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PART III. PHOSPHORUS FORMS AND LABILITY

PART III. PHOSPHORUS FORMS AND LABILITY

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In: Environmental Chemistry of Animal Manure

Editor: Zhongqi He



ISBN 978-1-61209-222-5

© 2011 Nova Science Publishers, Inc.



Chapter 10



SOLUBILITY OF MANURE PHOSPHORUS

CHARACTERIZED BY SELECTIVE

AND SEQUENTIAL EXTRACTIONS

John D. Toth1,*, Zhengxia Dou1 and Zhongqi He2

10.1. INTRODUCTION

Phosphorus (P) availability is governed by the P in soil or water that is made available by

desorption and dissolution processes for uptake by plants in terrestrial and aquatic ecosystems

(Sharpley, 2000). Solubility of manure P is a critical factor in evaluating manure‘s nutrient

value in agriculture and its role in eutrophication of surface waters. Kuo (1996) pointed out

that the quantity of labile P, the concentration of P in soil solution, as well as P buffering

capacity affecting the distribution of P between the solution and solid phases, are the primary

factors characterizing soil P availability. To evaluate P availability in soils, numerous soil

tests have been developed which extract varying amounts of P, depending on the types of

extractants used. The extractants include, but are not limited to, water or buffered salt

solutions, anion exchange resins, and diluted acids or buffered alkaline solutions with or

without a complexing agent (Kuo, 1996). Whereas the principles of these soil P extractions

could be applied to manure P research, the different physico-chemical properties of animal

manure should be recognized (He et al., 2003). For example, He et al. (2006a) observed that

the sequentially-extracted HCl fraction of animal manure, especially poultry litter/manure,

contained a large portion of organic P (Po) which would not have been measured by a soilbased protocol.

This chapter reviews solubility of P from food animal manures (in general, swine, dairy,

beef and poultry) and manure products (those undergoing further processing such as storage,

composting, pelletizing, etc.) determined through individual and sequential extractions with

*



Corresponding author – jdtoth@vet.upenn.edu

University of Pennsylvania, School of Veterinary Medicine, Kennett Square, PA 19348

2

USDA-ARS, New England Plant, Soil and Water Laboratory, Orono, ME 04469, USA

1



228



John D. Toth, Zhengxia Dou and Zhongqi He



the emphasis on methodological variations. Three broad classes of extraction methods, water,

other simple individual extractants, and sequential fractionation, are covered, followed by

examples of research on characterization of P forms in different types of animal manures,

effects of manure handling, processing, use of chemical amendments to bind P, and dietary

manipulation to reduce soluble P excretion.



10.2. WATER-EXTRACTABLE PHOSPHORUS

Water-extractable soil P has been used for several decades as a predictor of the potential

for losses of P in runoff from surface soil horizons (Pote et al., 1996; 1999) and in leachate

from soils enriched in P (Koopmans et al., 2006). Pote et al. (1996; 1999) found a high degree

of correlation between water-extractable P (WEP) and the biologically active P fractions

(r2=0.82 to 0.93) extracted from the 0-2 cm soil depth in four Ultisols. Water-extractable P

was as good a predictor of dissolved reactive P and bioavailable P as other standard soil test P

methods and iron oxide paper strips. More recently, WEP has become a critical part of

Phosphorus Indices. The concept was proposed by Lemunyon and Gilbert (1993) as a

phosphorus management tool to identify areas and practices enhancing the risk of water

pollution due to P movement off-farm. The P Index uses field, soil type, topographic,

background soil P level, and P application rate and method parameters divided into ―source‖

and ―transport‖ categories to rank a given field for P loss vulnerability. The P Index has been

developed into a flexible and widely applicable tool for nutrient management planning, and

has now been adapted for use in some form by most states in the US (Sharpley et al., 2003).

As a component of the P Index, WEP as a function of manure or manure product type,

management and application method is currently being employed to develop P source

coefficients (Elliott et al., 2006; Shober and Sims, 2007), which are weighting factors related

to potential for P loss in runoff and may differ depending on locally- or regionally-specific

manure types, application methods and farm management decisions.



