The Potential Use of Rock Phosphate in Sudan for Sustainable Agricultural Production

Inaam Saad

• Publisher: Page Publishing, Inc.
• Year: 2022
• Print ISBN: 9781662461217
• eBook ISBN: 9781662461224
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

Agriculture is primarily the uppermost important sector in Sudan, contributing significantly to the country's national economy. However, in spite of the country's vast cultivated area, productivity remains very low. The declining agricultural production is indicative of the need for adopting sustainable agricultural development approaches to the achieve country's food security. The attainment of sustainable agricultural production will require the proper use of all available resources such as land, water, and mineral resources, including the phosphate rock reserves.

Rock phosphate deposits of potential economic value occur at more than a hundred locations in sub-Saharan African countries, including Sudan. Several geological surveys in Sudan have proven the occurrence of abundant resources of phosphate deposits in different locations in the country.

For advocating the initiation of rock phosphate mining and phosphate fertilizer industry in Sudan, this book delivers sufficient and validated information on the underutilized resource of rock phosphate in Sudan and exposes its potential role in the development of sustainable agricultural production. The book presents an accurate image of the current status and prospective scenario in relation to rock phosphate deposits and the potential establishment of the fertilizer industry in Sudan.

The book presents a comprehensive literature review on the history, advantages, and impacts of using rock phosphate. It also sheds light on the rock phosphate deposits in Sudan, their locations, and geological and agricultural quality as compared to rock phosphate from other global resources. The book presents two detailed case studies on assessing the agronomic value of Kurun and Uro rock phosphate rocks as fertilizers and their environmental impact assessment. The book then highlights the potential benefits of phosphate mining and the initiation of the phosphate fertilizer industry in Sudan, proposes a plan for achieving that goal, and presents recommendations to bring the Sudanese phosphate fertilizer industry to life.

Chapter 1

Introduction

Agriculture is the fundamental economic activity in Sudan, the country has almost 17 million hectares of cultivated land, and the potentially arable area is much larger. In the past decades, agricultural production significantly contributed to Sudan’s national economy, it generated up to 40% of the gross domestic product (GDP) and contributed about 80% of the country’s exports. The agricultural sector employed about 80% of the working force in the country (Elgali 2010). Recently, there is a steady decline in agricultural contribution to the country’s GDP over the years; it had dropped to about 36% in 1988 from 40% in the early 1970s. Another study indicated a decline to 28% during the mid-1980s and early 1990s (Abbadi and Ahmed 2006), which may be attributed to the declining agricultural productivity.

Agricultural productivity is usually measured as the ratio of agricultural outputs to agricultural inputs (land, water, and workforce). The declining agricultural production highlights the need for adopting sustainable agricultural development approach in Sudan. The goal of sustainable agriculture is to meet society’s need for food and other agricultural products in the present day without sacrificing the ability of the coming generations to obtain their own same necessities. Sustainable agriculture is also the means for the appropriate consumption, imitation, and replenishing of the essential agricultural resources that include soil fertility. The soil is one of the most vital resource constituents of the agricultural production cycle, which is consistently altered by agronomy and other farming systems, whereby nutrients are being repetitively removed from and added to the soil.

Soil fertility is significantly manipulated by the soil formation process and the nature of the parent rock because soils are formed originally by the weathering of primary rocks and generally bear the geochemical fingerprints of their parent rock material. The parent rock contains most of the plant-required nutrients in variable concentrations and these nutrients become available for plant uptake upon soil formation (Christophe et al. 1990). Thus, the management fertility of agricultural soils and their nutrients content levels are among the most significant practices in achieving sustainable agricultural production.

Soils at the most important agricultural production regions in Sudan have moderate to low fertility levels and are subjected to an unceasing loss of fertility and continuous physical degradation as a result of erosion, overgrazing, deforestation, inappropriate agricultural practices, and desertification. Almost 30% of the country’s area is currently classified as desert, which subjects Sudan’s arable land to a significant risk of additional depreciation, depletion of essential nutrients, and consequently the loss of soil fertility and loss of productive capability of the land.

Attaining sustainable agricultural production goal in Sudan will require an innovative development and application of an integrated nutrient management approach which will imply the use of chemical fertilizers and other 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 1995). Implementing these approaches requires the assessment of the nutrient supply from the locally available material resources and the establishment of guidelines for their application (FAO 1989).

