Quick
Search: 		  
advanced search
 GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help 
Elements Don't get GSW? Talk to your librarian.
JOURNAL HOME HELP CONTACT PUBLISHER SUBSCRIBE ARCHIVE SEARCH TABLE OF CONTENTS
Elements; April 2008; v. 4; no. 2; p. 105-108; DOI: 10.2113/GSELEMENTS.4.2.105
© 2008 Mineralogical Society of America
This Article
Right arrow Abstract
Right arrow Figures Only
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (2)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Manning, D. A.C.
Right arrow Search for Related Content
GeoRef
Right arrow GeoRef Citation

Phosphate Minerals, Environmental Pollution and Sustainable Agriculture

David A.C. Manning*

* School of Civil Engineering and Geosciences, Newcastle University
Newcastle upon Tyne NE1 7RU, UK
E-mail: david.manning{at}ncl.ac.uk


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 PHOSPHORUS SOURCES
 PHOSPHORUS IN AGRICULTURE
 PHOSPHORUS IN REMEDIATION
 THE PHOSPHORUS CYCLE IN...
 REFERENCES
 
The availability of phosphorus in soils is controlled by the ability of plants to dissolve phosphate-bearing minerals, including apatite and feldspars. To satisfy the requirement of plants for phosphate, mineral dissolution competes with precipitation such as, for example, reactions involving lead or other heavy metals. Plants exude organic acid anions that very effectively enhance mineral dissolution but that may also liberate harmful solutes, such as aluminium. To make readily soluble chemical fertilisers, apatite in igneous and sedimentary rocks is mined and processed; in organic farming, phosphate-rich rocks are crushed and applied directly to the soil, relying on compounds produced by plant roots (exudates) to extract the phosphorus that plants need.

KEYWORDS: phosphate, fertiliser, apatite, struvite, soil


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
PHOSPHORUS SOURCES
 PHOSPHORUS IN AGRICULTURE
 PHOSPHORUS IN REMEDIATION
 THE PHOSPHORUS CYCLE IN...
 REFERENCES
 
Where would we be without phosphorus? This element is vital to all forms of life because it drives the fundamental cellular (ATP-ADP) energy cycles that enable living systems to function. Essential for photosynthesis, it is one of the major nutrients that a farmer adds to soil to allow crops to be produced year after year. Because of its geochemical behaviour, it plays a role in reducing the risks associated with metal contamination of soils; in contaminated soils, precipitation of soluble metals as poorly soluble phosphate minerals can decrease bioavailability dramatically. The function and behaviour of phosphorus in soils depends on its geochemistry and on how plants have adapted and evolved to ensure that they can access the phosphorus they require.


Figure 1
Lupins, producers of copious quantities of root exudates to combat phosphate deficiencies. PHOTO P. TREMBLAY


   PHOSPHORUS SOURCES
TOP
 ABSTRACT
 INTRODUCTION
 PHOSPHORUS SOURCES
PHOSPHORUS IN AGRICULTURE
 PHOSPHORUS IN REMEDIATION
 THE PHOSPHORUS CYCLE IN...
 REFERENCES
 
Phosphorus (P) is common within geological materials. The average continental crust contains 0.15% P2O5 (Rudnick and Gao 2003). Phosphorus is normally contained in apatite, Ca5(PO4)3(OH). If one assumes that all phosphorus is contained in this mineral, one can estimate that the crust contains, on average, 0.35% apatite. The assumption that apatite is the dominant crustal host for P is incorrect, however, because feldspars also contain phosphorus. The berlinite substitution, in which P replaces tetrahedral Si in a coupled substitution with Al that maintains charge balance, can be written as 2Si4+ = Al3+ + P5+; this substitution allows P to be accommodated within the feldspar lattice (London et al. 1990). Feldspars in granite can contain up to 1% P2O5 in this form, but more commonly they contain 0.2-0.3% (Kontak et al. 1996). Feldspars are the dominant mineral species in continental crustal rocks (40-50%). Simple mass balance suggests that feldspars account for 50-90% of the estimated crustal P2O5 content. From an economic perspective, only rocks with a high concentration of P2O5 as phosphate minerals are worth mining (Oelkers and Valsami-Jones 2008 this issue) because of the cost of extracting P in a useable form. However, from the point of view of plants and soil biota, feldspars may be a more significant source of P (see Parsons et al. 1998).

