Agronomic and Environmental Aspects of Phosphate Fertilisers Varying in Source and Solubility an...

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O R I G I N A L A R T I C L E

Agronomic and environmental aspects of phosphate

fertilizers varying in source and solubility: an update review

S. H. Chien L. I. Prochnow S. Tu

C. S. Snyder

Received: 8 April 2010 / Accepted: 17 July 2010 / Published online: 28 July 2010

Springer Science+Business Media B.V. 2010

Abstract This review discusses and summarizes the

latest reports regarding the agronomic utilization and

potential environmental effects of different types of

phosphate (P) fertilizers that vary in solubility. The

agronomic effectiveness of P fertilizer can be influ-

enced by the following factors: (1) water and citrate

solubility; (2) chemical composition of solid water-

soluble P (WSP) fertilizers; (3) fluid and solid forms

of WSP fertilizers; and (4) chemical reactions of P

fertilizers in soils. Non-conventional P fertilizers are

compared with WSP fertilizers in terms of P useefficiency in crop production. Non-conventional P

fertilizers include directly applied phosphate rock

(PR), partially acidulated PR (PAPR), and compacted

mixtures of PR and WSP. The potential impacts of

the use of P fertilizers from both conventional (fully

acidulated) and non-conventional sources are dis-

cussed in terms of (1) contamination of soils and

plants with toxic heavy metals, such as cadmium

(Cd), and (2) the contribution of P runoff to

eutrophication. Best practices of integrated nutrient

management should be implemented when applying

P fertilizers to different cropping systems. The ideal

management system will use appropriate sources,

application rates, timing, and placement in consider-

ation of soil properties. The goal of P fertilizer use

should be to optimize crop production withoutcausing environmental problems.

Keywords Phosphate fertilizers

Agronomic effectiveness Cadmium

P runoff Eutrophication

Introduction

Phosphorus (P) is an essential macronutrient that can

limit normal plant growth if not provided by the soil orby appropriate quantities of fertilizers. For soils low in

available P, the nutrient must be applied in either

organic or inorganic form to obtain optimal crop

yield. Organic P sources, such as crop residues and

animal manure, generally have low P content. These

organic P sources must be supplied in massive

amounts to provide adequate P rates, rendering their

application economically unfeasible in many loca-

tions and conditions. Furthermore, plants often absorb

S. H. Chien (&)

Formerly affiliated with IFDC, Muscle Shoals, AL, USAe-mail: [email protected]

Present Address:

S. H. Chien1905 Beechwood Circle, Florence, AL, USA

L. I. Prochnow

IPNI Brazil Program, Piracicaba, SP, Brazil

S. TuIPNI Southwest China Region, Chengdu, China

C. S. SnyderIPNI Nitrogen Program, Conway, AR, USA

123

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DOI 10.1007/s10705-010-9390-4


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about 50% of their seasonal P requirements by the

time they have accumulated 25% of their total

seasonal dry mass (Black 1968). Thus, plants early

P requirements are critical. Unlike nitrogen (N),

which can be top-dressed as fertilizer 23 times

during the growing season, generally P fertilization

entails a one-time basal application to the soil beforeor soon after plant emergence. Because organic P

must be converted to inorganic P through a relatively

slow mineralization process before it becomes avail-

able to plants, organic fertilizers may not meet plants

early P requirements, often resulting in lower crop

yield (Nachimuthu et al. 2009). Furthermore, unlike

conventional chemical N fertilizers which are all

water soluble, P fertilizers vary widely in solubility

that can influence the initial and residual P effects. For

this and other reasons, inorganic P fertilizers remain

the major sources of P application used by farmers inboth developed and developing countries. Inappropri-

ate or excessive use of P fertilizers has been linked to

environmental pollution, including heavy metal con-

tamination in soils, and P runoff can contribute to the

eutrophication of water bodies (Chien et al. 2009).

In this review, we extract and synthesize informa-

tion concerning the P source varying in solubility,

agronomic effectiveness, and environmental impact

of inorganic P fertilizer use from recent literature

reports, especially those published from 1999 to

2009.

Solubility characteristics of various phosphate

fertilizers

Solubility measurement methods

In general, the solubility of P fertilizers is measured

according to three categories: water-soluble P, citrate

(neutral ammonium citrate or NAC)-soluble P, and

citrate-insoluble P. Methods of solubility measure-ment, however, vary among countries. In the United

States, the AOAC procedure (AOAC 1999) is most

commonly used. In this procedure, a sample of fully

acidulated P fertilizer [single superphosphate (SSP),

triple superphosphate (TSP), monoammonium phos-

phate (MAP), or diammonium phosphate (DAP)] is

extracted with water to determine water-soluble

P. This step is followed by filtration and extraction

of the residue with NAC solution to determine

citrate-soluble P. The remaining P in the residue

after citrate extraction is then digested to determine

its total P content, which is termed citrate-insoluble

P. The combined water-soluble and citrate-soluble P

is considered as available P. The amount of citrate-

insoluble P can also be indirectly calculated by

subtracting available P from total P. In other coun-tries [e.g., the European Community (EC)], direct

NAC extraction of fully acidulated P fertilizers

(without pre-extraction with water) is often used to

measure available P. In general, the two procedures

(with or without pre-extraction of WSP) produce only

minor differences in the measured amount of avail-

able P. For example, Johnsonton and Richards

(2003a) reported that the average available P content

of four TSP samples was 20.0% P according to one

method and 19.4% P according to the other. There-

fore, it appears that the use of a single extraction stepor sequential extractions for determining available P

is not a major issue for fully acidulated P fertilizers.

Although the two methods provide similar results for

available P, there has been some concern within the

fertilizer-production community that these small

differences could contribute to challenges in manu-

facturing and labeling P fertilizers (Falls 1991).

Because the water-soluble P fraction represents a

significant portion of total P in most fully acidulated

P fertilizers, we refer to fully acidulated P fertilizer

interchangeably with water-soluble P (WSP) fertilizerhereafter.

In partially acidulated phosphate rock (PAPR)

fertilizers or compacted (PR ? WSP) fertilizers,

which contain a mixture of un-acidulated phosphate

rock (PR) and partially acidulated WSP, the citrate

solubility of the un-acidulated PR may be decreased

by the Ca common-ion depressive effect on apatite

solubility due to the presence of water-soluble

monocalcium phosphate (MCP). Therefore, the avail-

able P in these types of P fertilizers can be

underestimated if a single citrate extraction step isused (Chien 1993). Compacted (PR ? WSP) fertil-

izers are produced by mixing PR with WSP at certain

P ratios. The mixture is compacted, crushed, and

screened until the desired granular sizes are reached

(Menon and Chien 1996). Similarly, the citrate

solubility of apatite in directly applied PR can be

underestimated if the PR contains a significant

amount of free carbonates (calcite and/or dolomite).

Free carbonates are more soluble than apatite,

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resulting in the same Ca common-ion depressive

effect (Chien 1993). In this case, a sequential second

extraction and measurement of citrate solubility

should be performed to determine the actual available

P in the PR after the first citrate extraction has

removed the free carbonates. In one study, for

example, the citrate solubility of Huila PR (Colom-bia) increased from the first extraction (0.6% P) to the

second extraction (1.9% P) because this PR contained

6% CO2 in the form of free carbonates rather than

apatite-bound carbonate. Consequently, the second

citrate solubility measurement was better correlated

with crop response to PR than the first citrate

solubility measurement (Chien and Hammond 1978).

Chien (2003a) reported that PAPR produced from

PR sources containing low Fe2O3 ? Al2O3 content

with 50% acidulation by H2SO4 had the same water

solubility as compacted (PR ? WSP) fertilizer madefrom the same materials at a 1:1 total P ratio.

However, when the PR sources had high

Fe2O3 ? Al2O3 content, the water solubility of the

resulting PAPR fertilizer was lower than that of the

compacted fertilizer. This effect occurred because

partial acidulation of PR with high Fe2O3 ? Al2O3content produces WSP and soluble Fe and Al ions

that precipitate WSP as water-insoluble FeP and Al

P. However, un-acidulated Fe2O3 ? Al2O3 minerals

do not adsorb WSP in physically compacted prod-

ucts. Furthermore, the compaction process is prefer-able to the wet granulation process, which requires

drying. During drying, WSP can be precipitated with

the associated free carbonates in the PR, resulting in

reduced water solubility. As will be discussed below,

compaction is preferred to partial acidulation for PR

sources that have high Fe2O3 ? Al2O3 content

(Chien 2003a).

Expression of phosphate solubility

The water solubility of fully acidulated P fertilizerscan be expressed as a percentage of fertilizer material,

total P, or available P. Johnston and Richards (2003a)

pointed out different conventions for expressing the

water solubility of P fertilizers might result in a

distorted view of P water solubility. They compared

the conventions used to express water solubility for

two US and two UK TSP products. Table 1 shows that

the UK2 and US1 samples differed by 11.2% if water

solubility was expressed as a percentage of total

P (95.2 - 84.0% = 11.2%), 6.4% if water solubility

was expressed as a percentage of available P (96.9 -

90.5% = 6.4%), and only 1.5% if water solubility

was expressed as a percentage of TSP (18.9 -

17.4% = 1.5%). Therefore, the convention used to

express P solubility should be considered when

comparing the water solubility of fully acidulated Pfertilizers. This consideration is relevant to the

fertilizer P regulations adopted by different countries.

