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IV. Phosphorous Management for Poultry Wastes

IV. Phosphorous Management for Poultry Wastes

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36



J. T. SIMS AND D. C. WOLF



search on the fate of P from poultry manures in soils; management programs for

P are discussed in more detail in Section VI.



A. PHOSPHOROUS

CONCENTRATION

AND FORM IN SOILS

AMENDEDWITH POULTRY

WASTES

Phosphorus levels in soils amended with animal manures for many years are

commonly well in excess of the critical values used by soil testing programs to

identify soils that will respond to fertilizer P. Soil test summaries can provide a

broad view of the impact of animal-based agriculture, or any other crop production system, on soil test P, as described in Section I. Because soil test results are

normally accompanied by geographic locations and crop management information, the magnitude and distribution of soils that are excessive in P are becoming

more apparent. A recent survey of four regional soil testing committees representing 34 states was conducted by Sims (1993) and found that the major environmental issue related to soil P was animal waste management, and poultry

waste management in particular. Soil test extractants do not measure, however,

the amount of total soil P, the distribution of soil P between organic and inorganic forms, or the biological availablility of soil P. Indeed it is fair to state that

from the point of view of soil testing programs, the process of measurement and

interpretation of P for environmental purposes is in its infancy. In fact, until

fairly recently many soil testing laboratories did not measure the actual value of

soil P once a defined “very high” value was reached. Actual P values were not

determined because there was no need from a fertilizer recommendation standpoint and because of the time and expense of the additional dilutions and laboratory analyses that would have been required on many samples. Although researchers have measured actual values and studied the fate of P in heavily

manured soils, it is only recently that new instrumentation (as with heavy metals

and pesticides) has made routine measurement of actual soil test P values possible. As an example, the University of Delaware Soil Testing Program only

began measuring and reporting actual soil test P values to farmers in 1991; prior

to that the highest value reported was 100 mg Plkg (by the Mehlich 1 soil test,

0.05 N HCl

0.025 N H,SO,). Results of the first annual soil test summary

after initiation of this procedure showed that, on a statewide basis, the percentages of samples from commercial cropland rated as low, medium, high, and

excessive in P were 11, 21, 28, and 40%,respectively. In Sussex County, site of

the poultry industry, 29% were rated as high and 48% as excessive. Of the

samples from Sussex County in the excessive category, 63% ranged from 67 to

134 mg Plkg, 30% from 135 to 268 mg P/kg, and 7% exceeded 268 mg Plkg.

Of the samples that indicated manure had been applied, 66% were in the excessive range. The critical value for soil test P in Delaware is 35 mg Plkg; beyond



+



POULTRY WASTE MANAGEMENT



37



this point no fertilizer or manure P is recommended, with the possible exception

of a small amount of starter fertilizer placed in a band at planting. Clearly a large

percentage of the cropland in the state and county can be considered nonresponsive to P. Further, the Delmarva peninsula is bordered on the east by the Delaware Bay and a national estuary, the Inland Bays of Delaware, and on the west

by the Chesapeake Bay, water bodies that are highly sensitive to eutrophication.

In this instance the information available from soil testing programs and the

continued inputs of P in poultry manure/litter certainly suggest the need for an

environmental P management program.

Unfortunately, it is rare to find long-term studies that document the rate of accumulation of P in soils amended with any type of manure/litter or the fate of

the added P. It is even more unusual to find studies that determine the amount of

time required for normal crop uptake to reduce soil P to a “medium” or “low”

soil test value. Sharpley et al. (1984) reported that the application of 67 Mg/ha/

year of beef feedlot manure for 8 years to a Pullman clay loam (Torretic Paleustolls) used for continuous grain sorghum production increased total P in the surface 30 cm from 353 mg/kg in an untreated check plot to 996 mg/kg. Available

0.025 N HCI) increased from 15 to 230 mg P/kg;

P (Bray PI, 0.03 N NH,F

critical values for Bray P (breakpoint between medium and high) typically range

from 30 to 50 mg P/kg. Meek er al. (1982) found that application of feedlot

manure to a Holtville soil (Typic Torrifluvents) used for a variety of crops (sorghum, lettuce, barley) for 3 years at 90 Mg/ha/year increased Olsen P (0.5 N

