Soil and Water Quality An Agenda for Agriculture (1993) / Chapter Skim
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7 Phosphorus in the Soil-Crop System
7 Phosphorus in the Soil-Crop System
Pages 283-312
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From page 283... ...
Development of strategies to reduce phosphorus loadings to surface water requires an understanding of phosphorus inputs and outputs and the transport mechanisms that deliver phosphorus to surface water. Simplistic solutions may exacerbate trade-offs.
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From page 284... ...
SOURCES OF PHOSPHORUS Phosphorus can enter surface water from a variety of sources including municipal wastes, industrial wastes, animal feedlots, and runoff from croplands. Point Sources Point sources of pollutants, such as municipal wastewater treatment facilities or industrial wastewater outlets, were formerly the major sources of phosphorus input to surface waters, with agricultural and other diffuse or nonpoint sources playing a relatively minor role (Bjork, 1972; Sawyer, 1947~.
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From page 285... ...
In 1979, an estimated 28 to 40 percent reduction in the diffuse phosphorus load to Lake Erie was required to meet water quality goals (Logan et al., 1979~. Agricultural Sources of Phosphorus The potential for phosphorus delivery to surface waters varies widely among different agricultural practices, and cost-effective solutions should target the systems with the greatest potential phosphorus delivery reductions per dollar spent on control measures.
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From page 286... ...
Not all of the dissolved reactive phosphorus in lake water is completely available for algal growth (Sharpley and Menzel, 1987~. The relative difference in dissolved reactive phosphorus and bioavailable phosphorus in water is greater in waters with low levels of phosphorus and is less in solutions with higher dissolved reactive phosphorus concentrations (Sharpley and Menzel, 1987~.
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From page 287... ...
~ Estimates of the fraction of sediment-bound phosphorus that is available for biological uptake vary according to the methods used to obtain the estimate, and the estimates obtained by different methods are difficult to interpret, making some standard means of obtaining bioavailability estimates desirable (Sharpley and Menzel, 1987~. The bioavailability of phosphorus in sediments as measured by a variety of methods usually does not exceed 60 percent of the total phosphorus in the sediment (Sonzogniiet al., 1982)
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From page 288... ...
Phosphorus associated with sediments may remain a problem years after excess phosphorus inputs cease. Lake Trummen in Sweden experienced nuisance algal blooms 10 years after nutrient inputs were reduced.
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From page 289... ...
Total phosphorus can serve as a useful proxy for total bioavailable phosphorus, and reductions in total phosphorus loadings should be the goal of phosphorus control programs. PlIOSPHORUS IN THE SOIL-CROP SYSTEM Like nitrogen and other plant nutrients, the phosphorus added to the soil-crop system goes through a series of transformations as it cycles through plants, animals, microbes, soil organic matter, and the soil mineral fraction.
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From page 290... ...
Phosphorus is relatively enriched in finer soil fractions, so it is perhaps expected that phosphorus adsorption is correlated with the clay content of soils (Solis and Torrent, 1989b) and with the soil surface area (Olsen and Watanabe, 1957~.
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From page 291... ...
Mass Balance Phosphorus is added to croplands in crop residues and manures, in synthetic fertilizers, and from phosphorus-bearing minerals in the soil (Figure 7-1~. Part of the phosphorus entering the soil-crop system is removed with the harvested crop; the balance is immobilized in the soil, incorporated into soil organic matter, or lost in surface or subsurface flows to surface water or groundwater.
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From page 292... ...
A more detailed analysis of phosphorus inputs and outputs to croplands helps to identify opportunities for reducing phosphorus loadings to surface water from farming systems. Phosphorus Inputs The phosphorus in fertilizer-P is the single most important source of phosphorus added to croplands in the United States (Table 7-1~.
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From page 293... ...
Phosphorus Outputs The fraction of total phosphorus inputs lost to erosion and runoff can be substantial, but it is difficult to estimate the amount. Larson and colleagues (1983)
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From page 294... ...
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From page 295... ...
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From page 296... ...