For example, the minimum water solubility of

EC-type TSP products has been decreased from 93

to 85% as a percentage of available P in the 2004 EC

Commission Regulations (European Community

2004). In Brazil, the minimum water solubility of

fully acidulated P fertilizers is about 90% of available

P (Chien et al. 2009).

When comparing the solubility of various PR

sources that have different total P contents, solubilityis more appropriately expressed as a percentage of

rock than as a percentage of total P. The latter

convention gives misleading comparisons when PR

sources of varying P content are used. For example,

Chien and Hammond (1978) showed that Pesca

(Colombia) PR was lower in both total P and citrate

solubility as a percentage of rock than Central Florida

PR (8.6 vs. 14.2% for total P and 0.8 vs. 1.4% for

citrate solubility as a percentage of rock). However,

when solubility was expressed as a percentage of total

P, the citrate solubility of Pesca PR was the same asthat of Central Florida PR (4.0%).The agronomic

effectiveness of Central Florida PR, however, was

greater than that of Pesca PR, consistent with the

citrate solubility values expressed as percentages of

rock rather than with those expressed as percentages

of total P (Chien and Hammond 1978). Lehr and

McClellan (1972) demonstrated that when total P

content of North Carolina PR was diluted by half

from 13.0 to 6.5% with quartz, the citrate solubility

Table 1 Chemical analysis of two US and two UK TSPproducts (Johnston and Richards 2003a)

Source US1 US2 UK1 UK2

% total P 20.7 21.0 20.4 19.9

% water-soluble P 17.4 17.9 19.1 18.9

% available P 19.2 19.3 19.1 19.5Water-soluble P as % of total P 84.0 85.5 90.4 95.2

Water-soluble P as % of available P 90.5 92.6 96.6 96.9

Nutr Cycl Agroecosyst (2011) 89:229255 231

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remained relatively constant (5.7%) when the solubil-

ity was expressed as % of material (PR ? quartz)

whereas the solubility increased from 9.1 to 16.0%.

Thisis due to the fact that apatite mineral in a PRhas its

maximum solubility fixed by a solubility-product

constant (Chien and Black1976). Thus, PR solubility

can be mathematically increased for one PR sourcecontaining low total P content as compared to another

PR source containing high total P content when the two

PR sources have the same reactivity. Therefore, the

solubility of PR in NAC, 2% citric acid (CA), or 2%

formic acid (FA) is better expressed as a percentage of

rock. The use of PR solubility measurements from the

second extraction and the expression of solubility as a

percentage of rock are key principles used in a recently

developed Phosphate Rock Decision Support System

(PRDSS), which compares the relative agronomic

effectiveness (RAE) of PR to that of WSP (Smalbergeret al. 2006; Chien et al. 2009). This comparison

procedure will be discussed below.

Chemical and mineralogical composition

and solubility characteristics of phosphate

fertilizers

In fully acidulated commercial-grade P fertilizers, the

P compounds within the WSP fraction are mostly in

the forms of MCP [Ca(H2PO4)22H2O] in SSP and

TSP, NH4H2PO4 in MAP, and (NH4)2HPO4 in DAP.In general, commercial-grade SSP, TSP, MAP, and

DAP fertilizers are not 100% water-soluble. Nor-

mally, at least 8590% of the total P in these

fertilizers is water-soluble, and the remaining P is

citrate-soluble. However, some poor-quality SSP

sources may contain as little as 5060% water-

soluble P. Single superphosphate also contains a

significant amount of CaSO4 as a result of the

acidulation of PR with H2SO4. The chemical and

mineralogical composition of the remaining water-

insoluble P fraction depends strongly on the source ofthe PR and the processes used during acidulation.

The most common water-insoluble P impurities are

the generic P compounds H8 [(Fe,Al)3NaH8(PO4)6

6H2O] in SSP and H14 [(Fe,Al)3NaH14(PO4)84H2O]

in TSP (Prochnow et al. 2003a, b, c).

According to Prochnow et al. (2003a, b), studies

prior to 1999 showed that about 20% of the total

P content in Australian SSP fertilizers was in the

form of residual apatite and two water-insoluble

P compounds [Ca(Fe,Al)H(HPO4)2F22H2O and

(Fe,Al)(K,Na)H8(PO4)66H2O] and that water-insol-

uble P occurred in the forms of MgAl(NH4)2H

(PO4)2F2, AlNH4HPO4F2, and FeNH4(HPO4)2 in

MAP fertilizers produced from North Carolina,

Florida, and Idaho PR sources. The compound

MgAl(NH4)2H(PO4)2F2 was the most abundant ofseveral citrate-insoluble P compounds that remained

after citrate extraction of these MAP fertilizers.

Prochnow et al. (2003a) identified Fe3NaH8(PO4)6

6H2O as the major water-insoluble P compound in

three Brazilian SSP fertilizers. The water solubility of

these SSP products as a percentage of available P was

46, 80, and 86%.

As will be discussed below, the water-insoluble

but citrate-soluble compounds (and citrate-insoluble

P impurity compounds) that are present in fully

acidulated P fertilizers do have some agronomicvalue when compared to 100% WSP compounds such

as MCP. In terms of providing P to plants, agronomic

performance differs among various fertilizers with

low percentages of water-soluble P. Some of these

fertilizers are very good sources of this essential

nutrient.

Agronomic aspects of phosphate fertilizers

varying in source and solubility

Effects of the chemical and mineralogical

composition of P fertilizers on their reactions

and agronomic effectiveness in soils

Major solid water-soluble P fertilizers include SSP,

TSP, MAP, and DAP. All of these compounds are

readily soluble in soil and provide P in the soil

solution for plant uptake. However, water-soluble P

in the form of orthophosphate can be readily

converted to water-insoluble P through reactions

with soil minerals. In acidic soils, FeAl oxides canconvert P in the soil solution to water-insoluble Fe

AlP on the surface of mineral particles. In calcar-

eous soils, P in the soil solution can be surface-

adsorbed by CaCO3. Furthermore, P in the soil

solution can precipitate with cations as amorphous

FeP and/or AlP compounds in acidic soils and as

CaP compounds in alkaline or calcareous soils. All

of these reactions can result in decreasing P avail-

ability over time (Hedley and McLaughlin 2005;


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Syers et al. 2008). Several terms, such as P sorption,

adsorption, retention, fixation, precipitation, and

immobilization, have been used to describe this

process (Chien et al. 2009). In general, only about

1020% of applied P is taken up by the first crop.

However, Syers et al. (2008) concluded that the

efficiency of water-soluble P fertilizer can be up to

90% when evaluated over an adequate time scale (at

least a decade) using the balance method to

calculate P recovery. The balance method simply

considers yield and P uptake relative to the amount of

P applied. Thus, it takes into account residual P from

previously applied fertilizer. This method differs

from the traditional difference method, which

compares crop yield and P uptake between soils with

and without added P. Consequently, P recovery

calculated by the balance method is higher than that

calculated by the difference method.

In general, all high-quality WSP fertilizers should

be equally effective in providing available P to plants.

Therefore, only a limited amount of research has beenreported in the literature regarding the agronomic

effectiveness of different fully acidulated P fertilizers

in terms of source (e.g., SSP, TSP, MAP, and DAP)

and solubility. Most published reports on this topic

have focused on water-insoluble and partially water-

soluble P sources [e.g., PR, PAPR, and compacted

(PR ? WSP) fertilizer]. However, because P reaction

products vary in solubility (Hedley and McLaughlin

2005; Syers et al. 2008), different sources of fully

acidulated P fertilizers may not be equally effective

under certain conditions. For example, Lu et al. (1987)

reported that SSP was more effective than DAP for

maize grown in a calcareous soil in terms of dry-

matter yield, P uptake, and available P (Table 2),

regardless of the P placement method (broadcast,

incorporation, or deep placement). In their study,

sulfur (S) was not limiting for plant growth. The major

initial reaction product of MCP, which is the main P

component of SSP, is dicalcium phosphate dihydrate

(DCPD; CaHPO42H2O) in calcareous soils, whereas

the principal reaction products of DAP are octacal-

cium (OCP) [Ca8H2(PO4)53 H2O] and hydroxyapa-

tite (HA) [Ca10(PO4)5(OH)2]. In addition, various

types of Ca-NH4-P, such as Ca(NH4)2(HPO4) H2O

and CaNH4PO4H2O, may be produced when DAP

reacts with calcareous soils (Lindsay 1979). Because

DCPD is more soluble than other CaP compounds,

SSP performs better than DAP in calcareous soil, as

reported by Lu et al. (1987). However, some research-

ers have claimed that the nitrification of NH4N inMAP and DAP fertilizers to NO3N (which increases

acidity levels around fertilizer granules in the soil) and

the root absorption of NH4N (which increases

rhizosphere acidity) may increase the dissolution of

precipitated CaP compounds. This increased disso-

lution enhances the P availability of SSP or TSP

(Terman 1971).