NaHCO,, pH 8.5) from 9 mg/kg to 68 mg/kg. Olsen P values 5 years after

cessation of manure applications and continued cropping were 65 mg P/kg. Critical values for Olsen P normally range from 20 to 30 mg P/kg. Similar long-term

studies with poultry manure/litter are uncommon. Robertson and Wolford (1970)

reported that the application of 26 Mg/ha/year (wet weight basis) of poultry

(layer) manure for 5 years to a Breckenridge sandy loam resulted in yields

equivalent to those due to commercial fertilizer and increased soil test P (Bray

P1) from 50 mg P/kg in a check plot to 147 mg P/kg; 52 Mg/ha/year of manure

increased soil test P to 189 mg P/kg. Mitchell etal. (1992) reported that soil test

P (Mehlich 1) in the surface horizon of an Esto loamy sand soil that had received

broiler litter for 20 years at about 7 Mg/ha/year was 180 mg P/kg, more than

seven times the critical value used in Alabama. Soil test P in a nearby pasture

area was less than 10 mg P/kg. Poultry manure applications at recommended

rates can markedly increase soil test P levels even in the short term. Sims et al.

( 1991) conducted on-farm evaluations of best management practices for broiler

litter on eight farms in southern Delaware. Applications of broiler litter at recommended rates for corn, wheat, and soybeans increased soil test P (Mehlich 1)

levels by from 38 to 121 mg P/ kg in 2 years (Table VI), relative to an unamended

control soil.

The agricultural significance of soils with extremely high soil test P values



+



+



38



J. T. SIMS AND D. C. WOLF

Table VI



Increase in Soil Test P in Two Years from Applications of Broiler Litter at Recommended

Rates for Corn, Wheat, and Soybeans"



Crop rotation



Soil series



Irrigated cornlwheatlsoybeans

Sorghumlsorghum

Full-season soybeansldry land corn

Full-season soybeanslsoghum

Irrigated cornlwheatlsoybeans

No-till soybeanslirrigated corn

No-till soybeanslfull-season soybeans



Evesboro loamy sand

Rumford loamy sand

Kenansville loamy sand

Kenansville loamy sand

Evesboro loamy sand

Evesboro loamy sand

Evesboro loamy sand



P added

in

broiler

Soil test P

litter

(mglkg)

(kglhal

rotation) Initial Final

550

234

283

337

514

373

337



148

144



217

200



153

99

150



227

218

241

202

246

185

174



"Adapted from Sims er al. (1991). Soil test extractant was Mehlich 1 (0.025 N H I S 0 4 + 0.05

N HCI). In Delaware, a soil test P value of 35 mglkg is considered high and no fertilizer P is

recommended except for a low rate of banded starter fertilizer for certain crops.



relates primarily to the length of time required to deplete these soils back to

responsive levels. Studies on the long-term P-supplying capacity of high-P soils

are available, but are usually limited to only a few soil types or cropping systems.

In general, however, these studies show that it can take years or even decades to

reduce soil test P levels in high-P soils back to levels that could be characterized

as medium. For example, McCollum (1991) reported the results of a 30-year

field study conducted on a Portsmouth fine sandy loam (Typic Umbraquult) used

for corn and soybean production. The soil had an initial soil test (Mehlich 1)

level of -100 mg/kg; it required 16 years of continuous cropping to decrease

soil test P to the critical level of 20 mg P/kg. As stated earlier, soils with equivalent or much greater values of soil test P than the Portsmouth soil are common

on the Delmarva peninsula and other areas where poultry wastes are applied

annually. In a slightly different study, McCallister et al. (1987) applied 0, 1 1,

22, or 33 kg P/ha to a Sharpsburg silty clay loam (Typic Argiudolls) and grew

irrigated corn for 12 years. Despite excellent grain yields (12-year average of

9.9 Mg/ha), soil test P (Bray P1) levels in the check plots declined only slightly,

from approximately 15 to 8 mg P/kg; addition of 33 kg P/ha/year increased soil

test P to about 30 mg P/kg after 12 years. The authors attributed the long-term

P-supplying capacity of the Sharpsburg soil to rapidly available pools of Fe- and

Al-bound P in the surface horizon and to subsoil reserves of P.

