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This publication deals with the direct application of phosphate rock (PR) sources to agriculture. Phosphorus (P) is an essential plant nutrient and its deficiency restricts crop yields severely. 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 plant growth and adequate food and fibre production.

Manufactured water-soluble P fertilizers such as superphosphates are commonly recommended to correct P deficiencies, but 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 countries necessitates the addition of P not only to increase crop production but also to improve soil P status in order to avoid further soil degradation. Hence, it is imperative to explore alternative P sources. Under certain soil and climate conditions, the direct application of PR, especially where available locally, has proved to be an agronomically and economically sound alternative to the more expensive superphosphates. PR deposits occur worldwide, but few are mined (for use mainly as raw materials to manufacture water-soluble P fertilizers).

The Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture initiated a Coordinated Research Project called "The use of nuclear and related techniques for evaluating the agronomic effectiveness of phosphatic fertilizers, in particular rock phosphates". This was implemented by institutes of developing and industrialized countries from 1993 to 1998. The results obtained yielded new information on: chemistry of soil P; tests for available soil P; phosphate nutrition of crops; agronomic effectiveness of PR products; and P fertilizer recommendations with particular emphasis on PR use.

Within the framework of the integrated plant nutrition systems promoted by the Land and Water Development Division (AGL), FAO, and the national action plans for soil productivity improvement under the Soil Fertility Initiative for sub-Saharan countries, PRs are considered as potentially important locally available P sources. 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 studies.

Several organizations have conducted extensive research on the utilization of indigenous PR deposits in tropical soils in Latin America, Africa, Asia and elsewhere. In the past decade, considerable progress has been made in the utilization of PR sources for direct application in agricultural cropping systems worldwide. A wealth of information is now available but scattered in several publications.

Recognizing the need for the wider dissemination of the available information, the Joint FAO/ IAEA Division of Nuclear Techniques in Food and Agriculture together with the AGL convened a Consultants’ Meeting in Vienna in November 2001 in order to review advances in this field of research and development, and to elaborate a proposal for the production of this technical bulletin. Mr F. Zapata (IAEA, Vienna) and Mr R.N. Roy (FAO, Rome) implemented this task. Specialists in the sector were invited to contribute to the chapters of the publication.

This publication presents the results of studies on the utilization of PR products for direct application in agriculture under a wide range of agro-ecological conditions with a view to fostering sustainable agricultural intensification in developing countries of the tropics and subtropics. The subject matter is covered in 12 chapters, contributed by a team of scientists involved in PR research and working in a wide range of disciplines including geology, chemistry, soil science, agronomy and economics, etc. In addition, the publication includes a comprehensive bibliography.

Chapter 1 provides background information on: P as an essential plant nutrient; soil P status; the need for the application of P fertilizers; and the potential for using PRs in the context of food security, soil degradation and environmental protection. It describes generalities on the nature and variability of PR sources worldwide and provides an overview of past and current work and of the prospects for the utilization of PR products for direct application in agriculture. Chapter 2 deals with the type and distribution of PR geological deposits worldwide. It provides information on: inventory of reserves; PR production; and PR consumption for direct application. Chapter 3 describes the characterization of PR products in view of their wide variability for utilization as a raw material for the manufacture of P fertilizers and for direct application in agriculture. This includes information on: mineralogical composition of the phosphate-bearing minerals (main and accessory), in particular, the crystallographic features and empirical formulae of the apatite minerals; chemical composition (main chemical elements including micro-elements, heavy metals and radionuclides); solubility indices as indicators of reactivity; and physical properties (particle-size distribution, specific surface area, and hardness). Chapter 4 examines the approaches and methodologies for evaluating PR for direct application, such as solubility tests, soil incubation, greenhouse tests and field evaluation. For each approach, the chapter provides an overview of the objectives, methods and measurements with examples. It also provides a brief description of the advantages and limitations of the methods, and special considerations for interpreting results. Chapter 5 provides an up-to-date review of the main factors that affect the agronomic effectiveness of PR products. Selected case studies provide region-level examples of the influence of these factors and their interactions, and economic guidelines. Chapter 6 describes soil P testing methods for providing recommendations for PR application. It discusses principles and considerations for interpreting the results of ‘available P’. Chapter 7 focuses on the need for and the development and testing of a decision-support system to predict the agronomic effectiveness of PR for direct application, together with the use of P modelling. Chapter 8 examines current knowledge, and gaps therein, on the effects of other elements present in PR products. It covers beneficial and hazardous elements, such as calcium and magnesium, micronutrients, heavy metals and radionuclides. Chapter 9 describes approaches and technologies for improving the agronomic effectiveness of indigenous PRs. Chapter 10 examines the economic criteria to be considered in the production, marketing and distribution of PRs. It discusses macroeconomic and microeconomic issues, and includes case studies on the exploitation of PR deposits. Chapter 11 provides an overview of current regulations on PR for direct application. Legislative guidelines are proposed for consideration. Chapter 12 is an epilogue containing the main conclusions and issues for further consideration.

The aim of this publication is to provide the most up-to-date information on the direct application of PR in agriculture. It endeavours to do this in a technically focused document that leads on to practical guidelines to assist middle-level decision-makers, the scientific community, higherlevel extension workers, non-governmental organizations and other stakeholders involved in agricultural development. The ultimate goal is to maximize technology dissemination in a targeted way, particularly for policy-makers at all levels who may require information on the adoption of PRs as a capital investment to trigger sustainable agricultural intensification in the acidic soils of the tropics and subtropics.

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