Researchers often use SSP and/or TSP as standard

WSP sources in comparison with other CaP sources,

Table 2 Available P (measured by the Olsen method) in calcareous soil treated with DAP or SSP according to placement method(Lu et al. 1987)

Fertilizer Placement methoda Rate of P added (mg P/kg soil)

27 (mg P/kg soil) 55 (mg P/kg soil) 110 (mg P/kg soil) 220 (mg P/kg soil)

DAP Broadcast 9.4 13.6 18.4 32.9

Incorporation 5.2 6.7 12.3 27.0

Deep placement 6.3 10.6 20.8 33.6

SSP Broadcast 22.3 27.9 39.8 63.6

Incorporation 8.3 17.2 28.6 55.3

Deep placement 11.3 18.9 31.6 70.9

Control (no fertilizer P) (1.5 mg P/kg)

Fertilizer 9 placement 9 rate

LSD (0.05) 13.35

CV(%) 18.80

a Values for the incorporation method should not be directly compared to those for broadcast and deep placement because of theeffect of placement method on fertilizer concentrations in the soil


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such as PR, PAPR, or compacted fertilizer (PR ?

WSP). Use of poor-quality SSP or TSP as a P

reference to compare other CaP sources (e.g., PR or

PAPR) might result in misleading information and

over-estimate the relative effectiveness of those CaP

sources. Therefore, it is recommended that research-

ers check the quality of SSP or TSP before startingthe agronomic experiments. It also should be pointed

out that SSP contains significant amount of S that

may under-estimate the relative effectiveness of PR

against SSP in the S-deficient soils if S is not added to

the PR treatment. Similarly, the same problem could

also be encountered when comparing with other fully

acidulated P fertilizers (e.g., TSP, MAP, or DAP) that

contain little or no S. For example, Bationo et al.

(1995) showed that SSP performed better than TSP as

P source for pear millet grown on a sandy soil (pH

5.7) in West Africa. There is no reason to expect thatSSP is better than TSP as P source since both contain

the same WSP compound (MCP). In this study, no

other nutrients except N were applied. It is most

likely that the crop responded to P and S so that SSP

was better than TSP as reported.

Reactions and agronomic effectiveness of fluid

and solid WSP fertilizers in soils

Numerous studies have compared the agronomic

effectiveness of fluid versus granular or non-granularWSP fertilizers. For a valid comparison of fluid and

solid P fertilizers, the P should be supplied by the

same chemical compounds in both cases, and it

should be similarly placed. However, many reported

greenhouse and field trials do not comply with these

requirements, and the results often conflict. For

example, many studies have claimed that fluid

ammonium polyphosphates (APP) are agronomically

superior to ammonium phosphates because of the

different P compounds in the two P sources (poly-

phosphates in APP vs. orthophosphates in ammoniumphosphates). Most results in the United States and

other countries show equal P availability from these

two P sources to crops grown on most soils (Chien

et al. 2009). Ottman et al. (2006) also found no

significant differences in alfalfa yield obtained with

fluid APP and granular MAP on a calcareous soil

over 3 years. Some favorable results for APP on

neutral to alkaline soils may have been caused by

appreciable amounts of micronutrients, such as Fe or

Zn, in the APP (Engelstad and Terman 1980). It is

now recognized that the P in APP becomes available

to plants only when the polyphosphates are hydro-

lyzed to orthophosphate in the soil. This process

depends largely on soil biological activity. If the

hydrolysis of polyphosphate to orthophosphate is

slow, soil P fixation may decrease over time throughthe sequestration of polyphosphate, keeping it from

being adsorbed by soil minerals. This process may

increase the agronomic effectiveness of polyphos-

phates compared to orthophosphates.

Recently, renewed interest in research on fluid

versus granular forms of the same WSP fertilizers has

been reported, especially in Australia. Holloway et al.

(2001) showed that commercial fluid P fertilizers

(e.g., MAP, DAP and APP) were more effective than

the corresponding commercial granular P fertilizers

in increasing crop yield in calcareous and alkalinesoils. Lombi et al. (2004), Hettiarachchi et al. (2006)

used highly sophisticated instruments to study the P

mobility of surface-applied granular versus fluid

MAP in a calcareous soil at 60% of field moisture

capacity. They found that total and labile P from

liquid MAP diffused farther (1.35 cm) from the

initial site of P application than total and labile P

from granular MAP (0.75 cm). This difference may

explain the better agronomic performance of fluid

MAP compared to granular MAP in field trials

reported by Holloway et al. (2001), who claimed thatthe P diffusion and isotopic lability of granular MAP

were reduced compared to those of an equivalent

liquid MAP because precipitation reactions osmoti-

cally induced the flow of soil moisture into MAP

granules. In addition, a significant amount of the

initial P remained in the granules even after some

time of dissolution due to the presence of water-

insoluble FeAlP minerals in the granule and the in

situ precipitation of similar minerals resulting from

the diffusion of Ca and Al into the granule. In

contrast, there was significantly less P fixation fromfluid P fertilizers and hence a greater concentration of

labile P (Lombi et al. 2004). It should be noted that P

diffusion depends strongly on soil moisture content

(% of field capacity) and on the water content of

liquid P fertilizers. Furthermore, in studies conducted

in Australia, the researchers reported only the total P

content. They did not report the proportions of WSP

and water-insoluble P content when comparing

corresponding fluid and granular P fertilizers. For


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example, Lombi et al. (2004) compared commercial

granular MAP with commercial fluid technical-grade

MAP, but they did not report whether the two sources

contained the same P compounds or had the same

water solubility.

One potential benefit of applying fluid ortho-or

polyphosphate sources rather than solid P sourcesmay be that fluid P sources can increase the soil

concentration of available P with depth (Kovar 2006).

This increased concentration is especially important

under no-till cropping conditions because the down-

ward movement of orthophosphate is significantly

limited in soil-surface application. By increasing P

movement to subsurface depths, application of fluid P

fertilizers can enhance P recovery and response for

deep-rooted cereal crops like maize. Downward P

movement should also reduce the surface runoff risk

that is responsible for eutrophication in aquaticenvironments, as will be discussed below. In light

of the recently renewed interest in research compar-

ing the agronomic effectiveness of fluid and granular

WSP fertilizers in Australia (Evans 2008; Holloway

et al. 2006), it may be worthwhile to conduct similar

research on different soils in other countries.

Agronomic effectiveness of controlledrelease

coated WSP fertilizers

Compared to research on slow-release (mainlyN-polymers) and controlled-release (mainly coated

urea) N fertilizers, limited information is available in

the literature regarding controlled-release WSP fer-

tilizers (mainly MAP and DAP). Most reports on

controlled-release soluble P fertilizers concern poly-

mer-coated soluble NPK compound fertilizers and are

limited to laboratory studies (Shaviv 2000; Du et al.

2006).

Pauly et al. (2002) conducted a greenhouse study

with polymer-coated MAP and DAP (thin-coated,

1.8% by weight or thick-coated, 2.2% by weight) totest the growth response and P uptake of barley. Thin-

coated MAP showed greater dry-matter yield, P

uptake, net P fertilizer efficiency and net fertilizer

release efficiency than thick-coated and uncoated

MAP. The coating of DAP fertilizer did not consis-

tently improve any of the above-mentioned parame-

ters. Additional information on polymer-coated WSP

fertilizers can be found in a recent review paper by

Chien et al. (2009). More research, particularly field

trials, is needed to test whether polymer-coated WSP

fertilizers have the potential to improve P fertilizer

efficiency and crop production.

Agronomic effectiveness of water-insoluble

and partially water-soluble non-conventional P

fertilizer sources

According to the AOAC definition (AOAC 1999),

citrate-soluble P (including water-soluble P) in P

fertilizers is available to plants and is called available

P. However, some water-insoluble or partially water-

soluble P fertilizers that vary in citrate-soluble P may

be as agronomically effective as water-soluble P

fertilizers under certain conditions. Chien and Friesen

(1992) showed that a high-reactivity ground North

Carolina (NC) PR with only 4.2% citrate-soluble P as

a percentage of rock was equally effective for maizegrown on an acidic soil as almost 100% citrate-soluble

TSP. The less-reactive Jordan PR (2.9% P) and Togo

PR (1.8% P), however, were less effective than TSP

(Fig. 1). In Tanzania, new data indicate that the most

reactive Minjingu PR in Sub-Saharan Africa exhibits

good agronomic effectiveness (Smithson et al. 2003;

Msolla et al. 2005; Szilas et al. 2007a, b). In general,

the agronomic effectiveness of different PR sources

correlates well with the citrate solubility of the PR

(Chien 2003a; Truong 2004). In India, a combination

of local low-reactive Mussoorie PR (MPR) with Psolubilizing bacteria (PSB) was compared with DAP

with respect to productivity and P balance in a rice-

rapeseed-mungbean cropping system for 3 years. The

0

5

10

15

20

25

30

0 200 400 600 800 1000 1200

Rate of P2O5 Applied, lb/acre

Dry-MatterYieldofMaize,g/pot

TSP NC PR Jordan PR Togo

NAC Solubility, % P of Rock

NC PR = 4.2Jordan PR = 2.9Togo PR = 1.8

Fig. 1 Dry-matter yield of maize grown with P sources

varying in citrate solubility (Hartsells soil; pH 4.8) (Chien2003a)


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RAE of (MPR ? PBS) in relation to DAP as judged

by total productivity was 5365% in the first cycle but

reached 69106% in the third cycle of the cropping

system (Sharma et al. 2010).

