In order to ensure food security in developing countries, there is a need for the sustainable intensification of agricultural production systems towards supporting productivity gains and income generation. In this context, novel, soil-specific technologies will have to be developed, pilot tested and transferred to farmers in a relatively short time. Phosphorus (P) is an essential nutrient element for plants and animals. The appropriate and sound utilization of phosphate rocks (PRs) as P sources can contribute to sustainable agricultural intensification, particularly in developing countries endowed with PR resources.
PR is a general term used to describe phosphate-bearing minerals. PR is a finite, nonrenewable natural resource. Geological deposits of different origin are found throughout the world. Currently, few PR deposits are mined, and about 90 percent of world PR production is utilized by the fertilizer industry to manufacture P fertilizers, with the remainder being used to manufacture of animal feeds, detergents and chemicals. However, many PR deposits located in the tropics and subtropics have not been developed. One reason is that the characteristics of these PRs, though suitable for direct application, do not meet the quality standards required for producing water-soluble P (WSP) fertilizers using conventional industrial processing technology. Another reason is that the deposits are too small to warrant the investment needed for mining and processing. On a worldwide basis, there is ample supply of high-quality PRs for chemical processing and direct application for the foreseeable future.
PR is the primary raw material for producing P fertilizers. The phosphate compound in PRs is some form of the mineral apatite. Depending on the origin of the PR deposit and its geological history, the apatites may have widely differing chemical and crystallographic characteristics and physical properties. Distinctive suites of accessory minerals are also associated with PRs of various origins and geological histories. It is imperative to establish simple procedures for the standard characterization of PR sources, define quality standards for their direct application and categorize them. Well-known PR sources may be adopted as reference standards for comparison purposes.
The mineralogical, chemical and textural characteristics of phosphate ores and concentrates determine: (i) their suitability for various types of beneficiation processes to upgrade the ores and remove impurities; (ii) their adaptability for chemical processing by various routes; and (iii) their suitability for use as direct application phosphate rock (DAPR). The most important factors in the assessment for direct application are: grade, suitability for beneficiation, and the reactivity of the apatite. A comprehensive characterization matrix based on the integration of all the data obtained by various analytical methods brings out the beneficiation potential and probable best uses for a PR in fertilizer processing and as a direct application fertilizer.
There are various methods for evaluating PRs for direct application. The first approach uses empirical solubility tests with the PR being dissolved in chemical extracting solutions. The most common solutions are neutral ammonium citrate, 2-percent citric acid, and 2-percent formic acid, with the last option being preferred. The particle size of the PR and associated minerals in the PR can influence the result of the solubility test. Radioisotopic techniques can be used but they require skilled staff and special laboratory facilities. Soil incubation studies to evaluate PRs are relatively simple. However, closed incubation studies have limitations as reaction products are not removed and the results would have limited utility unless used as short-term observations. Greenhouse experiments are valuable in that they enable the performance of the PR to be evaluated under controlled conditions. However, the final evaluation of the agronomic effectiveness of PR sources in a network of field experiments conducted over a number of growing seasons at representative sites in the agro-ecological regions of potential interest is essential. A series of recommendations for undertaking such evaluation have been developed. Such evaluation is also required for the assessment of the economic potential of PR deposits for domestic consumption.
Factors that influence the agronomic effectiveness of PRs are: the reactivity of the PR, soil properties, climate conditions, crop species and management practices. High carbonate substitution for phosphate in the apatite crystal structure, low content of calcium carbonate as accessory mineral and fine particle size (less than 0.15 mm) enhance the reactivity of PRs and their agronomic effectiveness. Rapid chemical tests are available to measure the reactivity of PRs. Increasing soil acidity, high cation exchange capacity, low levels of calcium (Ca) and phosphate in solution and high organic matter content favour PR dissolution. High phosphate retention capacity of soils may facilitate dissolution of PR but the availability of dissolved P will depend on the concentration of phosphate maintained in solution. Soils of medium phosphate status are considered to be more suitable for PR application than severely phosphate-deficient soils. Increasing rainfall invariably results in improved agronomic effectiveness of PRs. High agronomic effectiveness of PRs may be realized with perennial and plantation crops and also with legumes. To achieve maximum agronomic effectiveness, the PRs should be incorporated into the soil. While only a maintenance rate of P application need be applied as PR where the soil phosphate status is medium or above, very high rates of application are required for severely phosphate-deficient soils. Application time can be nearer to planting in very acid soils (pH < 5.5) and 4-8 weeks before planting in less acid soils.
The potential for using local PRs for direct application varies for each country as the complexity of the interactions occurring between specific local factors is evident in the tropical and subtropical world. Countries with substantial reactive PR reserves, such as Mali, Madagascar and Indonesia, offer considerable scope for the direct application of PRs. In countries endowed with less reactive PR reserves, such as Venezuela and Brazil, it is feasible to modify the PRs in order to improve their performance. In some cases, the economic advantage to the farmer can be considerable, as evidenced in the example on Venezuela. The Indian case study reveals import replacement as the major economic benefit resulting from the use of local Mussoorie PR. The New Zealand case study illustrates a success story on the use of reactive phosphate rocks (RPRs) for pastoral agriculture in Oceania.