10.2.1. Methodology

A factor complicating WEP use in nutrient management planning is the diversity of

laboratory handling and analytical methods and the potential difficulty of comparing results

from different studies (Vadas and Kleinman, 2006). Across-study variability is found in use

of fresh or dried manure material, sample:extractant ratio, length of extraction (shaking),

filtration method, and choice of analytical instruments (Table 10.1). Self-Davis and Miller

(2000) proposed as a standard method a 20:200 ratio between the poultry manure (g wet

basis:mL) in their study and the water extractant, with 2 hr shaking, while Sharpley and

Moyer (2000), based on analyses of manures and manure composts suggested a 1:200 g:mL

sample dry weight:extractant ratio and 1 hr shaking. Several recent studies have examined

WEP methodology issues with the goal of a widely-applicable standard procedure. Increasing

the sample:extractant ratio in nearly all manure types increased WEP release. In poultry

litter, Vadas and Kleinman (2006) found a non-linear increase in WEP at decreasing

sample:extractant ratios, so 1:10<1:50<1:150 but 1:200=1:250, while Haggard et al. (2005)



Solubility of Manure Phosphorus ...



229



noted 5 to 13 times more WEP in 1:200 compared to 1:10 ratio poultry litters and pelletted

litter products. Similarly, Mamo et al. (2007) saw more WEP released from fresh, refrigerated

feedlot beef manure at higher extractant ratios. Kleinman et al. (2002) found relatively little

increase in WEP in dairy, poultry and swine manure extracts as the ratio was increased from

1:10 to 1:40, but substantially more WEP at 1:200 than at the 1:40 ratio, and suggested

dissolution of sparingly-soluble Ca phosphates as a reason. There was less difference between

the 1:200 and 1:40 g:mL extractant ratios in dairy than swine or poultry manures. The highest

correlation of WEP with runoff P was at a sample:extractant ratio of 1:100 from a range of

manures, poultry litters and biosolids (Kleinman et al., 2007).

Drying method of manure samples has given equivocal results for the amount of WEP

released relative to fresh manure. Air-drying or oven-drying tended to increase WEP release

for swine and some poultry manures (Vadas and Kleinman, 2006), but decrease WEP for

dairy manure (Vadas and Kleinman, 2006; Chapuis-Lardy et al., 2003). McDowell et al.

(2008) noted a decline of up to 61% in WEP released from air-dried manure compared to

fresh dairy manure. In broiler manure, Sistani et al. (2001) found no significant difference in

WEP between fresh and oven-dried (105°C) but less WEP released from freeze-dried, airdried or oven-dried at 65°C.

Increasing shaking time increased WEP release, but often by a small amount. Dou et al.

(2000) found 9% more inorganic P (Pi) released in water extracts of layer poultry manure and

11% more from dairy manure as shaking time increased from 1 to 16 hr. Mamo et al. (2007)

noted significantly greater WEP release at shaking times of 0.5 to 2 hr from beef manure at

1:200 and 1:100 g:mL sample:extractant ratios, but no difference at the 1:10 ratio. A

logarithmic relationship between shaking times up to 24 hr and WEP release from dairy, layer

poultry and swine manures was found by Kleinman et al. (2002), with 70% of total WEP

released into solution in the first hr.

As with shaking time, method of filtration had relatively little effect on measurement of

WEP. Kleinman et al. (2002) found up to 10% more WEP in dairy and poultry manure

extracts filtered through coarse paper (Whatman 1) compared to 0.45 µm membranes, and

suggested colloidal P not retained by the coarse filter paper could be hydrolyzed by acidic

colorimetric reagents or inductively coupled plasma spectroscopy (ICP). No differences as a

function of filtration type were noted in their swine slurry sample, nor by Mamo et al. (2007),

using coarse or fine filter paper or 0.45 µm membranes on beef manure extracts. Pore size of

the filtering medium is an important component of Haygarth and Sharpley‘s proposed classes

of WEP forms (2000), though many studies of WEP do not deal specifically with those

functional classes. Toor et al. (2006), in a review of the literature, noted that if centrifugation

and decanting is used without filtration, WEP may be substantially overestimated, although

Wolf et al. (2005) found no significant differences in WEP analyzed by ICP between

Whatman 40 paper filtration and centrifugation followed by decanting.

