1. Calcium and Magnesium
2. Sulphur
3. Incorporation of secondary nutrients in Granular Fertilisers
4. Bulk blending or Direct application
5. Addition to liquid fertilisers

Calcium, magnesium, and sulphur are designated as secondary nutrients. Plants require these nutrients in fairly substantial quantities for normal growth, but fertiliser sources generally are not applied at rates as high as those of the primary nutrients. Actual requirements and uptake of secondary nutrients vary with plant species and soil conditions. Deficiencies of magnesium and sulphur are becoming more common, the latter mainly because of reduction of polluting air emissions of SO₂, while those of calcium are lesser reported. ·

Calcium and Magnesium

Calcium is relatively abundant in soils, and calcium rarely limits crop production per se. Low levels of exchangeable calcium in soils result in increased soil acidity, which usually results in reduced growth of most crops. Using lime applications to correct soil acidity to recommended soil pH levels will provide sufficient calcium for crops because liming materials usually contain calcium (Table 1).

Magnesium deficiencies are found mainly on acid, sandy soils and on organic soils containing free calcium carbonate. Magnesium deficiencies also can be accentuated by high levels of available potassium or on soils that have been limed for many years with calcite limestone, which is very low in magnesium.

Products

Fertiliser sources of calcium include calcium nitrate, a highly soluble nitrogen product, and single and triple superphosphate. Phosphate rock (PR) also contains calcium; however, because its solubility is quite low, PR must be finely ground and applied to acid soils to be available to plants. Gypsum is an important source of calcium for groundnut. Although its solubility is somewhat limited, gypsum is sufficiently available for plants. Gypsum also is a soil amendment for reclaiming sodic soils for agricultural production.

The most economical way to correct magnesium deficiencies on acid soils is to apply dolomitic limestone. However, if soil pH levels are above 6.0, the sources listed in Table 2 can be used. Magnesium sulphate, magnesium oxide, polyhalite and potassium-magnesium sulphate are the most common magnesium sources [2]. Recommended application rates for magnesium range from 10 to 40 kg MgO/ha, depending on the magnesium requirement of the crop as well as soil test levels [3].

Benefits of Liming

Liming materials provide ample supplies of calcium (and of magnesium if dolomite is used) for crops and also increase the soil pH to recommended levels; thus, liming has additional benefits for plant nutrition [4]. For example, nitrogen fixation by legume crops and nitrification of ammonium-nitrogen to nitrate-nitrogen are microbial processes which are optimal in neutral soils. Similarly, phosphorus availability is highest in soils with a pH of 6 to 7; below this pH range, phosphorus reacts with aluminium and iron oxides to form insoluble compounds in soil, and insoluble calcium phosphates are formed above this range. Likewise, availability of most micronutrients is highest in soil with a pH range of 6 to 7. Except for molybdenum, the availability of the micronutrients decreases with an increase in soil pH. Toxicities of some micronutrients, especially manganese, also occur in very acid soils. Conversely, over-liming soils can result in micronutrient deficiencies.

Sulphur

Most of the soil sulphur is in the soil organic matter and is highest in the surface soil. Sulphur in organic matter must be mineralised to the sulphate form to become available to plants. Conversely, sulphate can be immobilised when bacteria decompose crop residues high in carbon. Sulphate-sulphur also can be converted to unavailable sulphides under reducing conditions caused by poor drainage and flooded soil conditions [5].

Recommended application rates for sulphur range from 10 to 50 kg S/ha, depending on the level of available sulphur in the soil and the sulphur requirements of the crop (rapeseed and cabbage are known to be high sulphur demanding soils). Knowledge of other inputs, such as sulphur contained in N-P fertilisers, atmospheric deposition of sulphur, and sulphate-sulphur in irrigation water, as well as crop needs and levels of available sulphur in soil should be considered when deciding if sulphur applications to soil are warranted. Because of continuous improvement of the air quality due to reduction of SO₂ emissions, the part of S from depositing is reducing continuously in todays agriculture and S-deficiency gets more and more visible.

Products

There are many sulphur sources for application to soil (Table 3). Normal superphosphate contains as much as 12% sulphur as a result of the manufacturing process using sulphuric acid. Most high-analysis phosphate fertilisers like MAP and DAP contain very little sulphur, although inclusion of sulphuric acid in the granulation process results in 2%-3% sulphur in the final products.

The main sulphur sources are ammonium sulphate, ammonium thiosulphate, potassium sulphate, polyhalite and magnesium sulphate (kieserite), which contain 17%-26% sulphur [6].

Ammonium sulphate is a fertiliser suitable for alkaline soils. The ammonium ion forms a small amount of acid in the soil, lowering its pH balance, while providing both N and S for plant growth. It is made by treating ammonia with sulphuric acid:
2NH₃ + H₂SO₄ -> (NH₄)2SO₄
and can also be made from gypsum (CaSO₄·2H₂O). Gypsum is mixed with an ammonium carbonate solution from which calcium carbonate precipitates as a solid, leaving ammonium sulphate in the solution.

