilizer, manure, or both, than from fields with incorporated fertilizer (Truman et al. 1993).
Rainfall interacts primarily with the 0- to 2-in. layer of surface soil (Oloya and Logan 1980, Sharpley and Smith 1989). As a consequence, there is a very good positive relationship between soil test P levels and concentrations of DP in runoff (Pote et al. 1996), as shown in Figure 1. Losses are exacerbated by stratification of surface applied P in no-till and conservation-tillage systems, where soil test P levels are greatest in the 0- to 2-in. layer (Triplett and Van Doren 1969, Robbins and Voss 1991).
There has been a general increase in soil test P levels in the United States since World War II because of P applications. A 1989 summary of soil test values showed that in several states, more than 50% and in some states 75% of soil test P samples tested high (PPI 1994). A recent soil test summary from 1997 (PPI/PPIC/FAR 1998) indicates that many agricultural soils remain in the high and very high categories. For many states, percentages of tests with high P levels are similar to 1989 percentages, but trends show decreasing numbers of tests with high P levels in some important agriculture states in the Midwest, such as Indiana, Illinois, Iowa, Minnesota, and Ohio. In other states, such as Arkansas, Wisconsin, North Carolina, and Delaware, soil test P levels continue to increase.
A nutrient budget analysis for Iowa and Wisconsin indicates that these up-or-down trends might be explained based on P use and removal. Assuming that all collectable manure in Iowa was applied to cropland, P removal by crops exceeded the inputs by 10% in 1996. In Wisconsin, P crop removal is only 84% of P inputs (Bundy 1998). In many cases, the problem of elevated soil test P levels is associated with regions where intensive animal production facilities exist and animal manure supplies exceed crop needs on available agricultural land. County-based estimates of the potential for P available in animal manure to meet or exceed crop removal are available to identify local soil test P problem areas (Lander et al. 1998). The potential for P loss from surface runoff and, in some situations, subsurface leaching, increases as soil test levels exceed critical soil test values established for crop needs (Sharpley et al. 1996).
Because commonly used soil test procedures (e.g., the Bray and Kurtz P-1, Mehlich III, and Olsen tests) were developed to provide indexes of P availability to plants, a more rigorous test is desirable to indicate the loss of DP for environmental interpretation. Soils adsorb or desorb P depending upon the P sorption saturation of the soil, which is defined by the following equation:
P sorption saturation = Extractable soil P X
100
P sorption capacity
More P is desorbed (or released) from soil and lost through runoff or leaching as P sorption saturation increases (Figure 2). A better relationship was found for DP concentration and P saturation than for DP concentration and soil test P indexes (Sharpley 1995). The P sorption saturation test provides an integration of soil characteristics, but it is time-consuming and costly. This method, like routine soil tests, does not predict total loss of DP, which depends on runoff volume. The P sorption saturation approach does indicate potential for loss of DP. The Dutch have designated a critical P saturation value of 25% (Van der Molen et al. 1998).
Phosphorus associated with eroded soil sediment is termed particulate phosphorus (PP). Eroded sediment tends to have a higher P concentration than its original source, but excessive soil erosion may dilute the concentration in the total eroded sediment. Particulate P can be 75% to 90% of the P transported in runoff (Schuman et al. 1973).
Although PP loss may be greater than DP loss, only a portion of PP is bioavailable phosphorus (BAP) because some of the adsorbed P will not desorb and is not available to plants. Phosphorus extracted by a NaOH solution is more closely associated with BAP concentration and availability to algae than is soil test P (Wolf et al. 1985). A simpler procedure using iron oxide-impregnated paper strips to determine the BAP directly related to algae growth was adapted by Sharpley (1993a, b) (Figure 3). The iron-oxide strips function as a P-sink, adsorbing P released from soil sediment and simulating P removal by algae.
Although specific analytical procedures provide indexes of P that are related to concentrations of DP, PP, and BAP loss in runoff water and eroded sediment, test results are not reliable indicators of P amounts lost from fields or arriving to surface waters. Test results do indicate a potential for loss, if combined with estimates of runoff or erosion potential.
Because P is considered to be immobile in the soil, there is generally little concern that it will be lost in tile drainage. There are, however, locations with sandy or organic soils where tile effluent has high concentrations of P (Duxbury and Peverly 1978). In the Netherlands, where sandy and organic soils with high water tables are prevalent, restrictions on P use are imposed. Studies in the United States have in general found very low concentrations of DP in tile drainage, but concentrations frequently exceed the projected critical value of 0.01 ppm of P for algae growth (Baker et al. 1975). In one study, P concentrations in tile effluent increased as rates of manure increased, indicating that excessive manure loading can contribute to DP loss even though economic loss is negligible (Hergert et al. 1981).