Vadas and Kleinman (2006) found acid digestion of extracts of dairy, poultry and swine

manure yielded on average 22% greater total WEP than inorganic WEP measured by

colorimetry; however, Wolf et al. (2005) reported 7% lower WEP from ICP analysis

compared to Murphy-Riley colorimetric analysis, and suggested interference from extraction

of colored compounds by the water extractant may be the cause.



230



John D. Toth, Zhengxia Dou and Zhongqi He

Table 10.1. Details of the various methods used to determine WEP in manures.



Sample

conditions



Extraction

time



Filtration



Analytical

methods†



Reference



Freeze-dried



Sample:extractant

ratio (DM basis

unless noted)

2:25



1 hr



Not specified



Fresh



1:60 to 1:250



1 hr, 1 hr

repeated



0.45 µm

membrane

Whatman 2



Angel et al.,

2005

Chapuis-Lardy

et al., 2003



Fresh



2:98 wet basis



1 hr



Whatman 42



Air-dried



1:10



1 hr



Whatman 42



Fresh



1:10 wet basis



2 hr



Fresh



1:10 to 1:200 wet

basis



2 hr



Centrifuged,

0.45 µm

Centrifuged,

0.45 µm



Fresh



1:10 to 1:200



1 min to 24

hr



Fresh



1:200



1 hr



Dried



1:100



1 hr



Oven-dried



1:10



1 hr



Fresh



1:10



1 hr



Fresh



1:10



Fresh



1:10 to 1:200



Not

specified

0.5 to 2 hr



Fresh or ovendried



1:333 or 2:98 wet

basis



1 hr



Oven-dried



1:15



30 min



Whatman 2V



Fresh



1:10



2 hr



Fresh



1:10 wet basis



2 hr



Ascorbic acid

method (Pi)

Ascorbic acid

method (Pi)



Fresh



1:10 wet basis



1 hr



Centrifuged,

0.45 µm

Centrifuged,

0.45 µm

membrane

0.45 µm

membrane



Fresh, oven-,

air-, or freezedrying



1:15



30 min



Whatman 2V



Ascorbic acid

method (Pi)



Centrifuged,

Whatman 1 or

0.45 µm

Centrifuge and

decant

0.45 µm

membrane

0.45 µm

membrane

Whatman 40

Centrifuged,

Whatman 40

0.45 µm,

Whatman 40 or

42

Centrifuged,

Whatman 42



Ascorbic acid

method (Pi),

ICP (Pt)

Ascorbic acid

method (Pi),

ICP (Pt)

Ascorbic acid

method (Pi)

Ascorbic acid

method (Pi)

Ascorbic acid

method (Pi),

ICP (Pt)

Ascorbic acid

method (Pi)

ICP (Pt)

ICP (Pt)

ICP (Pt)

ICP (Pt)

ICP (Pt)

Ascorbic acid

method (Pi)

Colorimetric

method (Pi),

ICP (Pt)

ICP (Pt)



ICP (Pt)



Chapuis-Lardy

et al., 2004

Codling et al.,

2000

Do et al., 2005

Haggard et al.,

2005

Kleinman et

al., 2002

Kleinman et

al., 2005

Leytem and

Thacker, 2008

Leytem et al.,

2004

Maguire et al.,

2006

Maguire et al.,

2006

Mamo et al.,

2007

McDowell et

al., 2008

Miles et al.,

2003

Moore and

Miller, 1994

Self-Davis and

Moore, 2000

Sims and

LukaMcCafferty,

2002

Sistani et al.,

2001



231



Solubility of Manure Phosphorus ...







Sample

conditions



Sample:extractant

ratio (DM basis

unless noted)



Extraction

time



Filtration



Analytical

methods†



Reference



Fresh



1:10 to 1:250



1 hr



1:200



Not

specified



0.45 µm

membrane



1:200



1 hr



Whatman 40 or

centrifuge and

decant



Ascorbic acid

method (Pi),

ICP (Pt)

Ascorbic acid

method (Pi),

acid digestion

(Pt)

Ascorbic acid

method (Pi),

acid digestion

(Pt)

Ascorbic acid

method (Pi),

ICP (Pt)



Toor et al.,

2007



1 hr, 1 hr

repeated



Centrifuged,

0.45 µm or

Whatman 40

0.45 µm

membrane



Fresh, ovenor air-dried



1:10 to 1:250



Fresh



Fresh



Vadas and

Kleinman,

2006

Vadas et al.,

2007



Wolf et al.,

2005



―Ascorbic acid method‖ refers here to a variety of colorimetric procedures, including the MurphyRiley method (1962) and He and Honeycutt‘s modified Mo blue method (2005), in which

ascorbate serves as reducing agent for the quantitative reaction of P with the color reagent. Details

of methodological approaches can be found in the individual citations. ―Colorimetric method‖ is

used when further details are not provided by the authors.