Due to the great role of phosphorus (P) in soil fertility and plant nutrition, many agronomists and soil scientists in Sudan have acknowledged the harmful effect of deficiencies of phosphorus on crop production and made significant efforts to estimating the phosphorus requirement of the major food, revenue crops, and fodders (Ageeb and Abdalla 1988, Eltahir 1999, Ibrahim & Adlan 1989). A great deal of research and studies have assessed the agronomic effectiveness of various phosphate fertilizers, including rock phosphate from the local deposits (FAO 2016).

Since the United Nations Ukrainian Fertilizer Seminar held in Kiev (1965), it became evident that guidelines for evaluating and establishing chemical fertilizer projects would be of practical help, particularly to the developing countries concerned with increasing their food supplies. The recommendation of the seminar was that the potential fertilizer value of local raw materials must be studied and improved to achieve the maximum economic benefits of natural resources (UN Fertilizer Industry Series 1968). The purpose of fertilizer use is to remove the limitation of crop production that would be caused by an inadequate supply of nutrients in the soil. Fertilizer application helps in achieving high crop production and reducing the hazards of food inadequacy (Kukkola 1989).

There are many commercially available phosphate fertilizers, such as rock phosphate, phosphoric acid, calcium orthophosphates, ammonium phosphates, ammonium polyphosphate, single superphosphate (SSP), triple superphosphate (TSP), and composed nitrogen-phosphorus-potassium (NPK). Researchers in Sudan recommend the use of phosphate fertilizers to correct P deficiencies.

Phosphate rock (PR), or rock phosphate, is usually defined as the product obtained from the mining and subsequent metallurgical processing of phosphate-bearing ores. PR usually contains a high concentration of P. In addition to the main phosphate mineral, PR deposits also contain other minerals. Such minerals include silica, clay minerals, calcite, dolomite, and hydrated oxides of iron (Fe) and aluminum (Al) in various combinations and concentrations (UNIDO & IFDC 1998). The phosphorus content or grade of PR is conventionally expressed as phosphorus pentoxide (P2O5). The higher the P2O5 content, the greater the economic potential of the rock. Based on the phosphate content, PRs can be used either as raw materials in the industrial manufacturing of WSP fertilizers or for direct application as a source of P in agriculture (FAO 1995, Oldeman 1994, United Nations Environmental Program [UNEP] 2000).

A major study commenced by the International Fertilizer Development Center (1982) did assess the performance of ground rock phosphate in Africa and suggested that some of these rock phosphates are suitable for direct application to revise soil phosphorus deficiencies. Reports from many other countries including Australia, Brazil, India, and Venezuela indicated the usefulness of the direct application of rock phosphate. Brinkmann (1986) discovered rock phosphate in western Sudan (Nuba Mountains). Other information from the Sudanese Geological Research Corporation ascertained the presence of great quantities of rock phosphate in the Nuba Mountains area and other locations. A feasibility study managed by the United Nations Environment Program (1989) had examined the quality of the rock phosphate from the Nuba Mountains and indicated valuable qualities and promising performance of these rock phosphates.

A great deal of chemical analysis reports, greenhouse experimentations, and field trials have proven the significant agronomic value of the two types of rock phosphate obtained from Kurun and Uro in the Nuba Mountains in South Kordofan (Saad 1993). It is evidently proven that the P content of these two rocks types is equivalent to the content of most of the PRs used for the production of phosphate fertilizers both regionally and globally.

Based on the information and research findings published by Saad (1993), which were supported by numerous subsequent studies and research conducted on the agronomic evaluation, technical and economic feasibility studies on the use of rock phosphate in Sudan (Abd Elhafeez 2001, Eltilib et al. 2003), the information gathered by Saad (1993) indicated the great potential and competence of Sudan to embark on a large-scale phosphate fertilizer industry, which will lead toward self-sufficiency and contribute significantly to the progress of the country’s national economy.