Conventional soil tests for available P estimation are used for formulating fertilizer recommendations for the application of WSP sources. However, these common soil tests can underestimate (e.g. Bray I and Olsen) or overestimate (e.g. Bray II and Mehlich I) P availability in soils fertilized with water-insoluble PR. Separate calibration curves are needed for these two types of P fertilizer. Thus, the issue of developing suitable soil P testing methods for PR application brings a new dimension to phosphate research. There is need to develop appropriate soil tests that reflect P availability closely across a wide range of soil properties, PR sources and crop genotypes. Furthermore, soil tests should be suitable for both PRs and WSP fertilizers. Two recently developed soil tests show promise in soils fertilized with WSP and with PR sources. These are the iron-oxide-impregnated filter-paper strip test and the mixed cation-anion exchange resin membrane test. In both cases, the P extracted simulates P sorption by plant roots without the chemical reaction involved in the conventional tests. Available P measured in soils treated with both PR and WSP sources has shown: (i) good correlation with plant response; and (ii) both PR and WSP sources follow the same calibration curve. Further field trials are needed in order to test their suitability for developing recommendations for DAPR.
In order to provide sound guidelines for DAPR, it is essential to predict their agronomic effectiveness, crop yield increases and profitability. Several factors affect PR dissolution in soil under a specified set of conditions. DAPR as a P fertilizer is a more complex issue that involves consideration of a number of factors and their interactions. The development of a decision-support system (DSS) is the most effective approach for integrating all these factors and for providing an effective means of transferring research results to extension services. DSSs have been constructed to utilize available information to predict whether a given PR will be effective in a given crop environment. This publication discusses different types of DSSs, including the approaches adopted to develop a PR-DSS in New Zealand and Australia. It describes joint efforts currently underway by the International Centre for Soil Fertility and Agricultural Development (IFDC) and the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture (FAO/IAEA) to develop a more global DSS for use in tropical and subtropical countries for a range of food crops. Moreover, the PR-DSS could be linked to the phosphate model of the Decision Support System for Agrotechnology Transfer (DSSAT) family to predict crop yields, and GIS-based maps of crop response to PRs could also be compiled.
Phosphate-bearing minerals show a complex structure as a result of their geological origin and weathering processes. Their mineralogical and chemical composition is extremely variable. DAPR studies have focused mainly on the use of PR as a P source for crop production in acid soils, while limited research has been conducted on the effect of other beneficial and hazardous elements associated with PR use. Available information has suggested that PRs may also have potential agronomic value by provide some secondary nutrients, such as Ca and magnesium, and micronutrients, such as zinc and molybdenum, for plant growth depending on the type of PR sources. Reactive PRs containing free carbonates (calcite and dolomite) have been shown to reduce aluminium (Al) saturation of acid soils by raising soil pH and hence decreasing Al toxicity to plants. Limited studies also suggest that plant uptake of toxic heavy metals, notably cadmium, from PR is significantly lower than from WSP fertilizers produced from the same PR. More research is needed to investigate secondary nutrients, micronutrients, liming effect, and hazardous elements associated with PR use.
Not all PR sources are suitable for direct application. However, it is possible to utilize several means to improve their agronomic effectiveness under a particular set of condition. Selecting the appropriate process requires a good understanding of the factors hindering the agronomic effectiveness. The biological means (e.g. phospho-composting, inoculation with vesiculararbuscular mycorrhizae, use of phosphate solubilizing micro-organisms, and use of P-efficient plant genotypes) are based on the production of organic acids to enhance PR dissolution and P availability to plants, and they hold good promise. The use of chemical means to produce partially acidulated PR (PAPR) is the most effective way of increasing the agronomic effectiveness of PRs and also saves energy. However, the production of PAPRs still requires fertilizer manufacturing plants. The physical means of dry mixing PR with a defined proportion of WSP fertilizer is promising and cost-effective. This simple approach should be promoted and evaluated under local soil and climate conditions.
Fertilizer legislation exists in many countries, especially developing ones. Its aim is to ensure that the fertilizer qualities meet the specifications set by the government in order to protect consumers interests. As the bulk of PR production is utilized for WSP fertilizer manufacturing, only limited information on the legislation for DAPR is available. The agronomic effectiveness of PR for direct application depends on several factors and their specific interactions. Regulations for DAPR should consider three main factors: PR reactivity (solubility), soil properties (mainly soil pH) and crop species. Current legislation on PR for direct application considers only the quality of the PR, namely: total phosphorus pentoxide (P2O5) content, particle size distribution, and solubility. Based on recent research findings, this publication proposes several guidelines relating to issues on PR solubility and their interactions in a complex soil-plant system and concerning the establishment and revision of existing legislation.
In addition to technical considerations, a number of socio-economic factors and policy issues will determine PR production, distribution, adoption and use by the farmers. In principle, the use of PR sources should be promoted in countries where they are indigenously available. In countries with no PR deposits, it would be advisable to have a wide range of PRs available on the market in order to encourage price competition.
In conclusion, the appropriate and sound direct application of PR can contribute significantly towards sustainable agricultural intensification using natural plant nutrient resources. Although there has recently been substantial progress in scientific knowledge and technological developments on DAPR, further exploration of the specific issues is imperative.