Taking into account correlation with runoff P measurements, interlaboratory repeatability

and ease and feasibility of laboratory procedures, Kleinman et al. (2007) proposed the

following as a standard protocol for WEP measurement for manures: 1:100 sample:extractant

ratio, 1 hr shaking, centrifugation followed if necessary by filtration through Whatman 1

paper, and analysis by ICP or colorimetry.



10.2.2. Variability in WEP Across Manure Types

Water-extractable P from a range of manures from food animal species and storage and

handling procedures (Table 10.2) were reported by Kleinman et al. (2005) and Sharpley and

Moyer (2000). Based on analyses of 24 fresh manure samples of each manure type WEP

ranged from 1.63 to 2.49 g P kg-1 for dairy, 2.85 to 5.10 g kg-1 for poultry litter, 6.08 to 8.52 g

P kg-1 for poultry manure, and 5.01 to 7.38 g kg-1 for swine slurry (Sharpley and Moyer,

2000). As a proportion of manure total P, WEP was lowest in beef manure (17%), broiler

litter and layer manure (12 and 20%, respectively), intermediate in swine manure and turkey

litter (28 and 34%), and at 70%, greatest in dairy manure (Kleinman et al., 2005).



10.2.3. Effects of Manure Handling, Composting and Addition of P-binding

Chemicals

With composting of manure, Sharpley and Moyer (2000) found little difference in WEP

in their dairy samples (mean of 2.09 g P kg-1 in fresh manure compared to 2.39 g kg-1 after

composting), but a significant decrease in WEP in the poultry manure composted with wood

chips and other plant material (mean of 7.30 g kg-1 in fresh manure compared to 2.11 g kg-1

after composting).



232



John D. Toth, Zhengxia Dou and Zhongqi He

Table 10.2. Water-extractable P results from selected studies

on animal manures and composts.



Animal species

Broiler poultry,

turkey, swine

Broiler poultry

Dairy

Dairy

Broiler poultry

Dairy

Beef

Broiler poultry

Dairy



Manure sample handling or management

Conventional, reduced mineral P or phytasecontaining diets

Conventional, low-phytate corn or phytasecontaining diets

Slurry with or without commercial chalk-clay

amendment

Diets with a range of P concentrations

Litter amended with Al- and Fe-based water

treatment residuals

Amended with polymers, Al or Fe-based

chemicals or coal combustion byproduct

Fresh or composted, receiving high-pH

amendments

Litter receiving Al, Ca, K or Fe-based

chemical amendments

Different aerobic and anaerobic digestion

systems



Broiler poultry

Dairy, layer poultry,

swine

Dairy, beef, broiler

and layer poultry,

turkey, swine

Dairy, beef, broiler

and layer poultry,

swine

Dairy, broiler and

layer poultry, turkey,

swine

Swine



Laboratory sample:extractant ratios

Laboratory sample handling and analytical

methods

Fresh manure and manure under different

storage conditions



Dairy, beef, swine,

poultry

Broiler poultry



Fresh, stored slurry, composted



Broiler poultry

Beef, layer poultry,

swine

Dairy



Reference

Angel et al., 2005

Applegate et al., 2003

Chapuis-Lardy et al.,

2003

Chapuis-Lardy et al.,

2004

Codling et al., 2000.

Dao and Daniel, 2002

Dao, 1999

Do et al., 2005

Güngör and

Karthikeyan, 2005,

2006

Haggard et al., 2005

Kleinman et al., 2002

Kleinman et al., 2005



Laboratory sample handling and analytical

methods



Kleinman et al., 2006



Fresh or with different manure storage

conditions, laboratory sample handling and

analytical methods

Different grain-based diets



Kleinman et al., 2007



Breeder manure, with or without phytase,

under different storage conditions

Litter amended with Ca-based liming

materials

Fresh or stored, sample:extractant ratios,

shaking time, type of filter paper

Comparison of 4 dairy feeding systems,

confinement vs. grazing



Leytem and Thacker,

2008

Leytem et al., 2004

Maguire et al., 2004

Maguire et al., 2006

Mamo et al., 2007

McDowell et al.,

2008



233



Solubility of Manure Phosphorus ...