As mentioned above, soil testing results can identify an accumulation of P to



POULTRY WASTE MANAGEMENT



39



excessive levels in soils and perhaps provide a general indication of their longterm supplying capacity for P. Soil tests, however, provide little information on

the forms of P present in soils. Sequential fractionation methods can partition

soil P into differentially soluble pools that can then be correlated to soil properties for use in management programs, as illustrated by the study of McCallister

et al. (1987). Several other examples illustrate the value of understanding the

form of P in waste-amended soils. McCoy et al. (1986) fractionated P in a Sassafras sandy loam soil amended with 100 kg P/ha in sludge compost and found

that approximately 82% of soil P was in the Al- or Fe-bound form, 3% was found

as Ca-P, and 15% as residual, undefined forms of P. The poor plant availability

of P in the compost was attributed to the sludge treatment process that used A1

and Fe to precipitate P as insoluble compounds. Sharpley et al. (1984) reported

that the long-term effect of adding feedlot manure to a Pullman clay loam was to

increase the relative amount of total inorganic P, not organic P. Application of

67 Mg/ha/year for 8 years increased total inorganic P from about 180 mg P/kg

in a check plot to 900 mg P/kg; total organic P increased from approximately

200 to 425 mg P/kg. The authors stated that although feedlot waste is regarded

as an organic soil amendment, most (78%) of the P in the waste was in the

inorganic fraction, hence the greater increase in soil inorganic P with time. The

increase in available P (Bray P) in this soil paralleled the increase in total inorganic P, suggesting that the inorganic forms of feedlot waste may be more important for plant P nutrition and/or that organic P is rapidly mineralized and

converted to inorganic P. Other authors have reported similar increases in inorganic P when organic wastes are added to soils (Chang et al., 1983; Sims, 1992).

From an environmental point of view, fractionation of soil and sediment P has

been used to characterize the biological availability of P to algae, a primary

factor in the eutrophication process. Sonzogni et al. (1982) suggested that bioavailable P is primarily a combination of dissolved inorganic P and nonapatite,

inorganic, particulate P (P adsorbed by Fe and A1 oxides). They cited several

fractionation schemes that could be used to characterize biologically available P

in soils and sediments and stated that extraction of soils with 0.1 N NaOH at a

wide solution: soil ratio (500: 1 to 1000: 1) for 17 hours could be used for a rapid

estimation of biologically available P. Vaithiyanathan and Correll (1992) reported that the discharge patterns of P from an Atlantic Coastal Plain watershed

into a nearby river were related to the forms of soil P present in the watershed.

Over 75% of the total P in agricultural soils in this watershed was found as

inorganic P; 98% of this P was occluded and nonoccluded Fe phosphates. They

also reported that 94% of the P exported from the agricultural fields was found

as particulate P; hence the biological availability of Fe-bound P in transported

sediments is likely to have a major effect on eutrophication of downstream lakes.

Previous work by Dorich et al. (1985) that related P sequentially extracted by

NH,F (ALP), NaOH (Fe-P), and HCl (Ca-P) to algal availability of P under



40



J. T. SIMS AND D. C. WOLF



laboratory conditions provides supportive evidence for the importance of Febound P. Their studies showed that NaOH-extractable P was highly correlated

with algal uptake of P during a 14-day laboratory incubation ( r = 0.95). Studies

on the distribution of P among various inorganic and organic fractions in soils

amended with poultry wastes are rare, however. The studies described here

strongly suggest that information on the distribution and biological availability

of P could be an important component of an environmental management program

for P in agricultural areas dominated by the poultry industry.



B. PHOSPHOROUS

RETENTION

AND MOVEMENT

INSOILS

AMENDEDWITH POULTRY

WASTES

Accumulations of P to such high levels in the surface horizons of agriculture

soils raise two other questions of environmental importance. First, what is the

capacity of these soils to adsorb the additional P that may be added in manures,

litters, and fertilizers? And second, what is the nature and magnitude of P loss

from these soils by erosion, runoff, drainage, and leaching to groundwaters?

