Indian Journal of Fertilisers, Vol. 13 (7), pp.14-23 (10 pages)
Management of Phosphorus for Sustainable Crop Productivity and Environmental Quality H. Pathak1, T.J. Purakayastha2, Smita Barkataky2, Rajendra Kumar Yadav2 and Pooja Pande2 1 ICAR – National Rice Research Institute, Cuttack, Bhubaneswar, Odisha 2 Division of Soil Science and Agricultural Chemistry, ICAR - Indian Agricultural Research Institute, New Delhi Abstract Phosphorus (P) is the energy currency of plants due to its role in adenosine triphosphate (ATP) synthesis. There is a strong need to evolve innovative management strategies for enhancing P use efficiency in view of large dependency on import of rock phosphate and phosphatic fertilisers, higher cost and inherently low P use efficiency. Phosphate rock is main raw material for phosphatic fertiliser industry; the reserves of this non-renewable natural resource have been declining rapidly due to excessive mining. Countries like Jordan, Egypt, Peru and Morocco are the largest exporters of phosphate rock and India is among the largest importers of the same. More than 70% of P produced is used as a fertiliser and only a small portion of applied P fertiliser is taken up by plant system, with remaining getting fixed in the soil as calcium phosphate, iron phosphate and aluminium phosphate, depending upon the pH of the soil and reactive soil constituents. Environmental implications of fertiliser P include its role in eutrophication of water bodies and possible contamination of soil-plant system with heavy metals. Key words : Phosphorus, P use efficiency, phosphate rocks, calcium phosphate, iron phosphate, aluminium phosphate, eutrophication
Introduction Phosphorus (P) is one of the 17 essential nutrients and is the primary nutrient along with nitrogen (N) and potassium (K). It influences plant growth by helping plants in effectively utilizing sugar and starch, process of photosynthesis, cell division and nucleus formation. Phosphate compounds act as storehouse of energy produced during the process of photosynthesis and carbohydrate metabolism. Adequate P results in rapid growth and early maturity; it also enhances the quality of vegetative crop growth (McKenzie and Middleton, 2013). Globally, the sources of P include geological sedimentary rocks. In 2011 the extent of mining was to the tune of 191 Mt (Scholtz and Wellmer, 2013), which corresponds to 25 Tg P yr-1. Most of the P mined is added to agricultural soils as P-fertiliser (18 Tg P yr -1 ; Heffer and Prud’Homme, 2010) and a small quantity used in the industry especially for the manufacture of detergents and in food industry. Overall, 82% P is required for manufacture of fertilisers; 7% as a
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nutrient in feedstock; and 11% in pharmaceuticals and industry. India has deposits of only 306 Tg of all types and grades of phosphorite, which account for 0.19% of the world’s resources. Much of Indian phosphate rocks are low-grade P having less than 2530% P 2O 5 and are not suitable for manufacture of phosphatic fertiliser. India is mostly dependent on imports to an extent of 90% for meeting its domestic P requirements; country imports rock phosphate, phosphoric acid and also P-fertilisers from Jordan, Morocco, Egypt, Peru, Senegal, USA, Russia, Saudi Arabia and China. Phosphorus deficiencies in Indian soils are widespread and to sustain the enhanced agricultural production, country had to go in for increased consumption of Pfertiliser. The P 2O 5 consumption increased from 0.053 million tonnes (Mt) in 1960-61 to 6.98 Mt in 2015-16 (FAI, 2016) and is expected to rise further to 14.0 Mt by 20302031. Since burgeoning demand is sending the prices of phosphate rock upwards globally, India has to spend large amounts of its valuable foreign exchange for import of Pfertiliser. Vast tracts of croplands Indian Journal of Fertilisers, July 2017 14
in the country have been experiencing physico-chemical and nutrient imbalances, leading to the abysmally low efficiency of applied P-fertiliser. Keeping added P in soil solution for prolonged periods and solubilization of fixed P are essential pre-requisites for making P positionally available to the plants. While use of microorganisms has shown promise, their impact has not been uniform on a large scale. Enhancing P use efficiency and preventing P from ending up as a pollutant in our aquatic resources, both freshwater and marine, continue to be the major challenge. Phosphorus is highly reactive and 13th most abundant element in the Earth crust. It is a main pillar of deoxyribose nucleic acid (DNA) and adenosine triphosphate (ATP). Besides its use as a fertiliser nutrient, P finds uses in detergent, cleaning agent making and industry (e.g., match box industries). More than 70% of the phosphorus produced is used as fertiliser. Only a small portion of the applied fertiliser-P is taken up by plant system; depending upon the soil pH most of the applied P reverts to sparingly soluble
