Commercial Fertiliser Specification

Commercial fertilisers are assigned an NPK value which is the % N, P2O5 and KjO by weight. Another specification is the BPL (bone phosphate of lime) value, which is the P content expressed as % Ca3(P04)2. Available phosphoric acid or APA is a measure of the P2O5 available to plants, as indicated by an empirical solubility test. Unavailable phosphoric acid is usually expressed as the portion of the fertiliser which is insoluble in neutral ammonium citrate. In the United States, available P = total P - citrate - insoluble P (Chapter 14.1). 

Solid fertilisers are usually produced in granular rather than in powder form. Bulk blending of these by mechanical mixing is often practised to produce compound or mixed fertilisers with specific N P K ratios. The principal materials used in bulk blending are ammonium nitrate, ammonium sulphate, urea, superphosphate, ammonium phosphates and potassium chloride. Under EEC regulations, mixed fertilisers are required to have a minimum of 3% N, 5% P2O5 and 5% K2O with the sum of these not less than 20%. 

Controlled-release fertilisers can be made by coating the granules with paraffin wax or urea formaldehyde. This limits the access of water and reduces the rate of solubilisation [34]. Slow-release fertilisers based on mixed cation polyphosphates have been patented [35]. 

Matrix-dispersed fertilisers have also been produced commercially. Expanded vermiculite, perlite, various gels, polymers and waxes have all been used as dispersing media. Granular fertilisers and soil conditioners can be produced from peat and water-soluble phosphates [36]. 

Micronutrient fertilisers can be made by incorporating snitable micronutrient compounds into bulk-blended fertilisers. Boron phosphate, BPO4, is an excellent slow-release source of boron. Relatively insoluble crystalline mixed-metal potassium polyphosphates can also be used. Micronutrient phosphate glasses also have valuable slow-release characteristics (Section 12.9). The required metals are introduced into the very sUghtly water-soluble glasses in the form of their oxides, for example, ZnO, CuO, MnOj, M0O3 or BjOj [37] (Table 12.26). 

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Sodium nitrate is used as a fertiliser and in a number of industrial processes. In the period from 1880—1910 it accounted for 60% of the world fertiliser nitrogen production. In the 1990s sodium nitrate accounts for 0.1% of the world fertiliser nitrogen production, and is used for some specific crops and soil conditions. This decline has resulted from an enormous growth in fertiliser manufacture and an increased use of less expensive nitrogen fertilisers (qv) produced from synthetic ammonia (qv), such as urea (qv), ammonium nitrate, ammonium phosphates, ammonium sulfate, and ammonia itself (see Ammonium compounds). The commercial production of synthetic ammonia began in 1921, soon after the end of World War I. The main industrial market for sodium nitrate was at first the manufacture of nitric acid (qv) and explosives (see Explosives and propellants). As of the mid-1990s sodium nitrate was used in the production of some explosives and in a number of industrial areas. 

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