The book consists of seven chapters. The first one comprises the introduction and the general ideas presented in the book. The second chapter presents a comprehensive literature review covering all of the book’s topics about the history, advantages, and impacts of using rock phosphate. Chapter 3 sheds light on the rock phosphate deposits in Sudan, their locations, their geological and mineral composition, and their quality as compared to rock phosphate from other global resources. Chapter 4 presents a detailed case study on assessing the agronomic value of Kurun and Uro phosphate rocks as fertilizers. The picture would not be completed without taking into account the impact on the environment and human health; therefore, chapter 5 presents the detailed outcome of an environmental impact assessment study on the use of Kurun and Uro rock phosphate. The theme of chapter 6 is to highlight the potential benefits of phosphate mining and the initiation of the phosphate fertilizer industry in Sudan and propose a plan for achieving that goal. Part one of the last chapter presents a summary of data, facts, and findings in the book; and the second part presents suggestions and recommendations to bring the Sudan phosphate fertilizer industry to life.

Chapter 2

Rock Phosphate Nature and Importance

Phosphorus is an important macronutrient for living matter and approaches nitrogen in importance; it occurs in nature almost entirely as phosphate in both organic and inorganic forms (Griffith et al. 1973). Phosphorus has both structural and metabolic functions, being a constituent of nucleic acids. While the inorganic ion has a key role in cell metabolism, energy is transferred in the metabolic reactions (Bieleski and Ferguson 1983). Phosphorus input is crucial for food production, and all plants need an adequate supply of it for successful growth. Phosphorus deficiency will result in a reduction in crop yield. Primary minerals that contain phosphate are widespread, and they are dominated by the apatite group containing complex calcium phosphate, rare minerals such as monazite occur in rocks, and others associated with metal ores such as A1 (varcite) and Fe (strengite) phosphate (Stewart and Sharpley 1987).

Phosphate can be found in the lattice of A1-Fe silicate clay minerals, and in this form, phosphate is irreversibly fixed (Vencent 1965). Although phosphate salts are soluble in acids, the apatite minerals weathered only slowly and reserves in the soil are low (Bieleski and Ferguson 1983). Some of the released phosphates may react with other weathering products, e.g., A1, Fe, and Cu to form secondary phosphate, depending on the pH and temperature (Stewart and Sharpley 1987). Other phosphates may become absorbed or fixed on the mineral surface of clays and hydrated Fe and A1 oxides in soil, hence, how and when phosphorus becomes available to plants (Vencent 1965).

Phosphorus is widely distributed in nature and occurs, together with N and potassium (K), as a primary component of plant and animal life. It serves a series of functions in 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.

Phosphorus is absorbed mainly during vegetative growth, and thereafter, most of the absorbed P is retranslocated 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 color (higher chlorophyll concentration) and reddish coloration (enhanced anthocyanin formation). It has been reported that the level of P supply during reproductive stages regulates photosynthate partitioning between the source leaves and the reproductive organs; this effect is 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, 1995). This nutrient is absorbed by plants from the soil solution as monovalent (H2PO4) and divalent (HPO4) orthophosphate anions, each representing 50% of total P at a nearly neutral pH (pH 6–7). At pH 4–6, H2PO4 is almost 100% of the total P in the solution. At pH 8, H2PO4 represents 20%, and HPO4 represents 80% of total P (Black 1968).

The physicochemistry of P in most mineral soils is rather complex owing to the occurrence of a series of instantaneous and simultaneous reactions, such as dissolution, precipitation, sorption, and oxidation-reduction. P-soluble compounds have very high reactivity, low solubility indices, and low mobility.

Under continuous cultivation of the arable land, P inputs, in particular water-soluble fertilizers, must be added to either maintain the 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, and so on are important factors to consider when attempting to formulate sound P-fertilizer recommendations and obtain adequate crop yield responses (FAO 1984, 1995).

2.1 Phosphate Rock Nature and Importance

Phosphate rock (PRs) or rock phosphate is an international term that describes any naturally occurring geological material that contains a high concentration of one or more phosphate-bearing minerals that are suitable for agricultural and/or the production of commercial phosphate fertilizers. The various phosphate minerals present in PRs have diverse origins and chemical and physical properties. The phosphorus content or grade of PRs is commonly reported as phosphorus pentoxide (P2O5). The principal phosphate minerals in phosphate rock are Ca-phosphates, mainly apatite (FAO 2006). Mined phosphate rock is usually defined as the product obtained from the mining and subsequent metallurgical processing of phosphate-bearing ores and primarily used in the production of phosphate fertilizers, besides other numerous usages and applications in agricu...

Table of contents

• Chapter 1
• Chapter 2
• Chapter 3
• Chapter 4
• Chapter 5
• Chapter 6
• Chapter 7