In deciding which gypsum to buy, the main points to consider are:
- the total cost of supply and application of gypsum, expressed on the basis of pure CaSO4.2H20 (Regyp – 90%+ pure),
- how quickly the gypsum dissolves in water (Regyp – up to four faster),
- how easily and evenly it spreads (Regyp – up to 30% more width).
Gypsum quality is assessed by considering two main factors: purity and fineness.
Gypsum may contain a variety of impurities. including water, soil, limestone, sodium chloride, cadmium and fluoride.
In NSW the purity of gypsum is defined in terms of percentage sulfur (S) on a wet weight basis, that is percentage S of gypsum as supplied. Most of the gypsum sold in NSW is calcium sulfate dihydrate (CaSO4.2H20), which has 18.6% S when completely pure.
Fineness refers to the size of the gypsum particles. It is important because it largely determines how quickly the gypsum dissolves in water. Lumpy mined gypsum may be unsuitable, not only because of difficulties in spreading, but also because it is very slow to dissolve.
There are two main sources of gypsum: from mining and as industrial by-products.
1. Mined gypsum, may contain a high proportion of impurities tmainly soil) and is often quite lumpy or coarsely crystalline. Its quality can be quite variable, as the gypsum type and content change with depth in the mine pit. Solubility in water tends to be low.
2. Industrial by-product gypsum can come from the manufacture of fertilizers and some power generation and industrial exhaust stack scrubber plants. Most industrial gypsum by-products have high purity and solubility. One industrial gypsum source is waste plasterboard.
Plasterboard and gyprock is manufactured using high quality mined gypsum from South Australia. Plasterboard can contain up to 95% of gypsum in the finished product. During the manufacturing process gypsum goes through the following stages:
- fine grinding to reduce the mean particle size to a mean of approximately 75um,
- a calcining process to produce plaster or hemi-hydrate (CaSO4 , ½ H2O), where combined moisture is driven off through heat. In some plants milling and calcining take place together (flash calcining),
- board making process, where water is mixed with the plaster to create a usable slurry. The slurry is poured onto face paper and the back paper is added in a continuing extrusion and forming process along a long belt conveyor. During the forming and drying process the plaster is re-hydrated and the gypsum crystals (CaSO4, 2 H2O) form in the core and bond with paper liner to give the plasterboard its strength.
What does this mean for agricultural applications?
- Due to the high quality gypsum used in the original manufacturing process, the gypsum purity of recycled is high ~90%+,
- particle size is not an issue like mined gypsum due to the plasterboard manufacture milling process,
- due to the porous particles and increased surface area from the milling and manufacturing process, recycled gypsum has increased solubility. Super Ag Gypsum has up to 4 times more than other mined sources;
- when spreading the gypsum you can use lower applications rates (up to 15%) due to its much higher purity than most NSW mined gypsum sources, saving you money;
- further application savings can be achieved through increased spreading widths over mined sources due to the larger particle size; and
These qualities mean that SUPER AG Gypsum is a high quality source of gypsum, sulphur and calcium, which will fully dissolve into the soil solution at a much faster rate and leave no un-dissolved gypsum particles.
Reference: NSW Agfacts AC.10
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