Phosphorus (P) is an element that is widely distributed in nature and occurs, together with nitrogen (N) and potassium (K), as a primary constituent of plant and animal life. P plays a series of functions in the plant metabolism and is one of the essential nutrients required for plant growth and development. It has functions of a structural nature in macromolecules such as nucleic acids and of energy transfer in metabolic pathways of biosynthesis and degradation. Unlike nitrate and sulphate, phosphate is not reduced in plants but remains in its highest oxidized form (Marschner, 1993).
P is absorbed mainly during the vegetative growth and, thereafter, most of the absorbed P is re-translocated into fruits and seeds during reproductive stages. P-deficient plants exhibit retarded growth (reduced cell and leaf expansion, respiration and photosynthesis), and often a dark green colour (higher chlorophyll concentration) and reddish coloration (enhanced anthocyanin formation). It has been reported that the level of P supply during reproductive stages regulates the partitioning of photosynthates between the source leaves and the reproductive organs, this effect being essential for N-fixing legumes (Marschner, 1993). Healthy animals and human beings also require adequate amounts of P in their food for normal metabolic processes (FAO, 1984, 1995a).
This nutrient is absorbed by plants from the soil solution as monovalent (H2PO4) and divalent (HPO4) orthophosphate anions, each representing 50 percent of total P at a nearly neutral pH (pH 6-7). At pH 4-6, H2PO4 is about 100 percent of total P in solution. At pH 8, H2PO4 represents 20 percent and HPO4 80 percent of total P (Black, 1968).
The physico-chemistry of P in most mineral soils is rather complex owing to the occurrence of series of instantaneous and simultaneous reactions such as dissolution, precipitation, sorption and oxidation-reduction. The P-soluble compounds have very high reactivity, low solubility indices and low mobility. Mineralization and immobilization of organic P compounds are relevant processes for P cycling in soils containing significant amounts of organic matter (Black, 1968; FAO, 1984).
Where a water-soluble P (WSP) fertilizer is applied to the soil, it reacts rapidly with the soil compounds. The resulting products are sparingly soluble P compounds and P adsorbed on soil colloidal particles (FAO, 1984). A low P concentration in the soil solution is usually adequate for normal plant growth. For example, Fox and Kamprath (1970) and Barber (1995) suggested that 0.2 ppm P was adequate for optimum growth. However, for plants to absorb the total amounts of P required to produce good yields, the P concentration of the soil solution in contact with the roots requires continuous renewal during the growth cycle.
Under continuous cultivation, P inputs, in particular water-soluble fertilizers, must be added to either maintain the soil P status of fertile soils or increase that of soils with inherent low P fertility. Therefore, soil, crop, water, P-fertilizer management practices, climate conditions, etc. are important factors to be considered when attempting to formulate sound P-fertilizer recommendations and obtain adequate crop yield responses (FAO, 1984, 1995a).
The present world population of 6 000 million is expected to reach 8 000 million by 2020 and 9 400 million by 2050. By then, the population of the developing world will probably be 8 200 million (Lal, 2000). Approximately 50 percent of the potentially arable land is currently under arable and permanent crops. A further 2 000 million ha has been degraded and land degradation continues through a wide range of processes, mainly related to mismanagement by humankind (Oldeman, 1994; FAO, 1995b; UNEP, 2000).
Against this global background, several developing countries will face major challenges in achieving sustainable food security. This is because of their available per capita land area, severe scarcity of freshwater resources, particular socio-economic conditions of their agriculture sector, and internal structures and conflicts (Hulse, 1995).
Enhancing sustainable food production will require a proper use of the available land and water resources, i.e.: (i) agricultural intensification on the best arable land; (ii) adequate utilization of marginal lands; and (iii) prevention and restoration of soil degradation.