Among crop species, rape is highly efficient in

utilizing PR. The exudation of malic acid and citric

acid by the plants roots is thought to be responsiblefor the dissolution of PR. Habib et al. (1999) were

probably the first to report that rape was able to

utilize a medium-reactive Ain Layloun PR (Syria) in

calcareous soils. In a follow-up study with nine

different PR sources varying widely in reactivity for

rape grown on an alkaline soil (pH 7.8), Chien et al.

(2003a) found that the RAE of PR increased from 0

to 88% as the 2% citric acid (CA) solubility of PR

increased from 0.9 to 5.7% P as a percentage of rock

(Table 3). These findings suggest that medium- to

high-reactivity PR sources can be used for rape inalkaline soils. This conclusion has important impli-

cations for countries that have medium- to high-

reactivity PR deposits and alkaline soils. For

example, several countries in North Africa (e.g.,

Tunisia, Algeria, and Morocco) have highly reactive

PR sources. Farmers in these countries cannot use

these low-cost indigenous sources of PR for their

current crops (e.g., wheat, barley, and maize) in

alkaline soils. The feasibility of growing a high-value

crop, such as rape for cooking oil, could translate into

a significant economic benefit for farmers if the cost/benefit ratio of using PR is higher than that of using

WSP fertilizers.

An interesting new field of PR research involves

the increased use of PR for organic farming world-

wide because chemical P fertilizers cannot be used on

certified organic farms (Nelson and Mikkelsen 2008).

Mixing elemental S, which is allowed as an input for

certified organic farming, with PR may provide a

source of both nutrients. The oxidation of elemental S

to H2SO4 may enhance PR dissolution for organic

farming, as reported by Evans et al. (2006), Evans

and Price (2009). It should be noted that the effectiveuse of PR with or without elemental S for certified

organic farming, as for conventional farming,

depends on the reactivity of the PR sources used.

Thus, organic farmers should be aware that not all PR

sources have the same reactivity. The general rule is

that the higher the reactivity of PR, the more likely it

is to be an effective P source for organic farming. For

example, an indigenous igneous PR from Ontario,

Canada, with very low reactivity has been marketed

for organic farming due to its high P content (Chien

et al. 2009). However, total P content is irrelevant toPR reactivity for direct application. In fact, most

igneous PR sources are high in P content but very low

in reactivity due to little CO3/PO4 substitution in the

apatite structure (Chien 2003a) and are therefore

unsuitable for direct application in organic farming.

Another example is an organic strawberry farm in

Canada that used a highly reactive PR imported from

North Africa. However, the crop was grown on

alkaline soils, making the PR ineffective as a P source

(Chien et al. 2009). The many factors affecting the

agronomic effectiveness of PR for organic farmingshould be considered more or less the same way as

for conventional farming. One major exception is in

cases where PR is used with composting of either

crop residues or animal manures. Composting is

likely to produce a neutral to alkaline rather than an

Table 3 Characteristics ofdifferent P sources and theirRAE values for rape grown

to maturity in an alkalinesoil (pH 7.8) (Chien et al.2003a)

a Solubility of P in 2%

citric acid as a percentageof rockb Based on CO3/PO4substitution in the apatitestructure as measured byX-ray diffraction

P source Total P 2% CAa Reactivityb RAE (%)

TSP 20.1 100 100

Gafsa PR (Tunisia) 13.1 5.7 High 88

Ain Layloun PR (Syria) 12.2 5.3 Medium 82

Chelesai PR (Kazakhstan) 7.4 3.6 Medium 74

Tilemsi PR (Mali) 11.4 4.5 Medium 72

El-Hassa PR (Jordan) 13.6 3.9 Medium 64

Kenegesepp PR (Russia) 13.0 3.4 Low 64

Kodjari PR (Burkina Faso) 11.0 2.6 Low 60

Kaiyang PR (China) 14.1 2.2 Low 42

Panda Hills PR (Tanzania) 10.8 0.9 Very low 0


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acidic soil environment (Chien et al. 2009). For

example, Kuo et al. (2004) reported that pH of a

composting pile dropped from initial 65 in the

mesophilic phase only for a few days, and soon after

the pH began to increase and stayed constant around

pH 78 in the thermophilic, cooling, and finally to

maturing phase of the composting. Therefore, in thecases of composting organic materials with PR, it is

likely that the chelation of organic matter by Ca ions

derived from apatite, similar to the mechanism of the

dissolution of apatite by neutral citrate solution, is

primarily responsible for PR dissolution rather than

soil acidity as in the conventional farming. More

research is needed to explore these PR reactions and

crop responses to the composting of PR in the field.

Lim et al. (2003) used a process called mechano-

milling in which six different PR materials were ball-

milled at high energy to allow enhanced chemical andphysical reactivity. Lim and Gilkes (2001) found that

milling increased the solubility of PR in 2% CA by

increasing the proportion of X-ray amorphous P and

reducing the size of the remaining apatite crystals.

However, the surface area of PR was reduced, and the

unit-cell a-value of apatite increased toward that of

fluorapatite, which has the lowest solubility and least

surface area among apatite minerals. The increased

unit-cell a-value of apatite was likely due to the

gradual driving-off of apatite-bound CO3 as CO2 gas

as a result of the high energy used during mechano-milling; this process is similar to the calcination of

francolite containing apatite-bound CO3 to fluorapa-

tite (Chien and Hammond 1991). Lim and Gilkes

(2001) attributed the increased solubility and agro-

nomic effectiveness of the processed PR to the

dominant amorphous materials formed during mec-

hano-milling. More work is needed to understand the

inconsistent effects of mechanical milling on the

physical and chemical reactions and agronomic

effectiveness of PR.

Although hundreds of agronomic trials of PR havebeen conducted worldwide, it is essential to integrate

all important factors affecting the agronomic effec-

tiveness of PR into a comprehensive system to better

understand these processes. One way of solving this

problem is to use PRDSS. Because it is designed to be

practical, PRDSS can be used in developing countries,

especially in countries endowed with indigenous PR

deposits, to help choose between WSP fertilizers and

PR. Recently, IFDC has developed and published its

own PRDSS model for PR sources varying in

reactivity for different crop species (Smalberger

et al. 2006). Based on this model, the FAO/IAEA

has posted the PRDSS on the IAEA web site

(http://www.iswam.iaea.org/dapr/srv/en/home). The

current PRDSS version can be used to estimate both

the initial and residual RAE of PR and the relativeeconomic effectiveness (REE) of PR compared to

WSP fertilizers. This PRDSS can be a useful tool to

researchers, extension workers, fertilizer companies,

and government decision makers who must determine

the feasibility of the agronomic use of PR, whether

locally produced or imported, compared to that of

WSP fertilizers. Figure 2 shows that the latest vali-

dation of the updated PRDSS based on initial and

residual RAE values is consistent with the predicted

and observed RAE values (within 10% of a 1:1 line)

across different PR sources, types of soil, and cropspecies (Chien et al. 2009). It should be noted that the

current residual RAE of PR in the updated PRDSS

version represents only the average residual RAE

values of a PR over several years with the same

residual crop. In the future, more work will be needed

to model a residual RAE of PR for a given residual

crop over a given period of time.

Sikora (2002) conducted a theoretical and experi-

mental study to calculate and quantify the liming

potential of different PR sources by laboratory titration

and incubation with an acidic soil (pH 4.2). Therelationship between the calcium carbonate equivalent

percentage (CCE) of PR and the percentage of dis-

solved P followed a quadratic model, where % CCE=

8.47 ? 0.0078 (% dissolved P)2 (r2 = 0.84). Thus, the

more P dissolved from PR, the higher the liming value

of PR. For example, 50% of the total P dissolved from

PR would be 28% as effective as CaCO3 in terms of its

liming effect. In a long-term(16 years) field study with

a pastoral soil treated with two PR sources at a P rate of

30 kg P/ha/year, Loganathan et al. (2005) found that

soil D pH (pH of PR-treated soilpH of control soil)values ranged from 0.28 at a 05-cm depth to 0.17 at a

2030-cm depth for North Carolina PR and from 0.15

to 0.11 at corresponding depths for Jordan PR. The

total liming values were 640 and 414 kg CaCO3/ha for

North Carolina PR and Jordan PR, respectively. The

results from this long-term field study show that the

continuous use of medium- to high-reactivity PR

sources can significantly reduce the rate of acidificat-

ion in pastoral soils.


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Under certain conditions, such as low PR reactiv-

ity, high soil pH, or short-term crop growth, the

agronomic use of PR may not be as feasible as that of

WSP. Mixing PR with WSP can sometimes be

agronomically and economically effective under

these conditions. Partial acidulation of low-reactive

PR (PAPR), which consists of un-acidulated PR and

acidulated WSP, is one way to achieve this goal.

Another way is to mix PR with WSP by dry

granulation (compaction). In one study, the agro-nomic effectiveness of a low-reactive Patos PR

(Brazil) compacted with SSP at a 50:50 P ratio was

equal to that of SSP based on the dry-matter yield of

wheat and ryegrass (Prochnow et al. 2004). This

method was effective because the WSP was able to

provide available P to the plants initially (starter

effect), resulting in better plant root development,

which in turn allowed the plants to utilize the PR

more effectively later in their development than if the

PR had been applied alone. Prochnow et al. (2004)

showed that P uptake from Patos PR in the presence

of WSP was higher than that from Patos PR alone,

indicating the beneficial starter effect of water-

soluble P on the effectiveness of Patos PR.