Studies on these issues with poultry manure/litter are available, but uncommon.

More research is available with other somewhat similar organic wastes (e.g.,

dairy manure, sewage sludges). Results from several studies will be reviewed

and used to describe our current understanding of the mechanisms involved and

the type of management practices needed to reduce P loss and transport to sensitive surface waters.

Phosphorus is retained in soils by a number of different mechanisms, collectively referred to as “P fixation”; it can also be immobilized in an organic form

if the C :P ratio of an added organic material is high, normally >300: 1. A

number of excellent review articles are available on the fast and slow processes

involved in the removal of P from solution (adsorption, precipitation) and the

factors controlling its reversion to a soluble form (desorption, mineral dissolution) (Barrow, 1980; Fixen and Grove, 1991; Olsen and Khasawneh, 1980;

Sample et al., 1980; Sanchez and Uehara, 1980; Sharpley and Halvorson,

1994). Sample et al. (1980) stated that the primary soil constituents involved in

P retention were the hydrous oxides of Fe and Al, the alumino-silicate minerals,

soil carbonates, and soil organic matter. Oxides of Fe and A1 are of greater

importance in the more weathered soils of humid regions; in areas of low rainfall

soil carbonates have a greater influence on P retention. Sample et al. (1980) also

described the mechanisms involved and the techniques used to study P fixation,

and stated that this process was a “continuum embodying precipitation, chemisorption, and adsorption, if the processes are viewed throughout the entire zone

of soil influenced by a fertilizer application and through a time span encompass-



POULTRY WASTE MANAGEMENT



41



ing an entire growing season or longer.” Precipitation was defined as the formation of discrete, insoluble mineral forms of Al-P, Ca-P, or Fe-P. Adsorption

was described according to the approach of Bache (1964) and Muljadi et al.

(1966) as having distinct stages related to the energetics of the chemical reaction

and the nature of the reactive site. Uehara and Gillman (1981) attributed differences in P adsorption among soils to variations in the specific surface area and

reactivity of soil colloids and their capacity to occlude P. Fixen and Grove (199 1)

characterized P bonding mechanisms on soil colloids as ligand exchange reactions in which phosphate replaces aquo and hydroxyl groups on oxide surfaces,

forming monodentate, bidentate, or binuclear bonds of progressively decreasing

reversibility. The authors also described the process of P desorption and the

hysteresis (lack of complete reversibility) commonly observed following P fixation. Various mechanisms have been proposed to explain hysteresis. Among

them are precipitation, occlusion within newly formed precipitates of Fe/AI hydrous oxides, and solid-state diffusion. Regardless of the process involved, it is

clear that reversion of P to a less desorbable form increases with time after application. The long-term conversion of soluble P to forms that are much more

slowly available has implications for plant P uptake and P desorption into runoff,

drainage, or leaching waters. It also implies that the contribution of P to eutrophication is usually best controlled by reducing particulate transport, as P will

primarily be found in soils in precipitated or adsorbed forms.

The role of organic matter in the retention and release of P will be of particular

importance in manured soils. Although the organic fraction (e.g., humus) is not

thought to have a major capacity to adsorb P directly, metal-organic matter (OM)

complexes (e.g., Al-OM, Ca-OM, Fe-OM) that form in soils amended with

organic wastes can play a much greater role. Organic matter has been shown to

have other effects on P fixation. For instance, organic acids were shown to compete for the same adsorption sites as phosphate anions and reduce the capacity of

soil minerals to retain P (Nagarajah et af., 1970). Solubilized organic matter may

also be redistributed to new sites in the soil where it can coat soil minerals and

reduce their importance in P fixation. Sharpley and Halvorson (1994) reviewed

P transport in agricultural runoff and emphasized the need for more research on

the biological availability of soluble and particulate organic P in runoff from

manured soils or soils with large amounts of crop residues. Subsurface transport

of P in artificially drained soils can also be affected by organic matter. Anaerobic

decomposition of organic matter can reduce Fe oxides, resulting in the release

of adsorbed P into drainage waters (Mitsch and Gosselink, 1986; Ponnamperuma, 1972). Much of the research on the role of organic matter as a source of P

and as a soil constituent that can affect P solubility and movement has been

conducted in organic soils. Further research is needed on the mineralization,

fixation, and desorption of P in soils amended with poultry manures and litters.