Elemental sulphur also is used, but it must be applied to soil in powder form below 100-micron size to be quickly oxidised to the sulphate form. This microbial process also results in the formation of acid, which reduces the soil pH. The micronutrient sulphates also provide available sulphur, but their application rate usually is so low that they do not provide adequate sulphur for crop needs on sulphur-deficient soils.

Incorporation of secondary nutrients in granular fertilisers

Calcium is most often incorporated in granular NPK grades as single superphosphate (SSP) which contains about 19%-22% CaO depending on the PR source. A grade such as 8-8-8 based on ammoniating solution, sulphuric acid, and SSP contains about 7% calcium. Also, polyhalite is known for its high content of CaO, next to its sulphate, potash and magnesium content.

Nitrophosphate products, either granular or prilled, contain calcium in the NP(K) grades and in the ammonium nitrate coproduct. NPK grades such as 13-13-13 retain some of the calcium originally in the phosphate rock because the calcium nitrate is never completely removed in the Odda process crystallisation step. The residual calcium is converted primarily to dicalcium phosphate when ammoniation is carried out to produce the NPK products. The separated crystalline calcium nitrate is dissolved and converted to ammonium nitrate solution and solid calcium carbonate by addition of ammonia and carbon dioxide. The ammonium nitrate solution is concentrated and prilled or granulated with addition of calcium carbonate to produce calcium ammonium nitrate (CAN). CAN typically contains 26% or 27% N and 20% to 24% calcium carbonate. ln some processes, a small amount of gypsum is mixed with the calcium carbonate prior to addition to the concentrated ammonium nitrate solution [7].In various cases calcium carbonate is replaced by dolomite in the CAN production, in order to introduce some 4% MgO next to the calcium.

Most of the magnesium-containing materials shown in Table 2 may be used for incorporation in granular fertilisers. Dolomite and kieserite are added to nitrophosphates to obtain grades such as 12-12-17-1.2 Mg and 13-9-16-2.4 Mg [8]. For granulation, magnesium chloride should be avoided in formulations using sulphuric acid because of the possibility of producing corrosive hydrogen chloride vapour or ammonium chloride aerosol. Magnesium nitrate should not be used in significant proportion in dry fertilisers because of its fire and explosion risk.

The predominant sources of sulphur in NPK products are sulphuric acid, which forms ammonium sulphate in the ammoniator-granulator, polyhalite, containing 19%S and SSP, which contains about 12% S as gypsum. Crystalline ammonium sulphate (24% S) is also frequently used. Gypsum is used, including natural gypsum, recovered gypsum from flue gas desulphurisation, and, where environmental regulations permit, phosphogypsum. Of course, all the sulphate salts listed as magnesium sources in Table 17.2 may also be used as sulphur sources.

Bulk blending or Direct application

In addition to secondary nutrients that are incorporated in granular mixtures, secondary nutrient materials are also used for direct application and bulk blending. Powdered sources are normally used for direct application only; bulk blending requires granules 2-5 mm in size, crystals, or compacted material. One has to pay extra attention to the specific gravity of blend components: they should not deviate more that 10% in order to prevent segregation. These large particle size materials are also excellent secondary nutrient sources for direct application. The following are some of the more important secondary nutrient sources for direct application and bulk blending:

Direct application only

Crushed dolomite, 20% Ca, 12% Mg
Basic slag, up to 33% Ca
SSP, 20% Ca
Ground phosphate rock, 30%-35% Ca
Nongranular TSP, 13% Ca
Calcium nitrate, 19% Ca
Fine potassium magnesium-sulphate, 11% Mg, 22% S Gypsum, 23% Ca, 19% S
Fine ammonium sulphate, 24% S
Polyhalite, 19% S, 6% MgO; 17%CaO and 14% K₂O

Bulk blending or direct application

Granular SSP and TSP
Pelletised dolomite
Granular gypsum
Ammonium sulphate, granular or coarse crystal
Potassium magnesium sulphate, compacted or coarse crystal
Polyhalite granular or coarse material
Calcium ammonium nitrate, granular, 9% Ca
Also, some of the magnesium sources listed in Table 2 may be applied directly.