Many studies have provided valuable information

on the factors affecting the agronomic effectiveness ofmixtures of PR and WSP, either by partial acidulation

or by compaction of PR with WSP. These factors

include the reactivity, degree of acidulation, and

degree of Fe and Al impurities of the PR source; the

effect of soil properties, such as pH and P-fixing

capacity; the starter effect of water-soluble P on PR

effectiveness; the crop species grown; and the initial

and residual P effects (Chien 2003b). Figure 3 shows

PAPR and compacted (PR ? TSP) products that

performed equally well using Huila PR (Colombia),

which contains low amounts of (Fe2O3 ? Al2O3)impurities (2.3%). The compacted product (PR?

TSP) was more effective than PAPR made with

Capinota PR (Bolivia), which contains a significant

amount of (Fe2O3 ? Al2O3) impurities (8.8%).

A

B

Fig. 2 Comparison of observed and predicted relative agro-nomic effectiveness (RAE) for (a) initial and (b) residualapplications of PR and WSP. The spread (10%) along theone-to-one line (dashed line) is shown by dotted lines (Chienet al. 2009)

B

A

Fig. 3 Dry-matter yield of maize grown with TSP and

modified PR products made from (a) Huila PR and (b) CapinotaPR (Chien et al. 2003b)


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Figure 4 shows that the RAE of PAPR increases with

the soils P-fixing capacity and that PAPR can be

more effective than SSP in soils with very high

P-fixing capacities. The chemistry of soil reactions as

explained by Chien (2003b) is as follows. When MCP,

which is the P component in SSP or TSP, dissolves in

the soil solution, it is hydrolyzed to DCPD and H3PO4in the meta-stable triple-point solution (Lindsay

1979). The released H3PO4 can lower soil pH to as

low as 1.5 and thus can dissolve Fe and Al minerals,

which in turn react with WSP to form water-insolubleFeAlP. These soil reactions decrease the P avail-

ability from SSP or TSP because of the P sorption

effect of the FeAl minerals. With PAPR, which

contains both WSP and PR, part of the H3PO4produced by MCP hydrolysis will be neutralized by

the unacidulated PR. This reaction of H3PO4 with

unacidulated PR not only reduces the P fixation of the

WSP component in PAPR by reactions with FeAl-

minerals but also allows additional P to be released

into the WSP pool. The results indicate that some of

the citrate-insoluble P of certain P fertilizers mightalso be available to plants. These studies raise doubts

about the ability of the AOAC procedure to accurately

extract plant-available P compounds (Chien et al.

2009).

Many studies on the agronomic use of apatitic PR,

PAPR, and compacted (PR ? WSP) products have

been reported in the literature since 1998. For the

readers information, we list some additional reports

categorized by the alphabetical order of country

name: (1) Djebel Onk PR (Algeria) by Nemeth et al.

(2002), (2) Kaiyang PR (China) and Gafsa PR by

Satter et al. (2006), (3) Jinxiang PR (China) by Xiong

et al. (2002), (4) China PAPR by Aye et al. (2009),

(5) Trinidad de Guedes PR (Cuba) and PAPR by

Rodriguez and Herrera (2002), (6) compacted

Jamakotra PR (India) with MAP by Nair et al.(2003); Begum et al. (2004), (7) Timemsi PR (Mali)

by Babana and Antoun (2005), Babana and Antoun

(2006), Somado et al. (2006) (8) Ben Guerir PR

(Morocco) by Rivaie et al. (2008), (9) Ogun PR

(Nigeria) by Akintokun et al. (2003), (10) Sokoto

PR (Nigeria) by Agbenin (2004); Sokoto and Singh

(2008), (11) North Carolina PAPR by McLay et al.

(2000), (12) North Carolina PR by Rajan (2002), (13)

Phalanowa PR (South Africa) by Loganathan et al.

(2004), (14) Eppawala PR (Sri Lanka) by Zoysa et al.

(1999), (15) Minjingu PR (Tanzania) by Mutuoet al. (1999), (16) Hahotoe PR (Togo) and PAPR

by Agyin-Birikorang et al. (2007), (17) Gafsa PR

(Tunisia) and Arad (Israel) by Gatiboni et al. (2003);

Mendoza et al. (2009), and (18) Riecito PR (Vene-

zuela) by Casanova et al. (2002). Several books on

these subjects have also been published since 1998 by

the IAEA (2002a, b, 2006), the IFDC (2003), and the

FAO (2004).

Calcination at high temperature is a non-conven-

tional process for increasing the solubility and

agronomic effectiveness of non-apatite PR containingcrandallite minerals in the form of CaFe-AlP,

which are not suitable for acidulation because of their

high FeAl content. In a recent study, Francisco et al.

(2008) characterized and conducted agronomic eval-

uations of two non-apatite PR sources (Sapucaia and

Juquia) from Brazil. After calcination at 500C for

4 h, the citrate solubility of these two PR sources

increased significantly from almost none to a max-

imum of 90 and 51% of total P, respectively. X-ray

diffraction showed that the crystalline crandallite

mineral was transformed to an amorphous form aftercalcination. Without calcination, both PR sources

were totally ineffective for upland rice grown on an

acidic soil (pH 5.4). The two calcined P sources and

the highly reactive Gafsa PR were 83, 89, and 100%

as effective as TSP, respectively, in increasing the

grain yield of upland rice. For flooded rice, the

corresponding values were 72 and 68% for the two

calcined P sources, respectively, and 0% for the

Gafsa PR. This study suggests that there is agronomic

Fig. 4 Relationship between soil P-fixing capacity and rela-tive agronomic effectiveness (RAE) based on dry-matter yieldand P uptake of maize grown with PAPR (PR acidulated at

50% with H2SO4) (Chien et al. 2003b)


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potential in calcined non-apatite PR sources that

cannot otherwise be used for direct application or

chemical acidulation.

Interest is growing in the use of magnesium

phosphates as P sources for crops in the form of

struvite (MgNH4PO46H2O) and dittmarite (MgNH4

PO4H2O), which can be recovered from municipal,

industrial, and agricultural wastewaters. Both are

water-insoluble but citrate-soluble P compounds.

Massey et al. (2009) found that recovered struvite

and dittmarite were as effective as TSP for wheat

grown in a neutral soil (pH 6.5), even when the soil was

limed (pH 7.6). These results support previous studies

showing recovered magnesium phosphates to be

effective in acidic soils, and they provide evidence

that recovered phosphates are also effective in slightly

alkaline soils (Johnston and Richards 2003b). More

research is needed to determine whether recoveredMg phosphates could become useful alternative P

fertilizer sources in conventional and organic farming

operations.

The new concept of effective P availability

of water-insoluble P compounds in fully

acidulated P fertilizers

Although water-insoluble P compounds often do not

provide P to plants as effectively as WSP compounds,

they do have certain agronomic values compared toWSP compounds (Prochnow et al. 2003a, b, c;

Prochnow et al. 2008). Johnston and Richards

(2003a) studied the agronomic effectiveness of

water-insoluble P residues isolated from four TSP

products produced in the US and the UK using

ryegrass (8 cuts) as the test crop, which was grown on

13 soils with widely varying properties. The relative

effectiveness (RE) of these water-insoluble P residues

with respect to MCP, as calculated by the substitu-

tion-rate method, ranged from 44 to 87% in dry-

matter yield. Johnston and Richards (2003a) also

advocated a new concept of calculating the effec-

tive availability of water-insoluble P residues com-

pared to WSP. Table 4 shows the calculated effective

P as a percentage of total P in the four TSP products.The concept takes into account the equivalent water

solubility of the water-insoluble P residues, which is

calculated from the RE of the water-insoluble resi-

due. For example, the total P of the water-insoluble P

residues of the US1 product was 3.3% (20.7 -

17.4% = 3.3%). Because its RE based on dry-matter

yield was 63% of that of WSP (MCP), the equivalent

available P in the water-insoluble P residues was

3.3% 9 0.63 = 2.1%. Thus, the total effective water-

soluble P was 17.4 ? 2.1% = 19.5%, and the effec-

tive water-soluble P as a percentage of total P was(19.5/20.7) 9 100 = 94%. The US and UK products

exhibited similar ranges of effective water-soluble P

as a percentage of total P (9497 and 9498%,

respectively), as shown in Table 4. However, the

range of actual water solubility values for the US

products (84.085.5%) was smaller than that of the

UK products (90.495.2%), as shown in Table 1.

Based on the calculated effective water solubility as a

percentage of total P, there was no systematic

difference between the US and UK products

(Table 4), but the calculated actual water solubilityof the US products (average 84.8%) was lower than

that of the UK products (average 92.8%), as shown in

Table 1. In short, Johnsonton and Richards (2003a)

concluded that there is little difference in the

agronomic effectiveness of TSP products with water

solubility values greater than 85% of the available

P2O5. This threshold value was officially adopted by

the EC in 2004.