In order to increase the intensification, diversification and specialization of agricultural production systems towards supporting productivity gains and income generation, innovative soil-specific technologies will have to be developed, pilot tested and transferred in a relatively short time. These technologies will address priority issues such as: (i) enhancing cropping intensity by exploiting genotype differences in adaptation to particular environments and nutrient use efficiency; (ii) increasing nutrient use efficiency and recycling through integrated nutrient sources management in cropping systems; (iii) soil and water conservation through crop residue management and conservation tillage; and (iv) improving water use efficiency through the development of efficient methods of irrigation, water harvesting and recycling (Lal, 2000).
In preventing and reversing soil degradation, the main sustainability issues will concern controlling soil erosion and associated sedimentation and risks of eutrophication of surface water and contamination of groundwater (UNEP, 2000). Similarly, enhancing soil carbon sequestration in cropland to improve soil quality and productivity and mitigate the greenhouse effect will also be a matter of concern (Lal, 1999).
The agricultural frontier is likely to expand into marginal lands with harsh environments that contain fragile soils with a lower productive capacity and a higher risk of degradation. The use of plant genotypes with adequate yield potential, efficient in nutrient use and tolerant to soil and environmental stresses (drought, acidity, salinity, frost, etc.) will be of strategic importance. Their use is becoming increasingly important in many international and national breeding programmes (Date et al., 1995; Pessarakli, 1999). This approach is currently being utilized for the sustainable management of P-deficient acid soils (Rao et al., 1999; Hocking et al., 2000; IAEA, 2000; Keerthisinghe et al., 2001).
The development and application of an integrated nutrient management approach in the agriculture of developing countries will imply the use of chemical fertilizers and natural sources of nutrients, such as phosphate rocks (PRs), biological nitrogen fixation (BNF), and animal and green manures, in combination with the recycling of crop residues (FAO, 1995a). The utilization of these technologies requires the assessment of the nutrient supply from the locally available materials applied as nutrient sources, their tailoring to specific cropping systems and the provision of guidelines for their application (FAO, 1998; Chalk et al., 2002). This is particularly the case of indigenous PR resources in the tropics.
Extensive tracts of land in the tropical and subtropical regions of Asia, Africa and Latin America contain highly weathered and inherently infertile soils. These areas generate low crop yields and are prone to land degradation as a result of deforestation, overgrazing and inadequate farming practices. In addition to socio-economic factors, the main constraints are soil acidity and low inherent N and P fertility (Lal, 1990; Formoso, 1999). While N inputs can be obtained from sources such as BNF, crop residues and other organic sources, P inputs need to be applied in order to improve the soil P status and ensure normal plant growth and adequate yields. Tropical and subtropical soils are predominantly acidic and often extremely P deficient with high P sorption (fixation) capacities. Therefore, substantial P inputs are required for optimum growth and adequate food and fibre production (Sanchez and Buol, 1975; Date et al., 1995).
Manufactured WSP fertilizers such as superphosphates are commonly recommended for correcting P deficiencies. However, most developing countries import these fertilizers, which are often in limited supply and represent a major outlay for resource-poor farmers. In addition, intensification of agricultural production in these regions necessitates the addition of P inputs not only to increase crop production but also to improve soil P status in order to avoid further soil degradation. Therefore, it is imperative to explore alternative P inputs. In this context, under certain soil and climate conditions, the direct application of PRs is an agronomic and economically sound alternative to the more expensive superphosphates in the tropics (Chien and Hammond, 1978; Truong et al., 1978; Zapata et al., 1986; Hammond et al., 1986b; Chien and Hammond, 1989; Chien et al., 1990b; Sale and Mokwunye, 1993).
Phosphate rock denotes the product obtained from the mining and subsequent metallurgical processing of P-bearing ores. In addition to the main phosphate-bearing mineral, PR deposits also contain accessory or gangue minerals and impurities. Although considerable amounts of accessory minerals and impurities are removed during beneficiation, the beneficiated ore still contains some of the original impurities. Such impurities include silica, clay minerals, calcite, dolomite, and hydrated oxides of iron (Fe) and aluminium (Al) in various combinations and concentrations, some of which may have a marked influence on the performance of a PR used for direct application (UNIDO and IFDC, 1998). Thus, currently PR is the trade name of about 300 phosphates of different qualities in the world (Hammond and Day, 1992).
