Phosphorus depletion – should the ACP countries be concerned? What are the current issues for the future research and policy?
P.O. Kisinyo, W.K. Ng’etich, C.O. Othieno, J.R. Okalebo and W.R. OpileDepartment of Soil Science, Chepkoilel University College, PO Box 1125-30100, Eldoret, Kenya.Contact Email: email@example.com
Phosphorus (P) is an essential element for plant and animal nutrition (FAO, 2004) and the second-most limiting nutrient after nitrogen (N) for plant growth and crop production in many agricultural areas in the tropics (Kamprath, 1984; Baligar and Fageria, 1997; Bekunda et al., 1997). Extensive tracts of land in the tropical and subtropical regions of Africa, Asia and Latin America contain highly weathered and inherently infertile soils. These areas experience low crop yields, and are prone to land degradation as a result of deforestation, overgrazing and inappropriate farming practices. In addition to socio-economic factors, the main constraints are soil acidity and inherently low N and P levels (Lal, 1990; Formoso and Cerri, 1999). Acid soils represent about 40% of the total arable land area in the world, being mostly found in the tropical and subtropical regions (Haug, 1984). They constitute about 68, 38 and 27% of the soils in Tropical America, Tropical Asia and Tropical Africa, respectively (Pandey and Gardener, 1992). Soils of these regions are extremely P-deficient, partly as a result of high P sorption associated with high levels of aluminium (Al) and iron (Fe) oxides. Therefore, substantial P inputs are required for optimum growth and adequate food and fibre production (Sanchez and Buol, 1975; Date et al., 1995).
Phosphorus is highly reactive with air and other oxygen–containing substances; therefore it is not found free in nature but it is widely distributed in many different minerals in the soil. While N inputs can be supplemented from sources such as biological nitrogen fixation, there is no biological replenishment of P. This means inputs need to be applied in order to improve the soil P status and ensure normal plant growth and adequate yields. Currently, the global food supply is dependent on continual inputs of artificial phosphate fertilizer to maintain soil fertility and to compensate for uptake with the harvested crops. In the developing world, farmers are becoming increasingly aware that use of inorganic fertilizer increases crop production (Sanchez et al., 1997; Okalebo et al., 2007). Given the demand for food to feed the growing human population, there is going to be more demand for P fertilizers.
Scientific relevance of phosphorous resources to crop production
Phosphorus exists in various mineral forms in the soil. Phosphate rock (PR), which is partially made of apatite (an impure tri-calcium phosphate mineral), is an important commercial source because of the high concentration of P minerals it contains (van Kauwenberg, 2006). While deposits of PR have been discovered in many parts of the world, the ones that account for most of world PR production are in Morocco and other African countries, the USA, the Near East and China. Approximately 90% of all the PR that is mined is used for food production in fertilizers, feed and food additives (Cordell, 2008b). Phosphate rocks can be used either as raw materials in the industrial manufacture of water-soluble phosphate (WSP) fertilizers or as P sources for direct application in agriculture. Experiments have shown that direct use of PRs was highly effective when applied to plantation crops in acid soils of the tropics. Under conditions where suboptimal yields were expected because of limits on additional inputs, i.e. extensive pasture systems, local PR was thought to be a possible suitable source of P (FAO, 2004). Direct use of PR has shown benefits similar to those achieved with WSP fertilizers (Waigwa et al., 2003; Kifuko et al., 2007; Opala et al., 2007). However, about 82% of PR mined is used for the production of WSP fertilizers (Prud’Homme, 2010).
An appropriate utilization of PR can contribute to sustainable agricultural intensification, particularly in developing countries endowed with these resources (FAO, 2004). However, there are various challenges as far as utilization of PR sources is concerned. There is no clear information on how long the existing PR deposits will last and this is needed for proper planning for their utilization. A report by Vaccari (2009) indicated that the PR reserves in the USA will be depleted within the next 40 years. Steen (1998) estimated that economically exploitable reserves could be depleted within 60-130 years. Cordell (2008a) suggests that this could occur in 50-100 years. In contrast, the recent report by the International Fertilizer Development Cooperation (IFDC) estimates that there are sufficient PR concentrate reserves to produce P fertilizers for the next 300-400 years (van Kauwenbergh, 2010). In a comparable way to fossil fuel, control of P resources is in the hands of a limited number of countries. Most of the known reserves are in Morocco, the USA and China. China, however, has put an export tariff on phosphate recently (Cordell, 2008a). These reports also suggest that estimates are not comprehensive, as they do not include deposits in all countries.