In environmental applications, the addition of a phosphate-rich material to metal-contaminated soils can help reduce the availability of metal contaminants, such as lead and cadmium, through the precipitation of sparingly soluble phosphate minerals. Phosphates thus exhibit a paradox: their mining, processing and use have the potential to generate serious pollution problems, including eutrophication of lakes, at and near mine sites, and especially in agricultural areas through overfertilization; yet, in other situations, they can be used to immobilise toxic elements and clean up contamination.


   PHOSPHORUS IN AGRICULTURE
TOP
 ABSTRACT
 INTRODUCTION
 PHOSPHORUS SOURCES
 PHOSPHORUS IN AGRICULTURE
PHOSPHORUS IN REMEDIATION
 THE PHOSPHORUS CYCLE IN...
 REFERENCES
 
The key to understanding the association between phosphate minerals and environmental pollution lies in appreciating their mineralogy and the geological setting of their formation. Apatite is the dominant mineral in phosphate ores. It may occur as carbonate-fluorapatite [Ca5(PO4,CO3,OH)3(OH,F)] in sedimentary rocks and as hydroxyl-fluorapatite [Ca5(PO4)3(OH,F)] in igneous rocks. By virtue of its chemical behaviour, apatite is generally associated with fluoride, which is a potential contaminant. Phosphate rocks also contain metal contaminants, including barium, cadmium and uranium. Little is known about the mineral host of these cations, although sequential extraction studies have compared their release (Pérez-López et al. 2007). The geological conditions that favour accumulation of apatite also favour accumulation of U, Cd and other elements. Sedimentary phosphate rocks contain many mineral impurities (quartz, clays, carbonates, etc.) and are often comparatively rich in organic matter, reflecting their original formation in high productivity zones in the ocean. Uranium is often associated with organic matter in sedimentary phosphate rock (e.g. Stamatakis 2004), but its association with the residual fraction in sequential extraction studies suggests that is it present as an oxide or another stable mineral phase (Eakin and Gize 1992). In general, igneous phosphate rocks have higher P concentrations and lower contaminant concentrations than sedimentary phosphates, reflecting the simplicity of their mineral composition (associated with primary silicate minerals) (FIG. 1). The purpose of adding fertiliser is to provide plants with a readily available source of P, one of the key nutrients limiting plant growth. Industrial phosphate fertilisers generally take the form of soluble phosphate salts. TABLE 1 summarises, in very general terms, the characteristics of a number of phosphate fertilisers and their available P under conditions appropriate for soil application. The dominant chemical phosphate fertilisers are superphosphate and triple superphosphate. Superphosphate is prepared by reacting phosphate rock with sulphuric acid, thereby producing a mixture of calcium phosphate and gypsum. Bearing in mind that hydration of the calcium salts may take place after acid neutralisation, the reaction can be written ideally as:

Formula
Normally, however, commercially produced superphosphate contains approximately 30% Ca(H2PO4)2·H2O mixed with 45% by-product gypsum (CaSO4·2H2O) and dicalcium phosphate (CaHPO4), along with impurities of geological origin, including iron oxides and acid-reacted clays (Roth 2004). In contrast, triple superphosphate is manufactured by reacting phosphoric acid with phosphate rock:

Formula
This material typically contains 80% hydrated calcium hydrogen phosphate salt and 20% impurities, but lacking gypsum, it is a much more concentrated form of soluble calcium phosphate.