compounds like calcium, iron and aluminium phosphates. Phosphorus availability is maximum in the pH range of 6.5 to 7.3. Phosphatic fertiliser, derived from the sedimentary rock phosphate and apatite ores (essentially geological reserves), besides being P source, also contains heavy metals [cadmium (Cd), lead (Pb), nickel (Ni), cobalt (Co) and chromium (Cr)], and radioactive elements (like 238U). Addition of the metal-containing P-fertiliser to the soil leads to the accretion of these heavy metals in soil, which over a period of time may accumulate in the soil or contaminate groundwater through run off or leaching. Another important problem associated with excess Pfertiliser use is that when the unutilized P-fertiliser is transported to surface water bodies like lakes, ponds and reservoirs, it leads to eutrophication - the process of natural aging of freshwater bodies brought on by nutrient enrichment (Shigaki et al., 2006). Excess P promotes the growth of aquatic algae, which kill the aquatic organisms by producing cyanotoxin. Rigorous growth of aquatic algae and other organisms depletes the oxygen available in water bodies and creates oxygenstarvation condition, which leads to death and decay of algae and other aquatic organisms producing foul smell conditions. So need of the hour is to improve the availability and reduce the fixation of phosphorus in soil by following 4R principle, modification of plant root morphology and root physiology, and employing microbes in dissolution of fixed P and converting it into the plant utilizable form. Sources and Distribution of Phosphorus Sedimentary rock phosphate can include loads of heavy metals, toxic elements and other precious elements in geological reserves. Its distribution is highly clustered in the world. Most of the rock phosphate in the world is widely distributed as phosphorite
deposits in marine ecosystem. Phosphorite is a non-detrital sedimentary rock, which contains higher amounts of phosphatebearing minerals. Totally 71 billion tonnes (71 Pg) of phosphorus is available for economic exploitation with current technology out of 300 Pg of phosphorus reserves available in the world. High clustering of (documented) phosphate rock reserves is noticed only in few countries, particularly in Morocco (78 to 85% of global P reserves). Out of 296.3 million tonnes (Mt) of rock phosphate deposits in India, Jharkhand (107.3 Mt), Rajasthan (88.0 Mt) and Madhya Pradesh (49.4 Mt), account for most of the P reserves (Anonymous, 2011). Currently economically exploitable P reserves are present only in Rajasthan and Madhya Pradesh. Apatite is an important group of phosphate-bearing minerals, usually an admixture of tricalcium phosphate and calcium hydroxide [3Ca 3(PO 4) 2.Ca(OH) 2 – hydroxyapatite], calcium carbonate [3Ca 3 (PO 4 ) 2 .CaCO 3 – carbonate apatite], [3Ca 3(PO 4) 2.CaCl 2 – chloroapatite] and [3Ca 3 (PO 4 ) 2 .CaF 2 – fluoroapatite] depending upon the dominance of hydroxyl (OH - ), carbonate (CO32-), chloride (Cl-) or fluoride (F -) ions in the solution during precipitation. About 24.23 Mt of apatite mineral is present in West Bengal (one of the leading states having apatite mineral in India) (Subba Rao et al., 2015). Phosphorus Fertility Status of the Indian Soils According to the systematic soil fertility map of Indian soils developed in 1967, nearly 4% of the soils were high in available P (Ramamurthy and Bajaj, 1969), which increased to 20% in 2002 (Motsara, 2002). Phosphorus fertility status varies across the states; however, soils of most of the Indian states fall in low to moderate class (Hasan, 1996; Pathak, 2010; Singh and Mishra, 2012). Hasan (1996) reported that nearly 49% of the districts of the country were low, 49% medium Indian Journal of Fertilisers, July 2017 15
