Ammonium nitrate and urea fertilisers are produced by spraying a solution of the fertiliser from a vibrating shower head into a ‘prilling tower’. As the droplets fall down the tower against a stream of cold air they become round and solid, producing prills 1–3 mm diameter. Nitrogen fertilisers in common use are:

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Ammonium nitrate (33.5–34.5% N). This is a very widely used fertiliser for top-dressing. Half the nitrogen (as nitrate) is very readily available. It is marketed in a special prilled or granular form to resist moisture absorption. It is a fire hazard but is safe if stored in sealed bags and well away from combustible organic matter. Because of the ammonium present, it has an acidifying effect.

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Ammonium nitrate lime (21–26% N). This granular fertiliser is a mixture of ammonium nitrate and lime. It is sold under various trade names. Because of the calcium carbonate present it does not cause acidity when added to the soil.

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Urea (46% N). This is the most concentrated solid nitrogen fertiliser and it is marketed in the prilled form. It is sometimes used for aerial top-dressing. In the soil, urea changes to ammonium carbonate which may temporarily cause a harmful local high pH. Nitrogen, as ammonia, may be lost from the surface of chalk or limestone soils, or light sandy soils when urea is applied as a top-dressing during a period of warm weather. When it is washed or worked into the soil, it is as effective as any other nitrogen fertiliser and is most efficiently utilised on soils with adequate moisture content, so that the gaseous ammonia can go quickly into solution. In dry conditions in the height of summer it is probably better to use ammonium nitrate. Chemical and bacterial action changes it to the ammonium and nitrate forms. If applied close to seeds, urea may reduce germination.

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Sulphate of ammonia (21% N, 60% SO ₃). At one time, as a fertiliser, this was the main source of nitrogen. However, sulphate of ammonia is seldom used now. It consists of whitish, needle-like crystals and it is produced synthetically from atmospheric nitrogen. Bacteria change the nitrogen in the compound to nitrate. It has a greater acidifying action on the soil than other nitrogen fertilisers. Some nitrogen may be lost as ammonia when it is top-dressed on chalk soils.

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Sodium nitrate (16% N, 26% Na). This fertiliser is obtained from natural deposits in Chile and is usually marketed as moisture-resistant granules. The nitrogen is readily available and the sodium is of value to some market garden crops. It is expensive and is not widely used.

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Calcium nitrate (15.5% N). This is a double salt of calcium nitrate and ammonium nitrate in prilled form. It is mainly used on the Continent.

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Anhydrous ammonia (82% N). This is ammonia gas liquefied under high pressure, stored in special tanks and injected 12–20 cm into the soil from pressurised tanks through tubes fitted at the back of strong tines. Strict safety precautions must be observed; it is a contractor rather than a farmer operation. The ammonia, as ammonium hydroxide, is rapidly absorbed by the clay and organic matter in the soil and there is very little loss if the soil is in a friable condition and the slit made by the injection tine closes quickly. It is not advisable to use anhydrous ammonia on very wet or very cloddy or stony soils. It can be injected when crops are growing, for example into winter wheat crops in spring, between rows of Brussels sprouts and into grassland. The cost of application is much higher than for other fertilisers, but the material is cheap, so the applied cost per kilogram compares very favourably with other forms of nitrogen. On grassland it is usually applied twice – in spring and again in midsummer – at up to 200 kg/ha each time. In cold countries it can be applied in late autumn for the following season, but the mild periods in winters in this country usually cause heavy losses by nitrification and leaching. At one time it was fairly popular in the United Kingdom. However, because the main marketing source ceased, this is no longer the case, although there is no reason why it should not be used again.

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Aqueous ammonia (12% N). This is ammonia dissolved in water under slight pressure. It must be injected into the soil (10–12 cm), but the risk of losses is very much less than with anhydrous ammonia. Compared with the latter, cheaper equipment can be used, but it is still usually a contractor operation.

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Aqueous nitrogen solutions (26–32% N). These are usually solutions of mixtures of ammonium nitrate and urea, and are commonly used on farm crops (liquid fertilisers).