PRs can be used either as raw materials in the industrial manufacture of WSP fertilizers or as P sources for direct application in agriculture.
The world phosphate industry is based on the commercial exploitation of some PR deposits. In spite of their extremely variable composition, PRs are the commercial source of P used as the raw material for manufacturing P fertilizers and certain other chemicals. Unlike other vital commodities, such as Fe, copper (Cu) and sulphur (S), there is little opportunity for substitution or recycling. Phosphate ranks second (coal and hydrocarbons excluded) in terms of gross tonnage and volume of international trade.
The fertilizer industry consumes about 90 percent of world PR production. Sulphuric acid and PR are the raw materials used in the production of single superphosphate (SSP) and phosphoric acid. Phosphoric acid is an important intermediate by-product that is used to make triple superphosphate (TSP) and ammonium phosphate. Highly concentrated, compound NPK formulations now form the mainstay of the world fertilizer industry (Engelstad and Hellums, 1993; UNIDO and IFDC, 1998).
PR is also used for industrial purposes and for the production of animal feed supplements and food products. Another important use is in the manufacture of elemental P and its derivatives, in particular sodium tri-polyphosphate, a major component of heavy-duty laundry detergents (Hammond and Day, 1992; UNIDO and IFDC, 1998).
PR deposits are widely distributed throughout the world, both geographically and geologically, and there are very large resources capable of meeting anticipated demand for the foreseeable future. Estimates generally indicate a total of 200 000-300 000 million tonnes of PR of all grades. Of these total estimates, a large proportion includes deposits composed of carbonate-rich PR, whose commercial exploitation depends either on the development of new beneficiation technology or on changes in economic conditions (British Sulphur Corporation Limited, 1987; Notholt et al., 1989).
About 80 percent of world PR production is derived from deposits of sedimentary marine origin, some 17 percent is derived from igneous rocks and their weathering derivatives, and the remainder comes from residual sedimentary and guano-type deposits.
Sedimentary PRs are mainly composed of apatites. These apatites exhibit extensive isomorphic substitution in the crystal lattice. Thus, they have a wide variation in chemical composition and accordingly show a wide range of properties. In sedimentary deposits, the main phosphate minerals are francolites (microcrystalline carbonate fluorapatites), which occur in association with a variety of accessory minerals and impurities (McClellan and Van Kauwenbergh, 1990a).
The phosphate content or grade of PR is conventionally expressed as phosphorus pentoxide (P2O5). In some low-grade commercial deposits, this may be as low as 4 percent. In the phosphate industry, the phosphate content of the rock is usually expressed as tricalcium phosphate and traditionally referred to as bone phosphate of lime (P2O5 × 2.1853 = BPL). The latter term is reminiscent of the time when bones were the principal source of phosphate in the fertilizer industry. Manufacturers of phosphoric acid and P fertilizers normally stipulate a minimum content of 28 percent P2O5, and most marketed grades of PR contain more than 30 percent P2O5 (65 percent BPL). To meet this requirement, most phosphate ores undergo beneficiation by washing and screening, de-liming, magnetic separation and flotation (Hammond and Day, 1992; UNIDO and IFDC, 1998).
As mentioned above, PRs mainly of sedimentary origin are suitable for direct application because they consist of fairly open, loosely consolidated aggregates of microcrystals with a relatively large specific surface area. They show a considerable proportion of isomorphic substitution in the crystal lattice and contain a variable proportion and amounts of accessory minerals and impurities. Thus, it is reported that these PRs are suitable for direct application to soils under certain conditions (Khasawneh and Doll, 1978; Chien, 1992; Chien and Friesen, 1992; Chien and Van Kauwenbergh, 1992; Chien and Menon, 1995b; Rajan et al., 1996; Zapata, 2003).
The practice of direct application of PR sources as fertilizers has several advantages:
PRs are natural minerals requiring minimum metallurgical processing. The direct application of PRs avoids the traditional wet acidification process for WSP fertilizers and circumvents the production cycle of polluting wastes such as phospho-gypsum and greenhouse gases, thus resulting in energy conservation and protection of environment from industrial pollution.