42



J. T. SIMS AND D. C. WOLF



1. Phosphorous Adsorption and Desorption

Amending soils with manures, litters, or other organic wastes has been shown

to affect the adsorption-desorption process for P. These processes are normally

studied by the use of adsorption isotherms that relate the amount of P added to a

soil to the concentration of P in solution after an equilibration period (usually 24

hours). Adsorption isotherms do not provide information on the mechanisms of

P retention, and, when conducted with whole soils, only indirectly indicate the

soil constituents involved in P retention. They do, however, provide reasonable

estimates of the potential for a soil to retain additional P and are useful for comparing the effects of management practices (tillage, manuring) or soil properties

(horizonation, texture, clay, AllFe oxides, etc.) on P adsorption. The Langmuir

equation is the most frequently used approach to estimate the “adsorption maxima” for soils:

= (1



kbC

kC)



+



where Q is the amount of P adsorbed per unit weight of soil, C is the equilibrium

concentration of P in solution, b is the maximum amount of P that can be adsorbed, and k is a constant presumed to represent the energy of bonding of P to

the surface of the solid phase.

Adsorption isotherms can also be used to determine the equilibrium concentration of P at the point of zero sorption (EPC,), as illustrated in Fig. 1 1. White

and Beckett (1964) suggested that the EPC, value provides an indication of the

potential of a soil or sediment to gain or lose P when placed in contact with

natural waters. If EPC, values exceed ambient concentrations of P in a stream or

lake (typically 0.01 to 0.1 mg Plliter) the soil or sediment would tend to desorb

P into solution, increasing the potential for eutrophication.

The adsorption and extractability of P in a Hayesville loam (Typic Hapludults)

amended with anaerobically digested poultry manure were examined by Field et

al. (1985). No significant effect of manure effluent on P adsorption maxima was

found, even at extremely high effluent rates (1000 mg Nlkg soil). Soil test extractable P (Mehlich 1) was linearly related to the rate of P addition, but decreased by approximately 52% after a 90-day incubation, suggesting that a rapid

process for the fixation of available P existed in this oxidic soil. Reddy et al.

(1980b) investigated the effect of manure (beef, poultry, swine) loading rate on

P adsorption and desorption in two soils, a Norfolk loamy sand (Typic Paleudults) and a Cecil sandy loam (Typic Hapludults). Poultry manure applied at a

P loading rate of 178 mglkg increased soluble P from 0.2 to 9.8 mg/kg and

Mehlich 1-extractable P from 49 to 214 mg/kg. Desorbable P measured by four

1-hour extractions was increased from 1.5 to 39.0 mglkg and the EPC, from

0.06 to 7.8 mg Plliter. The authors also measured changes in P adsorption



43



POULTRY WASTE MANAGEMENT



.-



0



2



4



6



0



10



12



14



16



Equilibrium P Concentration (mg/L)

Figure 11 Example of a P adsorption isotherm for high-P soil, illustrating the EPC, concept

(hypothetical data).



maxima with depth in the Norfolk soil following 5 years of application of swine

lagoon effluent. The EPC, values at a depth of 0-15 cm were 0.01, 1.7, 4.1,

and 22.0 for the Norfolk loamy sand and 0.01, 0.03, 0.16, and 0.88 mg P/liter

for the Cecil sandy loam for annual P loading rates of 0, 81, 161, and 322 kg P/

ha. Phosphorus adsorption maxima, as estimated by the Langmuir equation or a

single-point sorption isotherm (1000 mg/kg P), were also decreased by application of swine manure. The amounts of P adsorbed from the 1000 mg P/kg addition were, for the rates of 81, 161, and 322 kg P/ha, 58, 55, and 18 mg/kg in

the Norfolk soil and 220, 190, and 76 mg P/kg in the Cecil soil. The authors

attributed the decreased sorption at high manure rates to saturation of P sorption