Opportunities to exploit and to extend the lifespan of the diminishing PRs
There are a number of opportunities that can be exploited to extend the lifespan of the existing PR deposits, such as use of renewable P sources, integrated soil fertility management and use of crop germplasm with high P-use efficiency. One of the potential renewable sources of P is human excreta (urine and faeces), which represents a readily available form of P, provided safety measures and perception issues are addressed. Urine is the largest single source of recycling P from human excreta. It contains P, N and potassium in the correct ratios for soil fertilization. The large populations of urban areas in developing countries can produce substantial amounts of excreta. Studies in Zimbabwe and Sweden revealed that the nutrients emerging in one person’s urine are sufficient to produce 50-100% of the food requirement for another person (Cordell, 2008b). There are already existing guidelines focused on minimising health risk on collection, treatment and utilization of urine and faeces in crop production, which can be used to increase crop production (EcoSanRes, 2008). Many studies have focused on the use of excreta with appropriate controls to ensure food safety and public health (Heinonen-Tanski and Wijk-Sijbesma, 2005; Phuc et al., 2006; Cheng et al., 2008; Jensen et al., 2008; Vinnerås et al., 2008; Itchon et al., 2009; Winker et al., 2009). Researchers and policy-makers should be encouraged to improve on the existing guidelines on excreta utilization and overcome barriers regarding the negative perception associated with the use of human excreta in soil fertility replenishment.
Integrated soil fertility management should be enhanced to reduce the rates of inorganic P fertilizers used. Such technologies include combined use of inorganic P with organic (food wastes and manures) and liming materials (see box). Organic and/or liming materials improve P availability through reduction in soil P sorption and increase desorption of already-fixed P in acid soils. This allows high levels of crop production in such soils (Zhiguheba et al., 1998; Kifuko et al., 2007; Opala et al., 2010; Kisinyo, 2011). Also, use of plant materials high in P as green manure can reduce the amount of inorganic P resources required by farmers. One such plant is Tithonia diversifolia, which is quite promising (Ikerra et al., 2007; Opala et al., 2007; Bationo, 2008). Use of these materials will definitely reduce the amounts of P fertilizer applied and help save the diminishing PR resources.
Most ACP countries have acid soils with high P sorption; as a result, they need to use crop germplasm that is tolerant to aluminium (Al) toxicity and P-use efficient. Maize breeding programmes have developed germplasm tolerant to Al toxicity and low P (Dowswell et al., 1996; Parentoni et al., 2006). These genotypes have high P-use efficiency, even from sparingly soluble P forms. Their high P-use efficiency is related to their potential to enhance microbial colonization and symbiosis with P-solubilizing microorganisms in the rhizosphere (Oliveira et al., 2006). Such developments need to be extended to other crops grown. Also, discovery of new PR deposits is necessary to increase food production in these regions. Geologists Sheldon and Davidson pointed out 20 years ago that the rate of new resource discovery had been consistent over the 20th century (1987). They also suggested that tropical regions with deep soils had been inadequately explored: these regions occupy 22% of the earth’s land surface but contain only 2% of the known P reserves (Vaccari, 2009). Further analysis of the extent of P reserves in these areas should be undertaken.
Phosphorus deficiency limits crop production in many acid soils of the African, Caribbean and Pacific countries, mainly because of its fixation and inherently low P in soils of these regions. There is potential to use PR and existing and emerging organic sources in improving P availability, and thus improve crop production. In addition, using crop germplasm that is tolerant to aluminium (Al) toxicity and is P-use efficient is important. However, the contradictory information on how long the existing PR deposits will last makes it difficult to plan for its long-term utilization.
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