and 2% high in available P. Comparison of the above results with a similar survey by Ghosh and Hasan (1979) indicated 3% increase in low P fertility class, while medium and high categories exhibited a decrease of 2.7 and 0.3%; respectively. Pathak (2010) reported that the overall phosphorus fertility status in India had shown reduction during last 25 years. The GIS-based district-wise soil fertility maps of India, generated by the Indian Institute of Soil Science showed that the soils of about 51% districts were low, 40% medium and 9% were high in available P (Muralidharudu et al., 2011). Almost 80% soils in India are either low or medium in P fertility categories, which reinforces the need for P application in the form of organic manures, green manures and phosphatic fertilisers to maximize the crop yields. Agricultural crops’ fertilisation represents the greatest (more than 80%) use of phosphorus in India. Phosphorus exist in soil, both in organic and inorganic forms. Total P content ranges from 120 to 2,166 mg kg -1 soil. In most of the soils, inorganic P generally contributes 54-84% of the total P; organic P contributes the rest (Sanyal et al., 2015). Transformation and Abundance of Phosphorus in Soil In natural environments, biological availability of phosphorus is determined by the abundance of metal ions such as Ca 2+, Fe 3+ and Mg 2+, which cause phosphorus to precipitate at pH values above 7.0. The P-content in average soil is about 0.05% (w/w) but only 0.1% of the total P is available to plants (Scheffer and Schachtschabel, 1992). As only orthophosphate ions (H2PO4- and HPO42- depending on the pH) can be assimilated in appreciable amounts (Beever and Burns, 1980), the free inorganic P ions in soil play a central role in Pcycling and plant nutrition. Global distribution of soil “phosphorus retention potential” is based on an integrated model of variables (e.g.,
soil temperature and vegetation cover) that influence phosphorus solubility and precipitation and consequently its availability. However, large amount of the fertiliser-P added to the soil, which is not assimilated by plants or not retained by the soil eventually ends up in rivers, lakes through surface run off or leaching leading to eutrophication. It has been estimated that human activities have amplified rates of P cycling globally by ~400% relative to pre-industrial time, far more than those for carbon (~13%) or even nitrogen (~100%). To achieve P sustainability, farms need to become more efficient in the manner of using P while society as a whole must develop technologies and practices to recycle it. Phosphorus exists in soil both in inorganic (35 to 70%) and organic (30% to 65%) forms. It exhibits differential behaviour in the soil (Hansen et al., 2004; Turner et al., 2007; Shen et al., 2011). Phosphorus is always associated with oxygen (Thomason, 2002) as the phosphate anion; PO43– ion reacts with cations like H+, Na+, K+, Ca2+, Fe3+ and Al3+ (Goh et al., 2013). With H+, it forms phosphoric acid (H 3PO 4) and the orthophosphate anions (H2PO4 - and HPO42–). The relative distribution of these three ions is dependent on the soil pH (Lindsay, 2001). The orthophosphate forms (H 2PO4- and HPO42–) are soluble in the pH range of 5 to 9 and plants take up both these anions. Hinsinger (2001) reported that solubility of Fe and Al phosphates increases with rise in soil pH but solubility of Caphosphate decreases. But when pH exceeds 8, solubility of Ca phosphate also increases. Once applied to agricultural soils, P moves through the agricultural and animal production systems and can accumulate in the agricultural soils at an estimated rate of 12 Tg P yr-1 (Bouwman et al., 2009). Given an estimated fullchain nutrient use efficiency for phosphorus (NUE P ) of 12-20% (Bennett et al., 2001), around 3-5 Tg P yr-1 is excreted through human excreta and the quantity of P reaching the rivers by sewage
systems amounts to 1-3 Tg P yr -1 (van Drecht et al., 2009). Of the rest, the major losses occur in the steps from mining to preparation of mineral fertiliser and other P products and from mineral fertilisers to crop and livestock production. Once in surface continental water, part of P is trapped in sediments and about 9 Tg P yr -1 is delivered to coastal waters where it can contribute to eutrophication leading to localized hypoxic and anoxic conditions. Phosphorus Use Efficiency Phosphorus is the second most limiting nutrient after N in most of the soils and is unavailable to plants under most soil conditions (Vassilev et al., 1996). Continued long-term application of P fertilisers and organic wastes and manures can lead to accumulation of P in surface horizons due to the low crop use efficiency (