sites on A1 and Fe oxides by organic anions from manure mineralization. Singh

and Jones ( 1976) had previously reported a similar phenomenon when poultry

manure and other organic residues were added to a Mission silt loam (Typic

Vitrandept). In that study P sorption at equilibrium concentrations of 0.1 and

1.0 mg P/liter were approximately 100 and 600 mg P/kg in the untreated soil

and 50 and 275 mg P/kg in the poultry manure-amended soil. Desorption of P

was also greater in soils amended with poultry manure; for equivalent adsorption

values of 300 mg P/kg, 0.01 M CaCl, soluble P concentrations after 150 days of

incubation were approximately 0.1 1 and 0.69 mg P/liter for the check and poultry manure-amended soils, respectively. As noted by Reddy et al. (1980a), incubation of poultry manure decreased the P sorption capacity of the Mission soil

with time. Amounts of P sorbed at an equilibrium concentration of 1.O mg P/



44



J. T. SIMS AND D. C. WOLF



liter were approximately 525 mg/kg in the check and 300 mg P/kg in the poultry

manure-amended soil after a 150-day incubation period. Mozaffari and Sims

(1994) compared P adsorption maxima from the profiles of four soils that had

received broiler litter and P fertilizers on a regular basis for years with the

maxima from border areas separating these fields from drainage ditches (Fig. 12).

Phosphorus sorption maxima estimated from the Langmuir equation ranged

from 95 to 2564 mg/kg in cultivated soils and from 200 to 2000 mg/kg in field

border areas. Two clear trends were observed for P sorption. First, P sorption

was consistently greater in subsoils and was highly correlated with clay content

( r = 0.90). Second, when clay contents were similar, P sorption was usually

greater in field border areas than in cultivated fields, particularly in the upper

40 cm, suggesting that previous cultural practices (fertilization, manuring, liming) may have reduced the capacity of cultivated areas to retain additional P, as

has been seen in other studies (Barrow, 1974; Fox and Kamprath, 1970; Guertal

et af., 1991; Reddy ef al., 1980b).

Adsorption and desorption data have normally been used in the United States

to provide general estimates of the suitability of a site for continued P applications. However, in some countries where animal-based agriculture has resulted

in soils high enough in P to be a threat to groundwater and surface waters, more

stringent approaches have been taken. In the Netherlands, where 43% of the

grassland and 82% of the maizeland in areas with a manure surplus were esti-



-10



-20



A



-30

h



E



-40



~

~



0



W



5

5

a



2

n

Q)



-50

-60



-70



- Agricultural Field

Field Border



-80

-90

I



100



200



300



400



.



l



500



,



,



600



,



700



~



,



800



~



I



. ,



900



l/



,.



Il



,.



l



,



1000 1100 1200



P Adsorption Maxima (mg/kg)

Figure 12 Differences, with depth, in P adsorption maxima in agricultural fields and field border

areas for two high-P soils (Ev Is, Evesboro loamy sand; Sa sl, Sassafras sandy loam) that routinely

received poultry manure and fertilizer P.Adapted from Mozaffari and Sims (1994).



POULTRY WASTE MANAGEMENT



45



mated to be saturated with respect to P, regulations have established a critical

limit for the concentration of orthophosphate in groundwaters at 0. I mglliter

(Breeuwsma and Silva, 1992). Associated with this has been the definition of a

"critical degree of P accumulation in soils," defined as 25% of the P saturation

capacity of the soil, which is calculated as follows:

DPS = Pa,,/ PSC, x 100

where DPS is the degree of phosphorus saturation (%); P,, is the oxalate extractable P content, surface area basis; PSC, is the phosphate sorption capacity, surface area basis. Phosphate sorption capacity is not determined directly from the

Langmuir equation, but is calculated from an empirical equation developed by

Schoumanns et al. (1987) that combines laboratory data and constants obtained

from other adsorption equations (Freundlich and Elovich):