Figure 2
View larger version (10K):
[in this window]
[in a new window]

  		FIGURE 1 Comparison of cadmium and phosphorus contents in sedimentary and igneous phosphate rocks. DATA FROM HEFFER ET AL. 2006

 

View this table:
[in this window]
[in a new window]

  		TABLE 1 PHOSPHORUS YIELDS OF TYPICAL PHOSPHATE FERTILISERS UNDER SOIL CONDITIONS (based on Hart et al. 2004)

 


Figure 3
View larger version (11K):
[in this window]
[in a new window]

  		FIGURE 2 Experimentally determined solubilities of sparingly soluble phosphate sources determined after 1 and 2 months as a function of pH (Ahmed 2005). GRP = Gafsa rock phosphate; DCP = dicalcium phosphate

 
The availability of P from various fertiliser sources varies greatly (TABLE 1). The calcium hydrogen phosphate salts are highly soluble, but dicalcium phosphate, struvite and rock phosphate have lower solubilities, which decrease further as pH increases (FIG. 2). Rock phosphate, in particular, is a sparingly soluble, slow-release source of P.

So, what on Earth did plants do before man came along to make their lives easy by supplying soluble chemical fertilisers? Plants had evolved to produce root exudates, typically low-molecular-weight organic acids that corrode the primary mineral nutrient hosts within soils, thus releasing nutrients to the soil solution. Plant root exudates include acetate, oxalate, malate and citrate, organic acid anions known to make short work of feldspars and other silicate minerals (e.g. Manning et al. 1992; Drever and Stillings 1997; van Hees et al. 2002). In general, the availability of P is a key limitation to the ability of a soil to support plant growth, so plants have evolved a number of strategies to obtain the P that they need (Ryan et al. 2001). Many well-known and economically important crops (e.g. lupins and brassicas, including oilseed rape/canola; FIG. 3) produce organic acid root exudates in quantities that amount to 20-40% by mass of the photosynthetically assimilated C. One important mechanism that increases phosphate availability in soils is chelation of aluminium, which occurs as Al3+ in the soil solution of acid soils (Ma et al. 2001). Thus, plants that exude organic acid anions that can complex Al have higher aluminium tolerance. The production of citrate as a component of root exudates increases in response to phosphate stress (i.e. shortage of P) and enhances the availability of P from rock phosphate sources (Neumann and Römheld 1999). The corrosion of feldspars by citrate, well known in the geochemical literature (e.g. Drever and Stillings 1997; Manning et al. 1992), has not yet been linked by plant physiologists to the possible associated benefits of P supply.


Figure 4
View larger version (157K):
[in this window]
[in a new window]

  		FIGURE 3 Oilseed rape - one of the brassicas; many brassicas produce root exudates that facilitate P uptake

 
As alternative agricultural practices develop with the growth of organic farming, `non-chemical' sources of P have had to be identified (Stockdale et al. 2002). `Rock phosphate' is the most widely accepted material used as a mineral source of P. This is sedimentary phosphate rock, mined and exported directly for use in organic farming (e.g. Gafsa rock phosphate from Tunisia). Consistent with regulations that forbid chemical processing, it contains no readily soluble phosphate salts. The availability of P from this source depends entirely on the ability of rhizosphere processes to enhance apatite solubility and requires a sink for Ca if the dissolution reaction is to be promoted (Robinson and Syers 1990):

Formula
Given their low solubility and widespread occurrence in soils, especially in association with fungal hyphae (Burford et al. 2003), the calcium oxalate minerals whewellite and wedellite provide one possible Ca sink (Manning 2000). Similarly, the inherently low stability of organic acid anions within soil solutions means that they decompose ultimately to carbonate, which leads to carbonate mineral precipitation, a volumetrically more important calcium sink (Cerling 1984).