Being natural compounds, PRs can be used in organic agriculture.
Direct application enables the utilization of PR sources that cannot be utilized for industrial purposes in the manufacture of WSP fertilizers and phosphoric acid.
PRs suitable for direct application (reactive) can be more efficient than WSP fertilizers in terms of P recovery by plants under certain conditions.
Based on the unit cost of P, natural or indigenous PR is usually the cheapest.
Because of their extremely variable and complex chemical composition, PRs are sources of several nutrients other than P. They are usually applied to replenish the soil P status, but upon dissolution they also provide other nutrients present in the PR. Application of medium and highly reactive PRs to highly weathered tropical acid soils has a potential trigger effect on plant growth and crop yields as a result not only of P release but also of their effects on increasing exchangeable calcium (Ca) and reducing Al saturation. The resulting harvest products and residues have a better nutritional quality (higher P content than unfertilized plants). The incorporation of such organic residues enhances biological activity and soil carbon (C) accumulation, leading to improved soil physical and chemical properties. Thus, PRs have an important role in contributing to improving soil fertility and soil degradation control, in particular nutrient mining (depletion).
However, this practice also has some limitations:
Not all PRs are suitable for direct application. The effectiveness of some medium-to-low reactive PRs needs to be enhanced by biological and physico-chemical processes. Specific technologies must be developed and tested and their economics must be assessed on a case-by-case basis.
Not all soils and cropping systems are suitable for PRs of different origin. A standardized characterization of PRs is required as are guidelines for providing them.
There is a lack of knowledge on the main factors and conditions affecting the agronomic effectiveness of PRs and an inability to predict their effectiveness as well as a lack of assessment of socio-economic factors, financial benefits and government policies. In this respect, progress is being made in the development of decision-support systems (DSSs) for integrating all the factors that influence the use and adoption of PR technology.
The low grade of some PRs compared with high-grade commercial P fertilizers makes them more expensive at the point of application. This economic evaluation is very dynamic and it should be made at the time of exploitation of the PR deposit.
Sedimentary PRs show a very complex structure as a result of their different origin in nature and even within a particular geological deposit. Thus, they have extremely variable chemical constituents and may contain elements such as heavy metals and even radionuclides that upon dissolution of the PR in the soil may be harmful at some concentrations.
Several PR research projects have recently made considerable progress in addressing the issues mentioned above.
The direct application of ground, natural PR as a source of P for crops is a practice that has been utilized with varying degrees of popularity over the years. Numerous field and greenhouse experiments have been conducted during the past 100 years or more to assess the capabilities of these materials to supply P to crops and to define the most favourable conditions for their application. The results obtained have been reported as erratic and sometimes conflicting, leading to confusion and disagreement on the utilization of PRs (Khasawneh and Doll, 1978).
In general, experiments conducted in the past showed that PRs were most effective when applied to plantation crops in acid soils of the tropics. Moreover, under conditions where suboptimal yields were expected because of limits on additional inputs, i.e. extensive pasture systems, local PRs were thought to be a possible suitable source of P. However, no conclusive evidence could be obtained either for or against their adoption in the majority of cases.
The main reason for this situation was the lack of understanding of the various factors affecting the agronomic effectiveness of PRs. Since then, significant progress has been made on the evaluation of the main factors affecting their agronomic effectiveness. A milestone work on this topic was the comprehensive review by Khasawneh and Doll (1998). They examined the influence of the inherent PR factors (mineralogy, chemical composition, solubility tests and physical properties), soil factors (pH, soil texture, soil organic matter, soil P status, available P, P fixation, Ca content, etc.) and plant factors (growth cycle, P demand and pattern of P uptake, root system, rhizosphere properties, etc.).