Table 4 Calculatedeffective water solubility asa percentage of total P2O5in four TSP products(Johnston and Richards2003a)

a RE = Relativeeffectiveness with respect toTSP

Source US1 US2 UK1 UK2

% total P 20.7 21.0 20.4 19.9

% water-soluble P 17.4 17.9 19.1 18.9

REa of water-insoluble P residue (%) 63 82 46 59

Effective available P in water-insolubleP residue (%)

4.8 5.7 2.5 1.3

Total effective water-soluble P (%) 19.5 20.4 19.6 19.5

Effective water-soluble P as % of total P 94 97 94 98


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Environmental aspects of phosphate fertilizers

varying in source and solubility

Cadmium in soils and plants from application

of various P fertilizers

There is increasing concern over the use of Pfertilizers containing heavy metals, especially cad-

mium (Cd), for crop production. Heavy metal uptake

by plants is one possible avenue of entry into the

human food chain through direct or indirect con-

sumption. Numerous greenhouse and field studies

have been conducted with Cd because it is the heavy

metal of most environmental concern in terms of

adverse effects from long-term application of fertil-

izers. There have also been numerous review articles

on the issue of Cd impacts from the use of P

fertilizers (Alloway and Steinnes 1999; Grant et al.1999; Morvedt 2005). Grant et al. (1999) noted that

the Cd concentration of P fertilizers, application rate,

soil type, and crops grown are important factors when

considering the relationship between added Cd and

plant uptake.

The source of Cd in P fertilizers is PR. Depending

on the sources of PR, the Cd content associated with

P in apatite minerals can vary widely (Table 5). It

should be noted that the expression of Cd

concentration as mg Cd/kg of PR differs from that

as mg Cd/kg of P because P content varies among PR

sources. Because the Cd rate applied depends on the

P rate used, expression of Cd concentration as mg Cd/

kg of P is more appropriate in discussions of the Cd

issue than mg Cd/kg of P fertilizer.

Cd in PR originates from the substitution of Cd forCa in the apatite structure (Iretskaya and Chien

1999). If a PR source contains a significant amount of

Cd, all of the Cd will remain in SSP produced by

acidulation of the PR with H2SO4. In the production

of H3PO4-based P fertilizers, H3PO4 is first produced

by acidulation of PR with H2SO4 followed by

separation of H3PO4 and precipitation of phospho-

gypsum. As much as 3050% of the Cd in PR is

transferred to phosphogypsum and the remaining Cd

stays in the H3PO4 during the process (Wakefield

1980). Unlike SSP which uses 100% PR-P as Psource, TSP uses only 2430% of PR-P and 7076%

of H3PO4P (Davister 1998). Thus if H3PO4 is used

to acidulate the same PR, the resulting TSP may

contain 4050% of the TSP-Cd from the PR-Cd and

the remaining Cd from H3PO4Cd. Ammoniation of

Cd-containing H3PO4 will produce DAP and MAP

products that also contain Cd, but less Cd will be

present than Cd in TSP because no PR is used.

The chemical forms of Cd in acidulated P

fertilizers are Cd(H2PO4)2 and CdHPO4, which are

Cd analogs of the Ca and NH4 compounds found incommercial, fullyacidulated P fertilizers (Morvedt

2005). It has been suggested that the soil factors

affecting Cd availability may be similar to those

affecting the P availability of CaP. For example, the

dissolution of both CdP and CaP increases with

decreasing soil pH. Thus, Cd uptake by crops is

higher in acidic soils than in alkaline soils (Iretskaya

and Chien 1999). The plant availability of Cd in P

fertilizers also depends on soil texture and plant

species. Cd uptake is greater in leafy vegetables than

in maize or wheat (Morvedt 2005). Grant et al. (1999)stated that Cd availability may be greater with MAP

use than with DAP use because MAP is initially

acidic whereas DAP is alkaline.

Because the actual rate of Cd application based on

the P rate from P fertilizers is rather low and Cd

uptake by most crop plants is rather limited, gradual

accumulation of Cd usually occurs in soils only after

long-term application of Cd-containing P fertilizers.

Therefore, long-term experiments may be the most

Table 5 Range of Cd concentrations in PR from various

sources (adapted from Alloway and Steinnes 1999)

Source of PR mg Cd/kg PR mg Cd/kg P

Russia (Kola) 0.2 1

South Africa 4 23

China (Yunam) 4 35

Jordan 6 27

Australia (Duchese) 7 50

Mexico 8 57

Egypt (Quseir) 8 61

Peru (Sechura) 11 84

Israel (Arad) 12 85

Tunisia (Gafsa) 38 108

Israel (Zin) 32 228

Morocco (Boucraa) 38 240

Australia (Christmas Island) 43 275

USA (North Carolina) 47 311

Banaba (Ocean Island) 99 563

Nauru 100 641


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meaningful to assess the environmental effects of Cd

in P fertilizers. Morvedt (2005) summarized the

results of long-term field experiments with TSP made

from Florida PR and applied at an annual P rate of

1530 kg/ha after [50 years. He concluded that

minimal increases occurred in total Cd concentrations

in soil in the short term due to the addition of Cd in Pfertilizers applied for crop production. Bolan et al.

(2005) calculated that at an annual application rate of

20 kg P/ha, it would take from 214 years (with Gafsa

PR containing 70 mg Cd/kg) to 2,250 years (with

DAP containing 10 mg Cd/kg) to exceed the thresh-

old limit of 3 mg Cd/kg soil that has been set for Cd

application in sewage sludge in New Zealand. These

authors concluded that although P fertilizer addition

represents the major source of Cd input to soils, at a

normal annual rate of fertilizer input (20 kg P/ha), the

rate of Cd accumulation appears to be slow.Nevertheless, some countries have enacted regu-

latory controls on Cd limits in P fertilizers to reduce

the potential long-term effects of Cd on soil and plant

quality. Table 6 shows the proposed limits of Cd in P

fertilizers in various countries. Cadmium inputs in P

fertilizers containing the maximum allowable Cd

limits range from 0.7 g/ha in The Netherlands to

6.9 g/ha in Japan. Efforts to establish allowable limits

of Cd in P fertilizers in the United States began in the

mid-1990s. Because fertilizer regulations in the

United States are enacted on a state-by-state basis,there is no standard limit of maximum allowable Cd

in P fertilizers for the whole country. The US

Environmental Protection Agency (EPA) has set

national cumulative ceiling concentration limits for

land application of biosolids at 85 mg Cd/kg of soil

and a maximum loading rate of 1.9 kg Cd/ha per 365-

day period (US EPA 2007). The Association of

American Plant Food Control Officials (AAPFCO),

comprising the fertilizer regulatory officials in eachstate, suggested in 2005 that the standard Cd limit in

P fertilizers should be 4.4 mg Cd/kg per percent of P

content (Morvedt 2005). This is equivalent to a Cd

input to soil of 8.8 g Cd/ha at a fertilizer application

rate of 20 kg P/ha, which is higher than the 0.74.0-g

Cd/ha range in maximum allowable Cd inputs

imposed by Western European regulations (Table 6).

Grant et al. (1999) suggested that reducing the

amount of Cd in P fertilizer might be one method of

reducing the long-term accumulation of Cd in the

soil. As noted above, several countries have imposedlimits on the total Cd concentration that may be

present in fertilizer materials. The selection of PR

sources that are low in Cd should reduce the

concentration of Cd in finished P products after the

manufacturing process. However, global reserves of

PR containing low levels of Cd are relatively limited.

Removing Cd from PR during processing may not be

economically viable at current fertilizer prices,

despite the technologies that may be available.

Restricting P production to low-Cd fertilizer will

certainly increase the cost of fertilizer and may limitthe amount of P fertilizer available for trade. This

could have important implications for food produc-

tion, particularly in developing countries (Grant et al.

1999).

Many researchers have proposed that Cd uptake

may be reduced by increasing soil pH with liming

because Cd availability decreases with increasing soil

pH (Grant et al. 1999). This recommendation is based

primarily on greenhouse and growth-chamber studies.

However, Grant et al. (1999) pointed out that the

efficacy of liming under field conditions is lessconsistent and of a smaller magnitude than that noted

in container-growth studies. Numerous studies have

shown little or no effect of liming on crop Cd uptake

under field conditions. The interaction between pH

and organic matter in their effects on Cd in soils may

explain these differences. Soil organic matter can

reduce the toxic effects of Cd in contaminated soils

under acidic conditions. In contrast, at pH 68, higher

Cd concentrations were measured for soil samples

Table 6 Cadmium limits for P fertilizers in various countriesand estimated Cd inputs to soils from P fertilizers at themaximum Cd limits (Morvedt 2005)

Country Cd limit

(mg kg-1 P)

Cd input (g ha-1

with 20 kg ha-1 P)

Australia 300 6.0

Austria 120 2.4

Belgium 200 4.0

Finland 50 1.0

Germany 200 4.0

Japan 343 6.9

Norway 50 1.0

Sweden 100 2.0

Switzerland 50 1.0

The Netherlands 35 0.7


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with higher organic matter content because of the

complexation of Cd by soluble organic matter (Grant

et al. 1999). However, more research is needed,

especially long-term experiments under field condi-

tions, to unravel these conflicting reports on the

effects of liming on Cd uptake as influenced by soil

properties, agro-climatic conditions, and crop species.Most published research reports and reviews on Cd

contamination associated with the use of P fertilizers

focus exclusively on commercial, fully acidulated P

fertilizers (SSP, TSP, MAP, and DAP). Little infor-

mation is available regarding the effect of the degree

of acidulation of PR on Cd uptake from P fertilizers

varying widely in water and citrate solubility.

