The appropriate and sound utilization of phosphate rocks (PRs) as sources of phosphorus (P) can contribute to worldwide development by fostering sustainable agricultural intensification, particularly in developing countries endowed with indigenous PR resources, besides minimizing pollution in countries where PRs are processed industrially. PR products are an agronomically and economically sound alternative P input to manufactured superphosphates and have the potential to remove the major constraint of low soil P in tropical and subtropical soils. As many developing countries in the tropical and subtropical regions import P fertilizers, the direct utilization of indigenous raw materials such as PRs would contribute not only to import substitution and energy and capital savings but also to fertility improvement of the impoverished soils and control of land degradation.
Although excellent world surveys of PR deposits and occurrences are available, a detailed re-assessment, particularly of those PR deposits that are most suitable for direct application, is highly desirable in view of the dynamic changes in the technology and economics of mining and processing as well as in their potential use in developing countries. The grade of direct application phosphate rock (DAPR) is not a critical factor from a technical standpoint. PRs with less than 20 percent P2O5 have been used on a local basis. However, grade (nutrient content) is an important economic consideration, particularly when evaluating transportation costs.
Advances in standard characterization, methods of evaluation and technologies for enhancing the agronomic effectiveness of PRs will help improve knowledge and management techniques for increasing adoption by farmers. With these scientific and technological advances, it would then be possible to identify specific management practices for effective and economic direct application of PRs with a view to fostering sustainable agricultural intensification in developing countries of the tropics and subtropics, while contributing to improved food security and controlling soil degradation. However, specific technologies need to be developed in each case and more research is needed in order to obtain conclusive results.
Technologies for the direct application of PRs have already been proved under a variety of agro-ecological conditions. However, their agronomic effectiveness depends ultimately on a wide range of factors and their interactions. Therefore, it is important to develop and pilot test decisionsupport systems (DSSs) in order to predict their effective and economic utilization. Further validation and refinement of these systems for more accurate prediction should be an ongoing process. The availability of a global phosphate rock decision-support system (PR-DSS) will be a useful research and extension tool for researchers, extension workers, progressive farmers, planners and agribusiness dealers. It will assist in promoting the use of PR resources in tropical and subtropical developing countries. It is also suggested that PR-DSS should be an integral part of a broader decision system for integrated nutrient management/soil productivity improvement or even of the Decision Support System for Agrotechnology Transfer (DSSAT).
A widespread promotion of DAPR would require: (i) standardized methods and procedures for their characterization and evaluation; (ii) reliable prediction of their agronomic effectiveness; and (iii) predictability of crop yield increases and their profitability. Recent advances in scientific knowledge and technological developments on the utilization of PR need to reach all stakeholders involved in agricultural development. Conventional and advanced information technology, particularly participatory approaches, should be used to disseminate the DAPR technologies to farmers.
It is now known that effective PR sources provide not only P for plant growth but that they may also supply secondary nutrients, such as calcium and magnesium, and micronutrients, such as zinc and molybdenum, depending on the chemical and mineralogical composition of PR. Reactive PRs or PRs containing free carbonates (calcite and dolomite) can also raise soil pH to partially reduce aluminium (Al) saturation of acid soils and decrease Al toxicity to plant growth, although their liming effect is generally lower than lime. One environmental issue regarding PR applications has been the potential uptake by plants of elements that are harmful to human health. Limited studies suggest that the uptake of toxic heavy metals, notably Cd, by plants from PR is significantly lower than that from water-soluble P fertilizers produced from the same PR. Moreover, a PR source with a higher reactivity and Cd content can release more Cd than a PR with a lower reactivity and/or low Cd content for plant uptake. In addition to PR reactivity and Cd content, Cd uptake by plants also depends on soil properties, especially soil pH, and crop species.
In view of the fact that very limited information is available in the literature regarding secondary nutrients, micronutrients, liming effect, and hazardous elements associated with PR application, it is hoped that future research on PR will expand to include these areas so that the benefits and risks associated with PR use in soils and plants can be assessed more precisely.
The bulk of the worlds PR production is utilized for P fertilizer manufacturing, thus only limited information on legislation for DAPR is available in literature. Regulations for the direct application of PRs should consider three main factors: PR reactivity (solubility), soil properties (mainly soil pH) and crop species. All current legislation on PR for direct application considers the quality of PR, namely: total P2O5 content, particle size distribution, and solubility. Issues associated with solubility measurements are complex and need careful consideration. It is difficult to develop universal legislation that all countries can adopt. Nevertheless, legislation to be adopted by a country or region should be based on and upgraded according to recent scientific findings in PR research. The guidelines proposed in this publication should provide useful information for establishing and revising legislation regarding PR for direct application.