Animal species

Broiler poultry

Broiler poultry

Broiler poultry

Broiler poultry

Broiler poultry

Broiler poultry

Dairy, poultry, swine

Dairy, broiler poultry

Dairy, broiler and

layer poultry, swine



Manure sample handling or management

Conventional, low-phytate corn or phytasecontaining diets

Receiving Al, Ca or Fe-based chemical

amendments

Bark or straw-based litter, dried at different

temperatures

Litter repeatedly amended with aluminum

sulfate

Fresh or dried by different methods

Fresh or granulated with chemical

amendments

Presence/absence of bedding materials,

different drying methods

Manure on porous sheet on field soil,

interacting with natural rainfall

Laboratory sample handling and analytical

methods



Reference

Miles et al., 2003

Moore and Miller,

1994

Robinson and

Sharpley, 1995

Sims and LukaMcCafferty, 2002

Sistani et al., 2001

Toor et al., 2007

Vadas and Kleinman,

2006

Vadas et al., 2007

Wolf et al., 2005



Vadas et al. (2007) and Robinson and Sharpley (1995) examined changes in WEP in

manures and litters exposed to actual or simulated variable field conditions. Vadas et al.

(2007) spread dairy manure and poultry litter on porous fabric sheets on soil outdoors and

monitored WEP trends over up to 18 months. Water-extractable P decreased over the first 2

months of the study and then stabilized at 10 to 20% of the initial WEP concentrations. In the

Robinson and Sharpley (1995) study, dried, ground poultry litter was spread across a leachate

collection apparatus, and simulated rainfall events were alternated with drying at different

temperatures. Forty percent of the total amount of Pi leached from the litter was lost in the

first simulated rainfall, and 80 to 95% after 5 rainfalls.

Both high and low pH amendments have been used to reduce WEP concentrations in

animal manures, generally poultry litter and manure. Moore and Miller (1994) treated poultry

litter with a range of Al-, Ca- or Fe-containing chemicals and incubated the samples for 1 wk

at ambient temperature. After incubation, WEP was determined on 1:10 dry matter

(DM):extractant ratio samples. Selected high- and low-pH amendments lowered WEP in the

litters from >2000 to <1 mg P kg-1. Among the most effective amendments were alum,

quicklime, slaked lime, and the Fe-containing chemicals. Gypsum and sodium aluminate

reduced WEP by 50 to 60%. Maguire et al. (2006) treated layer manure and broiler litter with

calcium oxide and calcium hydroxide at rates of addition of 2.5 to 15% wet weight and

incubated samples 1 d. Water-extractable P was measured at 1:10 manure:extractant ratio by

ICP. Water-extractable P was reduced compared to unamended controls by >90% with

amendment rates of at least 10% calcium oxide. High-pH, Al- and Fe-rich water treatment

plant residuals were used by Codling et al. (2000) to control solubility of P in poultry litter.

Both Al- and Fe-rich residuals applied at rates of 25 to 100 g kg-1 litter reduced WEP by 39 to

88% following 7 wk incubation. Sims and Luka-McCafferty (2002) expanded chemical

amendment studies to the farm scale; 97 poultry houses had alum applied to the litter on the

house floor at 0.09 to 0.135 kg bird-1 seven times over the 16 mo study. An equivalent

number of non-amended poultry houses served as controls. At the end of the study, WEP in



234



John D. Toth, Zhengxia Dou and Zhongqi He



the litter was determined on 1:10 litter:extractant samples and measured by both ICP and

colorimetrically. After alum treatment, soluble P declined compared to controls by 67 and

73% (ICP and colorimetric determination, respectively).

These and other laboratory- and field-based studies demonstrate that a variety of

chemicals and water-treatment byproducts can effectively tie up soluble P in poultry litters

and manures. Other treatments, including industrial byproducts, and their effects on P

solubility in other animal manure types are discussed in later sections.



