Ammonium sulfate [(NH₄)₂ SO₄] was one of the first and most widely used nitrogen (N) fertilizers for crop production. It’s now less common but especially valuable where both N and sulfur (S) are required. Its high solubility provides versatility for a number of agricultural applications.
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Production
Ammonium sulfate (sometimes abbreviated as AS or AMS) has been produced for more than 150 years. Initially, it was made from ammonia released during manufacturing coal gas (used to illuminate cities) or from coal coke used to produce steel.
Today, manufacturers make ammonium sulfate by reacting sulfuric acid with heated ammonia. To get the crystal size best suited for the application, reaction conditions are controlled by screening and drying the particles until achieving the desired size. Some materials are coated with a conditioner to reduce dust and caking.
Byproducts from various industries meet most of the current demand for ammonium sulfate. For example, the nylon manufacturing process produces ammonium sulfate as a co-product. In another, certain byproducts that contain ammonia or spent sulfuric acid are commonly converted to ammonium sulfate for use in agriculture.
Although the color can range from white to beige, ammonium sulfate is consistently sold as a highly soluble crystal with excellent storage properties. As described earlier, particle size can also vary depending on the intended purpose.
Chemical properties
Agricultural use
Growers apply ammonium sulfate primarily where they need supplemental N and S to meet the nutritional requirement of growing plants. Since ammonium sulfate contains only 21 percent N, other fertilizer sources more concentrated and economical to handle and transport often make a better choice for N-deficient fields. It provides an excellent source of S, which supports or drives numerous essential plant functions, including protein synthesis.
Because the N fraction is present in the ammonium form of ammonium sulfate, rice farmers frequently apply it to flooded soils, since nitrate-based fertilizers are a poor choice due to denitrification losses.
A solution containing dissolved ammonium sulfate is often added to post-emergence herbicide sprays to improve their effectiveness at weed control.
This practice of increasing herbicide efficacy with ammonium sulfate works particularly well when the water supply contains significant concentrations of calcium (Ca), magnesium (Mg) or sodium (Na). A high-purity grade of ammonium sulfate often works best for this purpose to avoid plugging spray nozzles.
Management practices
After addition to soil, the ammonium sulfate rapidly dissolves into its ammonium and sulfate components. If it remains on the soil surface, the ammonium may be susceptible to gaseous loss in alkaline conditions. In these situations, agronomists advise incorporating the material into the soil as soon as possible. Other options include an ammonium sulfate application before irrigation or a predicted rainfall.
Most plants can utilize both ammonium and nitrate forms of N for growth. In warm soils, microbes will rapidly begin to convert ammonium to nitrate in the process of nitrification [2 NH₄⁺ + 3O₂ → 2NO₃⁻ + 2H₂O + 4H⁺]. During this microbial reaction, acidity [H⁺] is released, which will ultimately decrease soil pH after repeated use.
Ammonium sulfate has an acidifying effect on soil due to the nitrification process, not from the presence of sulfate, which has a negligible effect on pH.
The acid-producing potential of ammonium sulfate is greater than the same N application from ammonium nitrate, for example. That’s because all of the N in ammonium sulfate converts to nitrate, compared with only half of the N from ammonium nitrate that converts to nitrate.
Non-agricultural uses
Source: Nutrient Source Specifics, No. 12, International Plant Nutrition Institute.
Ammonium sulphate is not just a fertiliser used in agriculture.
It is an industrial component also used as a flocculant, which is a substance that encourages flocculation, i.e. the combination or aggregation of suspended solid particles.
The flocculation process involves destabilising colloidal particles by adding the flocculant.
It also has all these industrial applications:
Ammonium sulphate is a salt formed by the reaction between Ammonia and Sulphuric acid. Commercially, it can be found in white to beige crystals or granules.
Its content is as follows:
It is the most accessible source of low-concentration Nitrogen, is widely used in agriculture, and is also a relevant component in the production of balanced fertilisation formulas.
It is widely applied directly to the soil as a single product
, is an excellent source of fertilisation
in crops which extract large quantities of sulphur from the soil,
such as forage crops, vegetables (cruciferous vegetables, onions and garlic), cereals (wheat and barley) and grasses (maize, sorghum and sugar cane), among others.