PSC, = {[S, (1



+ a In t)(c/c,)"] + P,,}Td7.1



where S, is the phosphate adsorbed at a reference concentration and time (e.g.,

50 mg P/liter for 24 hours), in mmol/kg; a is a dimensionless constant from the

Elovich equation; t is the reaction time (days); c, is the reference concentration

of phosphate (mg/liter); c is the phosphate concentration (mg P/liter) in solution

at equilibrium; n is a dimensionless constant from the Freundlich equation; Po,

is the oxalate-extractable P (mmol/kg); T is the thickness of soil layer considered

(depth in cm); d is the soil bulk density (g/cm3); and 7.1 is the factor that converts mmol/kg to kg P,O,/ha. In essence, this equation uses data from two rather

simple laboratory measurements (oxalate-extractable P and results of a singlepoint adsorption isotherm) to identify soils that have become sufficiently saturated with P to pose a threat to groundwaters.



2. Phosphorous Losses by Erosion, Runoff, and Leaching

Phosphorus becomes an environmental problem only when it is transported to

a surface water sensitive to eutrophication. Because of the affinity of P for soil

colloids, the dominant process involved in P transport from most agricultural

soils is erosion. The loss of soluble P in runoff, drainage waters, and groundwater discharge is normally an issue only in soils that have become excessive in

P. The most common situation where significant losses of soluble P by processes

other than erosion have been reported has been in soils amended with animal

wastes. Management practices to control losses of sediment-bound and soluble

P differ conceptually and practically and are discussed in Section VI. This section focuses on the processes involved in P transport to surface waters.

Erosion can be defined as the transport of soil from a field in water or wind;

runoff is water that runs off a soil surface instead of infiltrating. Studies have

shown that smaller, lighter soil particles, such as clays and humus, are prefer-



46



J. T. SIMS AND D. C. WOLF



entially transported in erosion and runoff. These particles have also been shown

to be enriched in P relative to the whole soil from which they were transported.

Other studies have shown that, from an environmental perspective, the biological

availability of soluble P and particulate P transported in erosion and runoff to

aquatic organisms is perhaps of greater importance than the total P load to a

water body. Sharpley et a / . (1992) stated that the biological availability of P is a

“dynamic function of physical and chemical processes controlling both soluble

P and biologically available particulate P.” They stated that key processes regulating soluble P transport include desorption-dissolution (release of P from vegetation and decaying organic residues); for particulate P, physical processes that

regulate the size and nature of particles transported and their chemical reactivity

for P were of more importance.

A large body of literature is available on the loss of P from agricultural fields

due to erosion and runoff. The management of transport processes for bioavailable P has been reviewed recently by Sharpley and Halvorson (1994). However,

limited research as been conducted directly investigating P losses from manured

fields. Three recent studies illustrate the nature of this problem. Mueller et al.

(1984) examined the effect of tillage on P losses when dairy manure was applied

to a Dresden silt loam soil (Mollic Hapludolls). Total P, dissolved molybdatereactive P, and algal available P (estimated by resin extraction of unfiltered runoff sample) were measured in runoff under three tillage systems (conventional,

chisel plow, and no-till) with and without the application of dairy manure at

the rate of 8 Mg/ha (dry weight basis). Results of this study, summarized in

Table VII, illustrate some of the difficulties in controlling P losses from manures



Table VII

Influence of Tillage and Dairy Manure Application on Runoff Losses of Total

and Algal-Available P from a Dresden Silt Loam”

Total P



Algal-available P



Treatment



Concentration

(mglliter)



Loss

(g/m2)



Concentration

(mg/liter)



Loss

(g/m2)



Conventional ( - DM)

Conventional ( + DM)

Chisel plow ( - DM)

Chisel plow ( + DM)

No tillage ( - DM)

No tillage ( DM)



3.6a

3.4a

2.2b

1.7bc

0.7d

1.5c



237a

158ab

92bc

I Od

71cd

133bc



0.73ab

0.76ab

0.49b

0.75ab

0.24~

1.14a



52b

39bc

20cd

7e

24cd

98a



+



aFrom Mueller er al. (1984). Means within a column followed by the same letter

are not significantly different at P = 0.01,

bDM, Dairy manure.



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IV. Phosphorous Management for Poultry Wastes

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