Another alternative source of agricultural P is wastewaters, from which P can be recovered as the poorly soluble mineral struvite, (NH4)MgPO4·6H2O (Gaterell et al. 2000; Parsons and Smith 2008 this issue). Wastewaters and sewage contain high levels of P in dispersed material of biological origin. Certain water-treatment processes (Jaffer et al. 2002) are designed to recover struvite as a crystalline (like granulated sugar) precipitate. Struvite not only acts as a source of P, but contains ammonium and magnesium, thus meeting some other plant nutrient needs. It is also low in the contaminants (trace metals, pathogens, etc.) that make sewage sludge unattractive as a fertiliser. Despite its potential as a crop nutrient source, only one study, involving trials on grass (Johnston and Richards 2003), has been published to test its effectiveness. However, a recent PhD thesis (Ahmed 2005) has shown that when struvite is applied, wheat growth is equivalent to that observed with corresponding applications of conventional chemical fertilisers. The recovery of wastewater phosphate as struvite has the potential not only to reduce effluent phosphate loads, but to provide a `sustainable' source of P that can be handled conveniently when used in agricultural applications.


Figure 5
View larger version (24K):
[in this window]
[in a new window]

  		FIGURE 4 Schematic cartoon to show the relationship between plant-root activity and the soil phosphate cycle

 

   PHOSPHORUS IN REMEDIATION
TOP
 ABSTRACT
 INTRODUCTION
 PHOSPHORUS SOURCES
 PHOSPHORUS IN AGRICULTURE
 PHOSPHORUS IN REMEDIATION
THE PHOSPHORUS CYCLE IN...
 REFERENCES
 
The availability of metal contaminants to biological or aquatic systems often depends on the solubility of the material hosting the contaminant. Lead, for example, is poorly soluble as galena (PbS) or metallic lead (e.g. in piping and roofing materials), but when these oxidize in a soil environment, more-soluble lead sulphate or carbonate minerals can form. Other sulphide-hosted metals show similar behaviour. In these circumstances, addition of phosphate to soil can reduce metal contaminant availability (Chen et al. 2003). Using lead again as an example, the solubility of lead phosphates such as pyromorphite is very low, thus providing a potential remediation strategy (Cotter-Howells 1996; Cao et al. 2003). Both artificial chemicals and natural materials (including meat and bone meal) can be added to soils to provide a source of phosphate. Chen et al. (2003) found that the application of phosphate salts and rock phosphate is equally effective in immobilising lead derived from carbonate sources, as pyromorphite forms, while Sneddon et al. (2006) used bone meal to immobilise lead. Similarly, in a study of metal contaminants in a shooting range, Spuller et al. (2007) found that addition of phosphate immobilised lead, but not antimony or copper, whose availability increased with phosphate treatment.


   THE PHOSPHORUS CYCLE IN SOILS
TOP
 ABSTRACT
 INTRODUCTION
 PHOSPHORUS SOURCES
 PHOSPHORUS IN AGRICULTURE
 PHOSPHORUS IN REMEDIATION
 THE PHOSPHORUS CYCLE IN...
REFERENCES
 
The soil phosphorus cycle can be described as the interaction of plants with both artificially added and naturally present minerals within a soil. FIGURE 4 summarises this interaction, in which plant-root exudation of organic acids, such as citric acid, increases P release from poorly soluble minerals such as feldspars and apatite. This is a major part of the soil-forming process, because it contributes to the decomposition of primary silicates and encourages the formation of pedogenic minerals - carbonates, clays and others. In the process, the bioavailability of potentially contaminating metals is decreased. Al is complexed with citrate, malate, etc., enabling plants to tolerate high levels of soil Al. Other metals, such as lead, are precipitated as insoluble phosphates. This is a system that sustains itself; we can enhance it by adding phosphate fertilisers and other materials to soils to reduce metal availability. With a better understanding of the cycle, we could find new methods for plants to contribute to bioremediation activities. What could be more attractive than a crop of lupins on a metal-contaminated site as a means of converting soluble lead salts to pyromorphite?