Hammond et al. (1986b) conducted an extensive review on the agronomic value of indigenous PR sources located in the tropics, highlighting the potential use of partially acidulated products (PAPR) throughout Latin America, Africa and Asia. More recently, an updated review focusing on the fundamentals of PR dissolution in soils, the concepts of agronomic effectiveness of PRs and approaches to assessing the economics of PR utilization was conducted by Rajan et al. (1996).
National projects on the use of PRs for pastures in different environments have been implemented for more than 50 years in New Zealand and Australia (Bolan et al., 1990; Rajan, 1991a, 1991b; Bolland et al., 1997). Field trials with reactive PRs (RPRs) were carried out in New Zealand in the early 1980s (Hedley and Bolan, 1997; 2003), and in the period 1991-96 in Australia with implementation of the National Reactive PR Project (Simpson et al., 1997; Sale et al., 1997a). In both countries, significant progress was made in determining the soil, climate and pasture conditions under which RPR products are effective substitutes for WSP fertilizers and in defining the level of reactivity required for RPRs to be effective.
Regional networks in Latin America (Red Latinoamericana de Roca Fosforica - RELARF) and in Asia (East and Southeast Asia Program of the Potash and Phosphate Institute of Canada) have also been operational for some time in the tropics and subtropics of these regions. They have held periodical meetings to report on their research results (Dahayanake et al., 1995; Hellums, 1995a; Johnston and Syers, 1996; Casanova and Lopez Perez, 1991; Casanova, 1995, 1998; RELARF, 1996; Zapata et al., 1994; Besoain et al., 1999). A number of studies have also been conducted in almost all countries of Africa, but the results are scattered in many individual reports of limited circulation. Truong et al. (1978) carried out a comprehensive study with several PR sources from West Africa. Proceedings of regional meetings on the use of fertilizers and local mineral resources for sustainable agriculture in Africa have been published (Mokwunye and Vlek, 1986; Gerner and Mokwunye, 1995). Reports synthesizing results from PR studies have recently been produced. A report by FAO (2001b) presents the results from field agronomic trials in West Africa while Appleton (2001) has provided a comprehensive review of local phosphate resources in sub-Saharan Africa in the context of sustainable development. In addition, several international meetings organized periodically by the Institut Mondiale du Phosphate (IMPHOS) have provided an international forum for reporting and exchanging information on phosphate research (IMPHOS, 1983, 1992).
In the 1970s, the International Centre for Soil Fertility and Agricultural Development (IFDC) began to conduct research with a focus on the use of indigenous PR deposits as a source of P for crop production in developing countries. This PR research has been an important component of the IFDCs programme for many years (Chien and Hammond, 1978, 1989; Chien et al., 1987b; Chien and Friesen, 1992; Hellums et al., 1990; Hellums, 1992; Chien, 1995; Chien and Menon, 1995b). This research has also included the development and evaluation of modified PR products as well their economic assessment (Hammond et al., 1986b; Menon and Chien, 1990, 1996; Hellums et al., 1992; Chien and Menon, 1995a; Baanante, 1998; Baanante and Hellums, 1998; Henao and Baanante, 1999). In the 1990s, the IFDC commenced research on environmental issues associated with PRs and P fertilizers because they contain varying amounts of potentially hazardous elements such as heavy metals (Hellums, 1995b; Iretskaya et al., 1998; Iretskaya and Chien, 1999).
In 1994, the World Bank, in consultation with centres operated by the Consultative Group on International Agricultural Research (CGIAR) and university research groups from industrialized countries, launched the initiative Development of National Strategies for Soil Fertility Recapitalization in sub-Saharan Africa (World Bank, 1994; Valencia et al., 1994; Buresh et al., 1997; Baanante, 1998). One of the outcomes was the document Framework for National Soil Fertility Improvement Action Plans. This initiative included the use of PR as a capital investment in natural resources in Africa, where the situation is paradoxical because the soils are extremely poor in P in spite of the existence of numerous PR deposits. It was postulated that a one-time massive application of PR rock would overcome the soil P-sorption capacity problem and replenish the soil P capital. Further small applications of water-soluble fertilizers would be then more available to crops and of increased efficiency. The World Bank mandated several research organizations to conduct case studies in Burkina Faso, Madagascar and Zimbabwe. Although the results from these case studies suggested that the available PR resources in these countries could be used as capital investment to replenish soil P status, they were not conclusive owing to the lack of a comprehensive evaluation of the factors that influence the use and adoption of PR technology in each country (World Bank, 1997).