It mainly contains Ammonium (NH
4+) and Sulphate (SO
4-2), and it is an acid pH product that is recommended for application in limestone and alkaline soils due to its strong acidifying effect.
Its use as a fertiliser is due to the fact that the need for sulphur is closely related to the amount of nitrogen available for the plant. Therefore, Ammonium Sulphate provides a balanced supply of both nutrients.
In conventional agriculture, it is still used en masse, especially for less profitable crops and as an important source of Nitrogen accompanied by Sulphur, a precursor in its assimilation.
It is therefore not an organic fertiliser, as its process is obtained through synthetic transformations and, as such, is not covered by European organic farming regulations.
In the field, it is known as a good fertiliser used in both extensive and intensive crops with a dual action, as it provides two macronutrients added to its Sulphur content that promotes the physical and chemical conditions of the crop soils.
Optimising the dose and application to crops in the soil, Ammonium Sulphate is a very good source of supply of this element, and is easy to mix and use.
The form of Ammonium is usually quickly assimilated by crops, although in large volumes it can become phytotoxic to the plant.
In fact, in hydroponics, its use is limited to a maximum of 15-20% of the total fraction of Nitrogen, the remaining being 80-85% Nitric Nitrogen.
Inorganic soil sulphur is absorbed by plants mainly as anion Sulphate and, due to its negative charge, it is not attracted by soil clays and inorganic colloids;
Sulphur remains in the soil solution, moving with the water flow and, therefore, is easily leachable
. In some soils, this leaching accumulates Sulphur in the subsoil, making it usable by deep-rooted crops.
The risk of leaching is higher in sandy soils than in clay-textured soils.
Soils with low organic matter content (<2%) commonly have Sulphur deficiencies, and every one percent of organic matter releases approximately 6 kg of it per ha per year.
In plants, Nitrogen and Sulphur have a very close relationship in their nutritional role.
This is because both nutrients are constituents of the 5 proteins (amino acids) and are associated with the formation of chlorophyll (involved in the photosynthesis process).
The obtaining of Ammonium Sulphate as an industrial product is the transformation of Gaseous Ammonia (NH3), mixed with water vapour to create saturation, and the incorporation of Sulphuric Acid.
This controlled reaction is transformed into the mixture of Sulphur (SO4) and Ammonium (NH4), which, because it is attacked with an acid, one more Hydrogen (H) is added to the formula.
All the Nitrogen in this fertiliser is presented in ammoniacal form.
Although it is highly digestible by the plant, it should preferably be absorbed in nitric form.
Its behaviour in soil is good and, because it has a positive charge, it is fixed in the clay-humus complex and does not leach so much to deep fields (as the nitric form does).
The presence of nitrifying microorganisms (nitrobacter and nitrosomonas, among others) facilitates its transformation to nitric.
As it has an acid reaction, it is used to acidify soils with an alkaline pH and a high presence of non-soluble Calciums and Magnesiums.
In addition, the Sulphur incorporated into the fertiliser improves the availability of Nitrogen and has a synergistic effect in its assimilation.
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There are three types of application of ammonium sulphate:
WINTER CEREAL
Irrigation: Half budding, start of sprouting
1500 – 2500 150 – 200 150 – 200 2500 – 5000 200 – 350 200 – 350 5000 – 7000 350 – 500 350 – 500
SPRING RICE CEREAL
SPRING MAIZE-SORGHUM CEREAL
Short cycle: Single application at 50cm
8000 – 10000 500 – 650 500 – 600 10000 – 12000 650 – 775 600 – 750 12000 – 14000 775 – 875 750 – 850
INDUSTRIAL SUNFLOWER CROP
INDUSTRIAL COTTON CROPS
MISCELLANEOUS INDUSTRIAL CROPS
OLIVE GROVE
VINES
CITRUS LIMON VERNA-LATE VARIETIES
January
July-August
October
CITRUS CLEMENTINES – SATSUMAS – NAVEL – SALUSTIANAS
Dressing 2: July/August
3000 – 4000 75 – 100 70 – 80 4000 – 5000 100 – 125 80 – 100
ALMOND
APPLE – PEAR – QUINCE
CHERRY – PLUM – PEACH – APRICOT – NECTARINE – SATURN PEACH
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