Addition to liquid fertilisers

Secondary nutrients are an important component of the liquid fertiliser market, and many source materials are available for use in clear solution and suspension formulations. Most of the materials for clear solutions may also be used in suspensions, but they are seldom used because they are generally more expensive. The polysulfide solutions should not be mixed with acidic materials – dangerous hydrogen sulphide gas may be evolved! The following list gives secondary nutrient sources for solutions and suspensions are [9]:

Calcium in solutions:

• Chelate solutions and powders, 5%-10% Ca
• Calcium nitrate, granular or solution
• Calcium nitrate plus ammonium nitrate solutions
• Calcium lignosulfonate solution

Calcium in suspensions:

• Fluid lime (limestone-clay suspension) Gypsum, 50% flowable emulsion

Magnesium in solutions:

• Epsom salts, MgSO₄•7H₂O
• Chelated solutions and powders, 1 %-6% Mg
• Magnesium sucrate solubilised granules
• Magnesium nitrate, flake or solution (7-0-0-6 Mg)
• Magnesium chloride, MgCl₂•6H₂O, flake or solution

Magnesium in suspensions:

• Magnesium oxide
• Potassium magnesium sulphate, fine
• Magnesium carbonate
• Fluid lime (dolomitic)
• Polyhalite fine

Sulphur in solutions:

• Ammonium thiosulphate, 12-0-0-265 solution Ammonium sulphate, crystals or 8-0-0-95 solution Epsom salts, 13% S
• Ammoniumpolysulfide,20-0-0-40S solution with aqua or anhydrous ammonia
• Potassium polysulfide, 0-0-22-22S solution
• Potassium thiosulphate, 0-0-25-17

Sulphur in suspensions:

• Elemental sulphur,52%, flowable emulsion
• Elemental sulphur, 90%, wettable powder or degradable granule
• Potassium magnesium sulphate, fine
• Potassium sulphate
• Polyhalite fine

References

1. Follett, R. H., L S. Murphy, and R. L. Donahue. 1980. Fertilizers and Soil Amendments, Prentice• Hall Inc., Englewood Cliffs, NJ, U.S.A.

2. Portsch, S. (Ed.). 1991. The Role of Sulphur; Magnesium and Micronutrients in Balanced Plant Nutrition, Potash and Phosphate Institute of Canada, Saskatoon, Saskatchewan.

3. Mortvedt, J. J., and F. R. Cox. 1985. “Production, Marketing and Use of Calcium, Magnesium and Micronutrient Fertilizers,” IN Fertilizer Technology and Use, 0. P. Engelstad (Ed.), 3rd. Ed., Soil Science Society of America, Madison, WI, U.S.A.

4. Adams, F. (Ed.) 1984. Soll Acidity and Liming, 2nd Ed. Agronomy Monograph 12. American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Madison, WI, USA

5.Tabatabai, A. (Ed.) 1986. Sulphur in Agriculture, Agronomy Monograph 27, · American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Madison, WI, U.S.A.

6. Beaton, J. D.; R. L. Fox, and M. B. Jones. 1985. “Production, Marketing and Use of Sulphur Products,” IN Fertilizer Technology and Use, 0. P. Engelstad (Ed.), 3rd Ed., Soil Science Society of America, Madi• son, WI, U.S.A.

7. R. Richmann. 1994. “The BASF Nitrophosphate Process,” lN Nitric Acid-Based Fertilizers and the Environment, Proceedings of an International Workshop, R. G. Lee (Ed.), pp. 203-210, SP-21, lFDC, Muscle Shoals, AL, U.S.A.

8. J. Platz. 1994. “Solid and Liquid Product Formulation Options with the Nitrophosphate Process,” IN Nitric Acid-Based Fertilizers and the Environment, Proceedings of an International Workshop, R. G. Lee (Ed.), pp. 233-238, SP-21, IFDC, Muscle Shoals, AL, U.S.A.

9. Farm Chemicals Handbook. 1995, Vol. 81, Meister Publishing Co., Willoughby, OH, U.S.A.

Links to related IFS Proceedings

109, (1969), Prilling of Compound Fertilisers, F E Steenwinkel, J W Hoogendonk

140, (1974), Modern Sulphuric Acid Practice, G T Ketjen

153, (1976), Slow Release Fertilisers, Particularly Sulphur-Coated Urea, L H Davies

264, (1987), Soil and Fertiliser Sulphur in UK Agriculture, J Keith Syers, R J Skinner, D Curtin

380, (1996), Ammonium Sulphate: An Innovative Process for Production, G N Brown

400, (1997), Cadmium and Other Minor Elements in World Resources of Phosphate Rock, S J Van Kauwenbergh

412, (1998), Magnesium Fertilisers in Soil and Plants: Comparisons and Usage, A P Draycott, M F Allison

497, (2002), History and Outlook for Sulphur Fertilisers In Europe, S P McGrath, F J Zhao, M M A Blake-Kalff

498, (2002), Sulphur Fertilisers: A Global Perspective, G J Blair

502, (2002), Sulphur Sources, their Processing and Use in Fertiliser Manufacture, D L Messick, C de Brey, M X Fan

746, (2014), Sulphuric Acid Technology: Past, Current and Future Developments, A Schulze

782, (2016), Granulation Technology with Flexibility to Produce a Range of Specialist Products, N Kargaeva

783, (2016), Granulation of Complex Fertilisers, H Kiiski, A Kells

820, (2018), Fluidised bed granulation of ammonium sulphate – a new process, C Renk, P Banik