Because Cd and P are present in the same apatite

structure, it is expected that acidulation of PR will

release soluble P simultaneously with soluble Cd.

Table 7 shows that both available P (water ? citrateextractable) and available Cd (extracted by DTPA)

increased with increasing acidulation levels in two

PR sources (North Carolina and Togo) with high Cd

content. Just as the total P content of P fertilizer is

irrelevant to crop P uptake, and just as crop uptake of

P is determined by the available P content of a

fertilizer (Chien 2004a), crop Cd uptake is related to

the available Cd content of the P fertilizer and not to

the total Cd content (Chien 2004b) However, laws

that regulate the maximum permissible Cd content of

fertilizers are based on total Cd, not available Cd.

This discrepancy is a serious issue that lawmakers

and environmental regulators may need to reconsider.

A study by Iretskaya et al. (1999) showed that Cduptake by upland rice was increased by raising the

degree of acidulation of Togo PR, which had

significant Cd content (Table 8). The results showed

that the Cd concentration in upland rice grains was

0.051 mg/kg Cd with 50% acidulation with H2SO4compared to 0.114 mg/kg Cd with 100% acidulation,

whereas the two P sources produced the same rice

grain yield (Table 8). The study also showed that a

high-Cd reactive North Carolina PR was as agro-

nomically effective as a fully acidulated SSP pro-

duced from the same PR in increasing rice grainyields, but Cd uptake was lower from the directly

applied PR (Table 8). The results showed that Cd

uptake by rice was more related to DTPA-extractable

Cd than to the total amount of Cd added. For

example, the total Cd added was only slightly less

from NC-PR than NC-SSP (70.5 vs. 79.0 mg Cd/kg),

but Cd uptake from NC-PR was much lower than that

Table 7 Properties of P and Cd contents of various P fertilizers (Iretskaya and Chien 1999)

P Source Total P (%) H2O-P (%) Citrate-P (%) (H2O ? Citrate)-P (%) Total Cd (mg kg-1

) DTPA-Cd (mg kg-1

)

NC-PRa 13.3 0 2.8 2.8 47.0 0.2

NC-SSPa 6.3 5.8 0.5 6.3 24.7 18.7

Togo-PR 16.0 0 2.0 2.0 54.0 1.7

Togo-PAPRb 12.3 4.8 1.8 6.6 35.7 6.8

Togo-SSP 9.2 8.7 0.5 9.2 31.5 19.7

a NC = North Carolinab PAPR = PR acidulated at 50% level with H2SO4

Table 8 Upland rice grain yield and Cd uptake by grain from various P sources applied at 200 mg/kg P to an acidic soil (pH 5.6)

(Iretskaya and Chien 1999)

P sourcea Cd rate added

(lg g-1)

Grain yieldb

(g pot-1)

Uptake of Cd by

grainb (lg pot-1)

Concentration of Cd in

grainb (lg pot-1)

NC-PR 70.5 25.3A 1.68B 0.066B

NC-SSP 79.0 24.5A 3.25A 0.135A

Togo-PAPR 57.8 27.6A 1.44B 0.051B

Togo-SSP 69.3 25.6A 2.88A 0.114A

a NC = North Carolina; PAPR = PR acidulated at 50% with H2SO4; SSP = 100% acidulation of PR with H2SO4b Means followed by the same letter within columns are not significantly different at P\ 0.05


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from NC-SSP (1.68 vs. 3.25 lg Cd/pot). The Cd

uptake data reflected the much lower content of

DTPA-Cd in NC-PR than in NC-SSP (0.2 vs. 8.7 mg

Cd/kg), as shown in Table 7. The results also suggest

that if PR and PAPR are as agronomically effective

as fully acidulated P sources, the former maycontribute less to Cd uptake by crops than WSP

sources.

McLaughlin et al. (1997) reported that increased

chloride (Cl) content in irrigated waters imposes a

high risk of producing crops with high Cd concen-

trations. It is believed that Cl ions form relatively

strong complexes with Cd2? in the form of CdCl1?

and CdCl20 in solution, and these reactions result in

enhanced Cd uptake. Based on these observations,

Chien et al. (2003b) hypothesized that if a WSP

fertilizer has a high Cd content, granulation of thisWSP fertilizer with KCl may result in greater Cd

uptake by crops compared to an equivalent bulk-

blended PK fertilizer. In granulated PK fertilizers,

KCl and Cd-containing P fertilizers are present within

the same granule and are thus in close contact,

thereby increasing the possibility of forming CdCl20

and CdCl1? complexes. These complexes would be

less likely to form when KCl and Cd-containing P

granules are physically separated in soil-applied bulk-

blended PK fertilizers.

The above hypothesis was tested and confirmed byChien et al. (2003b) in a preliminary greenhouse

study. In this study, all sources of P and K, whether

produced by granulation or bulk-blending, had the

same granule size (1.683.36 mm in diameter).

Upland rice and soybean were grown to maturity

and Brachiaria grass was cut 4 times in an acidic soil

(pH 5.2). The results showed that the agronomic

effectiveness in increasing crop yield was equal for

Cd-containing SSP and reagent-grade MCP (0% Cd),

whether granulated or bulk-blended with KCl. Con-

centrations of Cd in plant tissue samples of all crops

were much lower for MCP than for SSP. In all plant

tissue samples, Cd concentrations obtained with

granulated PK fertilizer (SSP ? KCl) were signifi-

cantly higher than those obtained with bulk-blendedPK (SSP ? KCl) fertilizers. One example using a

soybean crop is shown in Table 9. The bulk-blending

of Cd-containing P fertilizers with KCl can thus

reduce Cd uptake by crops compared equivalent

granulated PK fertilizers. Although PK fertilizers, not

NPK fertilizers, were used in the study, it was

expected that the inclusion of N would not affect the

results (Chien et al. 2009). If this assumption holds

true, the superiority of bulk blending compared to

granulation in decreasing Cd uptake would also apply

to NPK compound fertilizers. Due to the simplicity ofbulk blending and its relatively low investment and

operating costs, this process has become popular

worldwide. Researchers and fertilizer companies

should further develop this process for the future

production and use of NPK compound fertilizers

while minimizing Cd uptake by crops.

Use of P fertilizers to remediate soils

contaminated by lead

Lead (Pb) in its soluble ionic form is a toxic elementthat contaminates water and soil, mainly as a result of

human activities. Lead contamination in soils is of

concern not only because of its toxicity to humans and

animals but also because of its ease of exposure

through ingestion or inhalation. A soil is generally

considered contaminated with Pb when its total

concentration exceeds 400 mg Pb/kg, and remediation

is required at this level (Yoon et al. 2007). There are

many reports in the literature on the use of P fertilizers

Table 9 Seed yield of soybean and Cd concentrations in soybean seeds, straw and roots (Chien et al. 2003b)

PK source Seed yielda, b

(g pot-1)Cd concentrationsb

Seeds (mg kg-1) Strawc (mg kg-1) Root (mg kg-1)

Granulated (SSP ? KCl) 27.1A 0.54A 1.66A 1.34A

Bulk-Blended (SSP) ? (KCl) 29.5A 0.35B 0.88B 0.99B

Bulk-Blended (MCP) ? (KCl) 23.8A 0.06C 0.26C 0.27C

a Seed yield of control plants (no P and K) = 1.4 g/potb Means followed by the same letter within columns are not significantly different at P\ 0.05c Combined leaf, stem, and pod samples


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to immobilize Pb by converting less stable Pb

compounds, such as cerrusite (PbCO3), to more stable

Pb compounds with very low solubility and bioavail-

ability, such as chloropyromorphite [Pb5(PO4)3Cl]

(Lindsay 1979).

There are many studies on the use of different

WSP and PR sources that contain low amounts ofheavy metals and that are useful for immobilizing Pb

in soils. The following additional references have

been reported since 1998: Ma and Rao 1999; Cao

et al. 2002; Chen et al. 2003; Bolan et al. 2003; Lin

et al. 2005; Bosso et al. 2008; Miretzky and

Fernandez 2009. In general, WSP fertilizers (e.g.,

SSP and TSP) are more effective than PR in reducing

soluble Pb content in the Pb-contaminated soils

because of their higher water solubility and their

ability to react with Pb to form more stable PbP

compounds. However, PR may be more economicallyfeasible than WSP for large-scale soil remediation

operations because PR is usually more cost effective

(Ma and Rao 1999; Chen et al. 2003). It should be

emphasized that, as for the agronomic use of PR in

conventional or organic farming (Chien et al. 2009),

the effectiveness of PR in immobilizing soil Pb

depends strongly on the reactivity or solubility of the

PR used.

Application of phosphate fertilizers in relation

to eutrophication

Importance of P in water eutrophication

Eutrophication (natural and anthropogenic) is the

enrichment of surface waters with nutrients that were

previously limiting, stimulating aquatic plant life. In

the late 1960s, it became clear that changes were

occurring in many lakes and reservoirs, especially in

industrialized countries, resulting from increasing

nutrient loads. These increased nutrient levels pro-

moted undesirable algal growth. Among plant nutri-ents, P is perhaps more critical than N for algal

growth because some species of algae, such as blue-

green algae (Cyanobacteria), can biologically fix

atmospheric N2. However, P is essential for biolog-

ical N fixation and the support of algal growth.