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 PHOSPHORUS SOURCES
 PHOSPHORUS IN AGRICULTURE
 PHOSPHORUS IN REMEDIATION
 THE PHOSPHORUS CYCLE IN...
 REFERENCES
 

Ahmed SY (2005) Novel sources of phosphorus for wheat cultivation in the south of Libya. Unpublished PhD thesis, Newcastle University

Burford EP, Fomina M, Gadd GM (2003) Fungal involvement in bioweathering and biotransformation of rocks and minerals.Mineralogical Magazine 67:1127 -1155[Abstract/Free Full Text][CrossRef][Web of Science][GeoRef]

Cao RX, Ma LQ, Chen M, Singh SP, Harris WG (2003) Phosphate-induced metal immobilization in a contaminated site.Environmental Pollution 122:19 -28[CrossRef][GeoRef]

Cerling TE (1984) The stable isotope composition of modern soil carbonate and its relationship to climate. Earth and Planetary Science Letters 71:229 -240[CrossRef][Web of Science][GeoRef]

Cotter-Howells J (1996) Lead phosphate formation in soils. Environmental Pollution 93:9 -16[CrossRef][Medline]

Chen M, Ma LQ, Singh SP, Cao RX, Melamed R (2003) Field demonstration of in situ immobilization of soil Pb using P amendments.Advances in Environmental Research 8:93 -102[CrossRef][Web of Science]

Drever JI, Stillings LL (1997) The role of organic acids in mineral weathering. Colloids and SurfacesA120 : 167-181[CrossRef]

Eakin PA, Gize AP (1992) Reflected light microscopy of uraniferous bitumens. Mineralogical Magazine56 : 85-99[CrossRef][Web of Science][GeoRef]

Gaterell MR, Gay R, Wilson R, Gochin RJ, Lester JN (2000) An economic and environmental evaluation of the opportunities for substituting phosphorus recovered from wastewater treatment works in existing UK fertiliser markets. Environmental Technology 21:1067 -1084

Hart MR, Quin BF, Nguyen ML (2004) Phosphorus runoff from agricultural land and direct fertilizer effects: A review. Journal of Environmental Quality 33:1954 -1972[Abstract/Free Full Text][Web of Science][Medline]

Heffer P, Prud'homme MPR, Muirheid B, Isherwood KF (2006) Phosphorus Fertilisation: Issues and Outlook. International Fertiliser Society Proceedings 586, York, UK,32 pp

Jaffer Y, Clark TA, Pearce P, Parsons SA (2002) Potential phosphorus recovery by struvite formation. Water Research 36:1834 -1842[Medline]

Johnston AE, Richards IR (2003) Effectiveness of different precipitated phosphates as phosphorus sources for plants.Soil Use and Management 19:45 -49[CrossRef][Web of Science]

Kontak DJ, Martin RF, Richard L (1996) Patterns of phosphorus enrichment in alkali feldspar, South Mountain Batholith, Nova Scotia, Canada. European Journal of Mineralogy8 : 805-824[Abstract/Free Full Text][Web of Science][GeoRef]

London D, Cerny P, Loomis JL, Pan JJ (1990) Phosphorus in alkali feldspars of rare-element granitic pegmatites. Canadian Mineralogist 28:771 -786[Web of Science]

Ma JF, Ryan PR, Delhaize E (2001) Aluminium tolerance in plants and the complexing role of organic acids. Trends in Plant Science 6:273 -278[CrossRef][Web of Science][Medline]

Manning DAC (2000) Carbonates and oxalates in sediments and landfill: monitors of death and decay in natural and artificial systems. Journal of the Geological Society157 : 229-238[Abstract/Free Full Text][Web of Science][GeoRef]

Manning DAC, Gestsdóttir K, Rae EIC (1992) Feldspar dissolution in the presence of organic acid anions under diagenetic conditions: an experimental study. Organic Geochemistry19 : 483-492[CrossRef][Web of Science][GeoRef]