Immediate concerns are to manage and monitor P concentrations in tile drainage from areas where soil P concentrations are already very high, soil P sorption capacities are low, and subsurface transport is enhanced by tiles and surface ditches. Phosphorus-saturated soils could lead to prolonged loss of DP in tile effluents. In many situations, loss of Pin tile effluent will be of little consequence relative to surface runoff and erosion (e.g., in fine-textured soils that are judiciously fertilized in accordance with soil testing recommendations and that have low degrees of P saturation).
Because of the diversity in the agricultural landscape, there is a wide range in the potential loss of P from fields within the landscape. Contributing to the diversity are physical and chemical characteristics of the soils, landscape form, crop and plant vegetation, crop production cultural practices, P level of the soils, and method of P application. Most watersheds contain field sites that are different in one or more characteristics. To assess the potential risk of P movement to surface waters from various landforms subjected to different management practices, a Phosphorus Index was proposed by Lemunyon and Gilbert (1993). This index considered eight weighted factors: soil erosion weighted x 1.5; irrigation erosion x 1.5; runoff loss x 0.5; soil P test x 1.0; P fertilizer application rate x 0.75; P fertilizer application method x 0.5; organic P source application rate x 1.0; and organic P source application method x 1.0. Rating values for each level of these site characteristics were assigned (low = 1, medium = 2, high = 4, very high = 8), multiplied by their respective weighting values, and summed over the eight items. The resulting value provides the relative vulnerability of a site for P loss. This index has been applied in evaluating watersheds and although it may need refinement, it has been satisfactory in identifying P sources within a watershed that will require more management to minimize P loss in runoff and maintain crop productivity
A more recent approach is simulation modeling through computer programs. This approach permits evaluation on a watershed basis and predicts the effect of various P management and cultural practices on potential P losses. This approach will probably require more input than the Phosphorus Index, which can be assessed with locally available information.
Several management practices can affect P loss from agricultural fields. These practices include: soil test P level maintained; time, rate, and method of P application; tillage that affects erosion and amount of crop residue on the soil surface; amendments to manure and soil to reduce P availability; and vegetative filter strips adjacent to surface water. Feed and feed additives can reduce the amount of P in animal manure.
Applying manure or fertilizer P to frozen or snow-covered ground results in more P loss in runoff than when nutrients are applied to bare unfrozen ground. The P concentrations in runoff from the first rain after a surface-manure application are greater than that in runoff from subsequent rains. As time between manure application and rain increases, P concentrations in runoff decrease. Timing of manure and fertilizer applications are important if these materials are surface applied. Injecting manure and banding fertilizer P into the soil eliminates most of the potential for runoff P.
Because of the relationship between surface soil test P values and loss of Pin runoff and erosion, building and maintaining very high soil test P values becomes an environmental concern. Data from Iowa, obtained from a corn-soybean sequence grown on a soil with a very low subsoil level of P, show it may take a decade or more to reduce a very high soil test P to a responsive range (Figure 4). Also, economic returns are negative for maintaining very high soil test values (Figure 5) (Webb et al. 1992).
Tillage on soils where runoff and erosion may occur increases total P loss with variable BAP concentrations and amounts as previously cited. No-tillage or very reduced tillage that leaves crop residue on the surface of similar soils reduces total P and BAP loss but may increase DP in runoff water. Periodic inversion of P-stratified surface soils may be advantageous to reduce concentrations of P at the soil surface and potential for P loss.
Amendments such as aluminum sulfate, ferrous sulfate, or coal combustion products can be added to manure or soil to reduce soluble P. These amendments can affect other soil properties and should be investigated before applications of such amendments are recommended practice. An organic compound, polyacrylamide, has been used on western irrigated fields and has been shown to reduce loss of P and sediment from these fields.
Much of the P in corn grain is in the form of phytic acid, which is unavailable to monogastric animals (e.g., swine and chickens). Most grain P is excreted in manure. Phytase enzymes, added to feed rations, increase the availability of Pin corn grain and reduce the amount of P excreted. Corn genetic material with a low phytic acid P content has been identified. Feeding trials using this corn grain have shown increased P availability to animals and reduced P content in manure (Ertl et al. 1998). Reduction of P concentrations in manure could reduce P loadings of fields where manure application is based on the nitrogen requirement of the crop to be grown.
Vegetative filter strips between agricultural lands and surface waters can be effective in reducing the amount of sediment and PP entering surface waters but may increase the amount of DP in runoff waters.
Crop and soil management options exist to minimize potential P losses into surface waters.
1 Regis Voss is a Professor of Agronomy, Iowa State University, Ames, IA 50011.
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