Phosphorus enrichment of surface waters can result in

intensified cultural eutrophication problems and

overall degradation of water quality. Such water-

quality degradation may impair water use for

fisheries, recreation, industry, and drinking because

of the increased growth of undesirable algae and

aquatic weeds and the oxygen shortages caused by

their death and decomposition (Sharpley et al. 2003;

Foy 2005). The resulting anaerobic condition in

marine systems caused by this depletion of oxygen is

known as hypoxia. Loss of P from agriculturalpractices has been identified as a major cause of

impaired surface-water quality (US EPA 2009). The

use of WSP fertilizers is one of the potential

agricultural practices contributing to the eutrophica-

tion of water bodies (Alexander et al. 2008). These

contributions occur mainly through surface runoff of

P in dissolved water (Heathwaite et al. 1998; Preedy

et al. 2001; Sharpley et al. 1992; Torbert et al. 1999),

the presence of P in eroded sediments (Baker and

Laflen 1983; Daverede et al. 2003, 2004), and

leaching of P into the groundwater in sandy soils(Maguire and Sims 2002).

Effects of P fertilizer application on P runoff

and eutrophication

Many recent experiments investigating P losses have

focused on overland flow pathways, comparisons

between P fertilizers and other P amendments

(especially livestock manures and slurries), and

methods of P application (Bundy et al. 2001; Withers

et al. 2005). Some recent examples of P losses inoverland flows following the land application of P

fertilizers and livestock manures are shown in

Table 10. Data on P export in terms of total P loss,

dissolved P loss, and flow-weighted total P reflect the

amount of available P for the promotion of eutrophi-

cation. The results indicated that P export was less

when P fertilizers were incorporated instead of

broadcast; that P export increased with increasing P

fertilizer application rate; that there was more P runoff

from irrigated pasture (Australia); and that P export

was greater when P fertilizers were used than whenlivestock manures were used. Similarly, P runoff

from pastures in Britain was reported to be higher

with WSP than with manure (Preedy et al. 2001). In a

study near Ames, Iowa that compared P runoff losses

from ammonium polyphosphate fertilizer to those

from liquid swine manure with and without soil

incorporation, Tabbara (2003) observed higher con-

centrations and losses of all P forms with the fertilizer

treatment. The higher losses that occurred with the


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fertilizer were attributed to the greater water solubil-ity of P in the fertilizer.

More detailed discussions on the factors affecting

P runoff risks from soil treated with WSP fertilizers

are described below

Landscape and rainfall characteristics The runoff of

P from soil varies strongly with landscape char-

acteristics. On sloping lands in Yunnan Province,

China, P losses were greatest from farmlands with

slopes of 68128 and gradually decreased beyond this

slope range (Duan et al. 2005). Orchards cultivated onsloping lands lost far more P through runoff than did

paddy rice fields. This difference in the orchards and

paddy rice field was closely related to the amounts of

soil and water lost (Huang et al. 2004). Phosphorus loss

significantly increased during the fallow period and

was intensified by heavy rain-induced runoff events

(Udawatta et al. 2004).

Soil P accumulation increases the loss of partic-

ulate P when soil particles are detached and

transported in runoff. The amounts of P lost throughrunoff are also dependent on factors related to soil

erosion risk, such as soil erodibility, topography, land

cover, and cultivation practices (Sharpley et al. 2000;

Harrod and Theurer 2002). Phosphorus loss from

soils is highest after dry seasons or at the beginning

of rain events and gradually decreases as the rainy

season or rain event proceeds upland crops (Xu et al.

2007) and paddy rice fields (Jiao et al. 2007). These

observations indicate that the amount of P lost from

soil through runoff is related to the amount of soluble

P in soil that can be leached out by rainwater (Poteet al. 1999). Studies indicate that fertilizers left on the

soil surface are most prone to loss, especially if

applied to grassland soils, sloping lands, or wet soils;

if applied just before a storm or irrigation event; and

in many high rainfall areas (Torbert et al. 1999).

Fertilizer P sources and management The sources

of acidulated P fertilizers may also affect the export

of P to water resources. Nash et al. (2003, 2004)

Table 10 Recent examples of P losses in overland flow following application of P fertilizers as compared to livestock manures(Withers et al. 2005)

Site P Sourcea Amount ofP applied

(kg ha-1 P)

P export

Flow (mm) TPb loss(g ha-1 P)

Dissolved P loss(% of TP)

FWTPc conc.(mg L-1 P)

P loss(% of TPapplied)

UK Control 0 5.4 32 22 0.60

TSP(I) 90 5.0 45 40 0.70 \0.1

TSP(B) 90 5.7 1,332 86 23.2 1.4

Iowa APP(B) 158 68.2 24,010 11 35.2 13.4

APP(I) 158 57.9 10,630 11 18.4 4.2

UK Control 0 48 60 36 0.12

TSP 29 48 1,863 74 3.88 6.2

Slurry 29 48 1,804 45 3.76 6.0

USA Control 0 7.6 1,210 1,210 15.9 -

Manure 87 7.6 1,980 1,980 26.0 0.9

TSP 87 7.6 2,680 2,680 35.2 1.7Australia (pasture) Control 0 0

SSP 22 22 6,600 90 30 30

SSP 44 17.5 10,100 90 58 23

SSP 66 14.6 13,200 90 90 20

SSP 88 11.3 12,300 90 109 14

a I = incorporated, B = broadcast, APP = liquid ammonium polyphosphate, Slurry = cow manure, Manure = poultry manureb TP = total Pc FWTP = flow-weighted total P


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reported that P losses following the application of

DAP were higher than losses from SSP in soils to

which water was subsequently added to generate

overland flow. These authors showed that lower P

availability of SSP than that of DAP after P application

to soil could be attributed to the formation of water-

insoluble reaction products, such as CaHPO4, as aresult of the hydrolysis of water-soluble MCP in SSP

(Lindsay 1979). If this were the case, TSP, which also

contains MCP, should exhibit P export results similar

to those of SSP. However, data from Hart et al. (2004)

showed that both dissolved reactive P and particulate

P, which are related to bioavailable P for algal growth

and eutrophication (Sharpley et al. 1992), were higher

from TSP than from SSP at the same rate of P

application (35 kg P/ha). The greater P export from

TSP than from SSP was likely due to the greater water

solubility of TSP. Therefore, the greater P export fromDAP was likely due not to the formation of water-

insoluble P from SSP, as explained by Nash et al.

(2003, 2004), but to the greater water solubility of

DAP. Another possible factor may be the effect of

fertilizer particle size on P runoff. Many SSP

fertilizers are produced in an irregular, non-granular

form, whereas commercial-grade DAP is always in a

granular form. Soil P adsorption of non-granular SSP

may be faster than that of granular DAP, resulting in

less P export from SSP. More research is needed to

investigate the effects of different acidulated Pfertilizers (SSP, TSP, MAP, and DAP) that vary in

solubility, granular form, and particle size on P runoff

risks.

Kovar (2006) suggested that fluid P sources

applied to the soil surface would move into the soil

profile, where they may be less subject to loss

through runoff or erosion. This process would be

especially important for polyphosphates, which are

thought to be less subject to soil adsorption. Such soil

infiltration may help minimize the environmental

impact of P during the winter months while main-taining available P for plants in the following

growing season. Thus, polyphosphates may present

less runoff risk than orthophosphates when applied to

the surface.

Most research and review reports on the use of P

fertilizers in relation to eutrophication are exclusively

concerned with conventional fully acidulated P

fertilizers. Little information is available on the use

of non-conventional P fertilizers, such as reactive PR

or PAPR, to sustain crop productivity and their

influence on P runoff. Studies done in New Zealand

and the United States have suggested that the use of

reactive PR can not only sustain crop productivity but

also minimize runoff losses better than WSP sources

because of lower P availability from PR for algal

growth (Hart et al. 2004; Shigaki et al. 2006, 2007).

Table 11 shows that both cumulative total and

dissolved reactive P losses from surface runoff fromthe three soils were significantly lower with reactive

North Carolina PR than with TSP, indicating that

reactive PR may pose a lower eutrophication risk

(Shigaki et al. 2007). More work, including field

studies, is needed to validate this conclusion, espe-

cially for soils with relatively high P fertility.

A positive correlation between fertilizer P rates

and P losses through runoff has been observed by a

number of researchers (Zhang et al. 2003; Cao and

Zhang 2004; Xia et al. 2008). In a paddy field,

application of 0, 25, 60, 120, and 240 kg P/haresulted in losses of 0.13, 0.50, 0.94, 3.02 and 5.97 kg

P/ha to runoff water, respectively (Xia et al. 2008).

This result indicates that an overdose of P fertilizer

will impose a high risk of P runoff from paddy fields

to watercourses during rainy seasons or in water

drainage practices that are adopted in paddy rice

production for controlling overgrown rice tillers.

Large P losses (5.59.0 kg P/ha) in overland flow

from pasture soils in Australia were observed when P

Table 11 Cumulative loss of dissolved reactive and total P insurface runoff from three soils treated with phosphate rock(PR) or triple superphosphate (TSP) (Shigaki et al. 2007)

Soils P sources

Control North Car

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