Compound fertilisers

The main constituents for compound fertihsers used in the United Kingdom are urea, mono and diammonium phosphate and potassium chloride. These compound fertilisers, or compounds, supply two or three of the major plant foods (nitrogen, phosphoms and potassium). Other plant foods, e.g. trace elements, as well as pesticides, can also be added, although this is not commonly done now. The exception to this is sulphur which is increasingly being offered as part of compound fertilisers to overcome the deficiencies in certain parts of the country. 

Because of the use of more concentrated basic ingredients, compoimd fertilisers have become much more concentrated in the last 60 years. For example, in 1948 

Complex fertilisers are normally made by drying a wet slitrry, containing the appropriate raw materials, on a fluid bed system to produce granules, each containing the declared nutrients in the correct ratio, size 2-5 mm diameter. 

Some examples of compound fertihsers and possible uses are shown in Table 4.7. 

Fertilisers containing different amounts of plant food may have the same plant food ratios (see Table 4.8). 

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Markets. Industrial use of ammonia varies according to region. Eor example, industrial usage represents 20% of the ammonia production in the United States and Western Europe, 10% in the USSR, 1—10% in Asia, and 5% in Latin America and North Africa (79). Fertiliser ammonia consumed domestically in most countries is converted to straight or compound fertilisers such as urea, ammonium nitrate, diammonium phosphate, and various grades of mixed fertilisers. However, almost 29% of ammonia nitrogen in the United States is consumed as direct appHcation material. The use of nitrogen solution such as urea and ammonium nitrate (UAN) has also become popular in the United States and the USSR. 

Parrish, P. and OGILVIE, H. Calcium Superphosphates and Compound Fertilisers. Their Chemistry and Manufacture (Hutchison, 1939). 

Coarsely pulverised dolomitic limestone in the size range 0.2 to 1mm is frequently added to compound fertilisers at levels of up to about 10 %. It confers several benefits ... 

Compound fertilisers (including calcium nitrate and calcium ammonium nitrate) sometimes contain 1 to 2 % of dolomitic hydrated lime. The lime confers similar benefits to pulverised dolomitic limestone (see section 10.3). 

The nitrogen in many straight and compound fertilisers is in the ammonium (NH cation) form but, depending on the soil temperature, it is quickly changed by bacteria in the soil to the nitrate (NOj anion) form. Many crop plants, e.g. cereals, take up and respond to the NO3 anions quicker than the NH cations, but other crops, e.g. grass and potatoes, are equally responsive to NH and NO ions. 

It should be noted that a significant proportion of the nitrogen now supplied to farm crops comes from compound fertilisers in which it is usually present mainly as monammonium phosphate (MAP) or diammonium phosphate (DAP), as described in the section on phosphate fertilisers. 

Muriate of potash (potassium chloride). As now sold, it usually contains 60% K O. It is the most common source of potash for farm use and is also the main potash ingredient for compound fertilisers containing potassium. As a straight fertiliser it is normally granulated, but some is marketed in a powdered form. KCl is found in vast quantities all over the world and is mined from rock deposits left by dried-up oceans. It is nearly always found in conjunction with NaCl and the two are separated by a flotation process. 

Sulphur deficiency can be a problem in second and subsequent silage cuts. Compound fertilisers should be used to apply about 40 kg/ha SO if deficiency is confirmed by herbage analysis. 

NPK compound fertilisers for farmers who want nitrogen, phosphoms, and potassium. These are usually a mixture of ammonium nitrate, ammonium phosphate, and potassium chloride. 

This diagram shows the raw materials the factory needs to make 1000 tonnes of straight and 1000 tonnes of compound fertiliser a day, and the steps in the production process t/day stands for tonnes per day. 

While the trace-element content of guano and, to a lesser extent, of mined mineral salts, could make some contribution to the nutrient requirements of crops, these materials are no longer adequately available. We have now become largely dependent on highly purified compound fertilisers containing only nitrogen, phosphorus and potassium as nutrients, so that the rate of depletion of essential trace elements has been greatly accelerated. The natural cycle... 

The equivalent quantities of sulfur dispersed are considerably greater than the amount needed to produce enough sulfate-based compound fertiliser to meet the whole UK requirement. 

Fluorine is another potentially-toxic trace element which is dispersed by atmospheric pollution and it has long been recognised that damage to plants occurs and that there is a hazard to man and farm stock, in the vicinity of industrial plants processing fluoride-containing minerals. Such plants include factories for the production of aluminium, superphosphates and compound fertilisers based on the liberation of phosphoric acid from rock phosphate. In October 1976, ten cows had to be destroyed on two farms in the vicinity of the British Aluminium Company s aluminium smelter at Invergorden in the north of Scotland, and problems of fluoride toxicity have been commonly associated elsewhere with the production of aluminium. 

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