10.2.4. Dietary Manipulation Effects

Modifying the constituents of food animal diets has been proposed as a simple, at-thesource means to reduce manure P concentrations (Valk et al., 2000; Maguire et al., 2005). In

dairy cattle, excess P in the diet is largely contributed by mineral P supplementation, in the

misperception that high-P diets improve milk production and reproductive success (Wu et al.,

2000). In a study of P feeding levels and manure P concentration on commercial dairies,

Chapuis-Lardy et al. (2004) found WEP increased from 2.25 g kg-1 fecal DM in a herd with

0.39% diet P to 6.35 g P kg-1 in a herd fed 0.60% P. Mixed stepwise regression of feed and

fecal parameters‘ influence on fecal P excretion showed that diet P was the dominant factor,

although fecal Ca, pH and stage of lactation also were significant contributors.

Table 10.3. Other single-extraction solutions used on animal manures and byproducts.

Animal species

Poultry

Dairy

Dairy, poultry

Dairy

Broiler poultry

Broiler poultry



Manure sample handling or management

Three fresh samples extracted with sodium acetate buffer

(pH 5.0)

Manure extracted with dilute HCl solution

Mixed dairy and poultry manures, palletized, extracted

with NaHCO3

Fresh from 13 farms, extracted with sodium acetate

buffer (pH 5.0)

Litter extracted with range of dilute HCl solutions

Litter extracted with range of dilute HCl or buffer

solutions (pH 6.0)



Reference

Dail et al., 2007

Dou et al., 2007,

2010

Hadas et al., 1990

He et al., 2004b

Tasistro et al., 2004

Tasistro et al., 2007



In swine and poultry diets, phytase is now generally used as a diet additive that can

release P from phytate, which is a relatively unavailable P source for monogastric animals.

Phytase supplementation allows the reduction or elimination of mineral P in the diet,

otherwise necessary for proper P nutrition in swine and poultry. There has been some concern

that the P released by action of phytase may have higher solubility and pose more of a risk for

P losses in runoff when manure is land-applied (Angel et al., 2005; Applegate et al., 2003).

Angel et al. (2005) added antimicrobial agents to poultry and swine diets with added phytase

and concluded that post-excretion increases in WEP were due to microbial activity and not to

inherently higher WEP in phytase-supplemented diets.



Solubility of Manure Phosphorus ...



235



10.3. SINGLE EXTRACTIONS IN DILUTE ACID AND OTHER SOLUTIONS

In addition to water as a single extractant for animal manures and manure products, other

solutions have been used to evaluate P solubility to fulfill different research objectives (Table

10.3). In a study of the relative P availability in the soil from amendment with pelletized

manure with or without fertilizer P enrichment, Hadas et al. (1990) extracted ground, mixed

poultry and cattle manure formed into 4-6 mm pellets with 0.5 mol L-1 sodium bicarbonate

solution, noting that soil test P-Olsen P is extracted under similar conditions (Kuo, 1996).

Phosphorus release was greatest from ground pellets followed by broken and whole pellets.

Inhibition of P release was thought to be related to high concentrations of ammonium and

high pH in the intact pellets. Similarly, sodium acetate buffer (100 mmol L-1, pH 5.0) has

been tested as a single extractant to determine plant-available P in dairy manures because

extraction conditions are close to that for Morgan (1.24 mol L-1 sodium acetate buffer, pH

4.8) or modified Morgan P (0.62 mol L-1 NH4OH + 1.25 mol L-1 acetic acid, pH 4.8)

solutions, which have been used for soil P testing (He et al., 2004b). In the 13 dairy manures

tested, total P extracted by the acetate buffer had an average of 6221 mg P kg-1 DM with a

standard deviation (SD) of 1811. Total P in both H2O and NaHCO3 fractions had an average

of 6104 mg P kg-1 DM with SD of 1668. The average of total P in all three fractions was 6669

mg P kg kg-1 DM with SD of 1701. These data indicate that the amount of P extracted by the

single sodium acetate buffer solution (100 mmol L-1, pH 5.0) from dairy manure was equal to

the summed amount of P extracted by the series of H2O, NaHCO3, and NaOH solutions

frequently used in sequential fractionation methods (described in detail in later sections).