Within the framework of the Integrated Plant Nutrition Systems promoted by the Land and Water Development Division (AGL) of FAO and the national action plans of the Soil Fertility Initiative (SFI) for sub-Saharan countries, PRs are considered as important potential locally available P inputs that can be used gainfully (FAO, 2001a). The AGL has instituted several studies on the agro-economic assessment of PRs for direct application in selected countries. Results of practical utility and policy guidelines can be drawn from these and other studies.
In the period 1993-99, the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture implemented a research network comprising 21 institutions of developing and developed countries with the aim of evaluating the agronomic effectiveness of P fertilizers in particular PRs using nuclear and related techniques. The data obtained have demonstrated the potential of PRs to improve soil fertility and increase agricultural production under certain conditions. The results have been published in IAEA documents and scientific journals (Zapata, 1995, 2000, 2002, 2003; IAEA, 2000, 2002). Important follow-up activities of this project are a joint FAO/IAEA-IFDC undertaking to develop a DSS for the direct application of PRs and the preparation of a Web site and technical publications for the wider dissemination of results to professional technical staff, and decision-makers, including extension specialists and progressive farmers (Chien et al., 1999; Heng, 2000, 2003; Singh et al., 2003).
In view of recent developments and practical experiences on technologies for the direct application of PR and related appropriate technology, the IFDC, in collaboration with the Malaysian Society of Soil Science, the Potash and Phosphate Institute and the Potash and Phosphate Institute of Canada - East and Southeast Asia Program, organized an international meeting in Kuala Lumpur. The event attracted more than 100 participants from more than 30 countries from all over the world representing different national and international research networks on PR, producers, dealers and users of PR for direct application. The latest agronomic research results on the use of natural PRs and modified products as influenced by sources of PR, types of soil, management practices and cropping systems were reviewed, and updated information on the production and agronomic use of PR was gathered from the PR producers, dealers and users. The event also provided an international forum for discussing future trends on the use of indigenous or imported PRs for direct application to increase crop production and lower production costs (IFDC, 2003).
In conclusion, extensive research on the agronomic potential and actual effectiveness of PRs as sources of P has been carried out in Africa, Asia, Latin America and elsewhere. A wealth of information is available but it is scattered in several publications of meetings, technical reports, scientific and other publications. Overall, information on DAPR is limited and there remain areas and topics associated with DAPR where further attention is needed.
From the sections above, it may be also inferred that there is a need for a comprehensive publication to cover the key topics dealing with the utilization of PRs in agriculture, including the latest information on PR research, and to provide guidelines for the direct application of PR to the acid soils of the tropics and subtropics. This bulletin is an attempt to meet this need. It is conceived as a technically oriented document for a target audience comprising policy- and decision-makers, the scientific community, higher level extension workers, non-governmental organizations (NGOs) and other stakeholders involved in sustainable agricultural development at local, national, regional and international levels.
The chapters in this bulletin provide an overview of the scientific basis for PR use, and they present technical information on the most relevant issues related to the use of PR sources for direct application. They provide: comprehensive coverage of world PR deposits; characterization of PR sources; evaluation methodologies of PR sources for direct application; analysis of the biophysical and farming factors that affect the agronomic effectiveness of PR sources; and also analysis of the socio-economic conditions and other factors that ultimately influence the use and adoption of PR technologies as a capital investment to trigger agricultural intensification. The chapters also cover: development and use of DSSs for DAPR; soil P testing for PR application; available technologies for enhancing the agronomic effectiveness of indigenous PR sources; environmental issues; and legislation guidelines. Finally, in the light of current knowledge and available technologies, future researchable areas and priorities are defined in an epilogue. The bibliography section provides a comprehensive list of references.