296 I Soil and Water Quality: An Agenda for Agriculture TABLE 7-3 State and National Phosphorus Inputs and Outputs as Percentage of Total Mass of Phosphorus Inputs Percentage of Total Input Mass Inputs Outputs Recoverable Crop Harvested Crop Fertilizer-P Manure-P Residues Crop Residues Balance Alabama 83 161 121 86 Alaska 99 01 91 90 Arizona 75 242 112 87 Arkansas 69 239 429 49 California 76 222 132 84 Colorado 38 5112 6012 28 Connecticut 61 39<1 7<1 92 Delaware 61 354 194 76 Florida 94 6<1 3<1 97 Georgia 83 152 132 85 Hawaii 91 90 <10 99 Idaho 85 114 284 68 Illinois 86 59 379 54 Indiana 87 67 287 65 Iowa 76 1311 4411 45 Kansas 69 239 389 54 Kentucky 89 83 193 78 Louisiana 90 46 326 62 Maine 74 251 141 85 Maryland 76 214 184 78 Massachusetts 75 25<1 9<1 90 Michigan 86 104 204 75 Minnesota 80 128 368 56 Mississippi 82 135 315 63 Missouri 82 116 366 57 Montana 86 77 457 47 Nebraska 65 2113 5113 35 Nevada 84 151 351 64 New Hampshire 56 44<1 18<1 82 New Jersey 93 52 132 85 New Mexico 56 395 285 67 New York 66 312 202 77 North Carolina 82 163 153 82 North Dakota 91 37 357 58 Ohio 85 96 306 64 Oklahoma 88 93 233 74 Oregon 85 114 324 63 Pennsylvania 56 404 274 69 Rhode Island 100 0<1 6<1 94 South Carolina 87 103 183 78 South Dakota 74 1611 5411 35
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From page 297... ...
Since the solution phosphorus concentration is particularly important regarding potential water quality effects, an increase in EPC because of the increasing phosphorus content of the soil is an undesirable situation with regard to water quality. EPC increases with increasing phosphorus additions, regardless of the source of the added phosphorus.
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From page 298... ...
. Phosphorus levels in soil were measured, using the Mehlich 1-extractable soil phosphorus method, after 1 year of equilibration; each symbol is the average of 15 observations; solid symbols were not used in the prediction equation.
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From page 299... ...
The proportion of harvested phosphorus ranged from 34.4 to 39.8 percent of phosphorus inputs from fertilizer, seed, and precipitation. Management of phosphorus inputs for the prevention of unnecessary buildup of soil phosphorus levels should be part of programs to reduce phosphorus loadings to surface water.
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From page 300... ...
, but particulate phosphorus can be a long-term source of phosphorus once the particulate is delivered to surface water. Soluble Phosphorus Losses Soluble phosphorus losses are greater from pasturelands than fromcroplands (Baker et al., 1978~.
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From page 301... ...
As with soluble phosphorus, particulate phosphorus losses also increase with increasing fertilizer additions, with sediment-extractable phosphorus increasing approximately linearly with the fertilization rate (Sharpley, 1981~. Stream-suspended sediments in agricultural watersheds derive mainly from surface soils rather than from stream bank erosion on the basis of mineralogical and other characteristics (Wall and Wilding, 1976~.
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From page 302... ...
Phosphorus management to reduce unnecessarily high soil-P levels should be part of efforts to reduce phosphorus losses from cropping systems. Phosphorus applications in excess of that harvested in the crop leads to the buildup of soil-P.
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From page 303... ...
In that study, the clay content and surface area of soil were the variables that best improved phosphorus soil test yield predictions. As noted above, the clay content and surface area of soil are correlated with phosphorus adsorption capacity.
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From page 304... ...
have shown that corn and soybeans grown on Piedmont and Coastal Plain soils testing high in available phosphorus do not respond to phosphorus fertilizer additions. On the basis of the soil test data presented in Table 7-4, no response to phosphorus would be expected on approximately half of the soils in the southeastern United States.
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From page 305... ...
1990. Soil test summaries: Phosphorus, potassium, and pH.
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From page 306... ...
American Society for Agronomy, Crop Science Society of America, and Soil Science Societ,v of America.
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From page 307... ...
Despite weaknesses in the ability to predict crop responses to phosphorus applications, most states have soil test procedures that, although not perfect, can be used to establish the threshold levels of soil-P beyond which no crop response is predicted. Given the importance of reducing soil-P levels for reducing phosphorus losses to surface waters, such thresholds should be established.
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From page 308... ...
American Society for Agronomy, Crop Science Society of America, and Soil Science Society of America.
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From page 309... ...
Reduced-tiDage systems do, however, appear to increase the proportion of bioavailable phosphorus lost in soluble form. The effectiveness of reduced-tillage systems in reducing bioavailable phosphorus losses would be increased if parallel efforts were undertaken to reduce phosphorus concentrations in surface soils.
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From page 310... ...
Wetlands have been proposed to act as nutrient filters, but they may be only small sinks for nitrogen and phosphorus and may, in fact, be net exporters of some nutrients (Peverly, 1982~. Buffer strips and protection of riparian areas should help to reduce phosphorus loadings to surface waters.
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From page 311... ...
New Cropping Systems Immediate gains in reducing phosphorus loads to surface waters can be accomplished by simultaneous efforts to reduce soil-P levels, erosion, and runoff from current cropping systems. These improvements can be increased by efforts to incorporate buffer strips and protect riparian vegetation to trap phosphorus in the sediments and runoff from current cropping systems.
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