However, dairy manure contained too little HCl-extracted P to test the correlation with the

portion of sequentially extracted HCl-P. Poultry manure contained small quantities of NaOHextractable P and a relatively large amount of HCl-extractable P, thus providing an

opportunity to verify the correlation between acetate buffer-extracted P and sequentiallyextracted P (Dail et al., 2007). In one sample, acetate-extractable P was twice that extracted

by H2O, NaHCO3, and NaOH together, but less than the total extracted by H2O, NaHCO3,

NaOH, and HCl. In a second sample, acetate-extractable P was about 22 % higher than that in

the first three fractions but less than the total extracted in the four fractions, suggesting that

the NaOH-P fraction of animal manure was extractable by sodium acetate buffer. These

observations imply that, whereas sequentially extracted NaOH- and HCl-extractable P in soil

are considered less plant-available than H2O and NaHCO3 fractions, the counterpart P in

animal manure should not be assumed to be so.

Tasistro et al. (2004) argued that the normally alkaline pH of poultry litter limits the

solubility of P forms; however, low soil pH could increase P solubility from poultry litter

after field application. Therefore, they assumed that the use of WEP concentrations measured

at the original litter pH might lead to an underestimation of the risk of P contamination of

runoff water. They measured WEP in broiler and breeder poultry litter (1:200

manure:extractant ratio) at original pH values of 7.6 to 8.5, and at pH of 6 and 7 after

acidification with HCl. Their results show that WEP measured at lower pH increased by 24 to

69% compared to that of WEP extracted at unmodified pH. In a second experiment, Tasistro

et al. (2007) compared soluble P concentrations extracted from poultry wastes at three pHs: 1)

at natural pH, using deionized water (DIw); 2) after titrating DIw suspensions with 0.5 mol L1

hydrochloric acid ( HCl) to pH end-points 3.0, 4.0, and 6.0; and 3) at pH 6.0 with a variety



236



John D. Toth, Zhengxia Dou and Zhongqi He



of buffering compounds. Extracting solutions adjusted to pH 3 to 4 extracted approximately

20% more molybdate-reactive P (MRP) from low-P layer manures than solutions at pH 6, and

up to 40% more MRP from high-P layer manures than at pH 6. Compared to WEP, total

soluble P increased by 60 to 140% in the acidified extractant at pH 6. Buffers extracted more

soluble minerals than suspensions acidified with HCl, probably because of their complexation

ability. The 2-(N-morpholino)ethanesulfonic acid (MES) buffer showed minimal metal

complexation, suggesting that it was the most suitable buffer compound tested for extracting

P at a stable pH value of 6. These results highlight the importance of measuring WEP under

conditions similar to those encountered in the soil after litter application. Tasistro et al. (2007)

argued that soluble P from poultry manure measured at pH 6.0 rather than at unadjusted

original pH would be a more correct input to simulation models as the extraction would be

done at a pH representative of the environment from which runoff is most likely to occur.

However, to date there are no modeling data to support their claims.



Figure 10.1. Provisional indicator of overfeeding P in dairy cattle, based on 0.1% HCl extraction of

fecal samples. Inorganic P in extracts of 4.75 g kg-1 manure DM corresponds to a diet P concentration

of 0.40%. Adapted from Dou et al. (2010).



Although a one-hr water extract of dairy manure seemed initially to be a good choice for

an indicator of P overfeeding (Dou et al., 2002), additional research (Chapuis-Lardy et al.,

2004) suggested that other factors such as fecal Ca concentration, sample handling and pH

also had a significant impact on the relationship between diet P and fecal P. To overcome the

confounding effects of Ca concentration, etc., a series of dilute hydrochloric, acetic, and citric

acid solutions were chosen as extractants to determine which, if any, could improve the

correlation between dairy diet P and fecal P (Dou et al., 2007). Manure samples collected

from 25 commercial dairies were extracted 1 hr in acid solutions at a 2:98 sample wet

weight:extractant ratio. Inorganic P released from dairy manure in 0.1% HCl solution was

closely correlated with diet P concentration (R2=0.69 compared to R2=0.33 for water as

extractant). A fecal P overfeeding indicator was proposed using the 0.1% HCl extracting



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PART III. PHOSPHORUS FORMS AND LABILITY

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