Neumann G, Römheld V (1999) Root excretion of carboxylic acids and protons in phosphorus-deficient plants. Plant and Soil 211:121 -130[CrossRef][Web of Science]

Oelkers EH, Valsami-Jones E (2008) Phosphate mineral reactivity and global sustainability. Elements4 : 83-88[Abstract/Free Full Text][CrossRef]

Parsons I, Lee MR, Smith JV (1998) Biochemical evolution II: Origin of life in tubular microstructures on weathered feldspar surfaces. Proceedings of the National Academy of Sciences95 : 15173-15176[Abstract/Free Full Text][CrossRef][Web of Science][Medline]

Parsons SA, Smith JA (2008) Phosphate removal and recovery from municipal wastewaters. Elements4 : 109-112[Abstract/Free Full Text][CrossRef]

Pérez-López R, Álvarez-Valero AM, Nieto JM (2007) Changes in mobility of toxic elements during the production of phosphoric acid in the fertilizer industry of Huelva (SW Spain) and environmental impact of phosphogypsum wastes. Journal of Hazardous Materials 148:745 -750[CrossRef][Web of Science][Medline]

Robinson JS, Syers JK (1990) A critical evaluation of the factors influencing the dissolution of Gafsa phosphate rock.European Journal of Soil Science 41:597 -605[CrossRef][GeoRef]

Roth DL (2004) The Merck Index 13.2 Online. CambridgeSoft Corporation, Cambridge, MA

Rudnick RL, Gao S (2003) The composition of the continental crust. In: Rudnick RL (ed) The Crust, Vol. 3, Treatise on Geochemistry, Elsevier-Pergamon, Oxford, pp.1 -64

Ryan PR, Delhaize E, Jones DL (2001) Function and mechanism of organic anion exudation from plant roots. Annual Review of Plant Physiology and Plant Molecular Biology52 : 527-560[CrossRef][Web of Science][Medline]

Sneddon IR, Orueetxebarria M, Hodson ME, Schofield PF, Valsami-Jones E (2006) Use of bone meal amendments to immobilise Pb, Zn and Cd in soil: A leaching column study. Environmental Pollution 144:816 -825[CrossRef][Medline]

Spuller S, Weigand H, Marb C (2007) Trace metal stabilisation in a shooting range soil: Mobility and phytotoxicity.Journal of Hazardous Materials 141:378 -387[CrossRef][Web of Science][Medline]

Stamatakis MG (2004) Phosphate deposits of Neogene age in Greece. Mineralogy, geochemistry and genetic implications. Chemie der Erde/Geochemistry 64:329 -357

Stockdale EA, Shepherd MA, Fortune S, Cuttle SP (2002) Soil fertility in organic farming systems - fundamentally different?Soil Use and Management 18:301 -308[CrossRef][Web of Science]

van Hees PAW, Lundström US, Mörth C-M (2002) Dissolution of microcline and labradorite in a forest O horizon extract: the effect of naturally occurring organic acids. Chemical Geology189 : 199-211[CrossRef][Web of Science][GeoRef]




This article has been cited by other articles:


Home page
 ELEMENTSHome page
E. H. Oelkers and E. Valsami-Jones
Phosphate Mineral Reactivity and Global Sustainability
Elements, April 1, 2008; 4(2): 83 - 87.
[Abstract] [Full Text] [PDF]

Home page
 ELEMENTSHome page
J. D. Pasteris, B. Wopenka, and E. Valsami-Jones
Bone and Tooth Mineralization: Why Apatite?
Elements, April 1, 2008; 4(2): 97 - 104.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract
Right arrow Figures Only
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (2)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Manning, D. A.C.
Right arrow Search for Related Content
GeoRef
Right arrow GeoRef Citation

JOURNAL HOME HELP CONTACT PUBLISHER SUBSCRIBE ARCHIVE SEARCH TABLE OF CONTENTS
Copyright © 2009 by Mineralogical Society of America