Single Superphosphate (SSP) was the first commercial mineral fertilizer to be discovered, back in the 1840s, which led to the development of the modern fertilizers industry. Once the most commonly used fertilizer in the world, SSP is now largely replaced with other phosphorus (P) fertilizers, because of its relatively low P content.
The modern fertilizer industry was launched in the 1840s with the discovery that the addition of sulfuric acid to naturally occurring phosphate produced a superb soluble fertilizer, given the name superphosphate.
Initially, farmers would use ground animal bones to achieve this reaction, but the newly discovered natural deposits of rock phosphate (apatite) presented an opportunity to replace the limited supply of bones with a largely available substance that’s also easier to handle. Making SSP is similar to what naturally occurs with bones or apatite in acid soils. The basic technique has changed very little in the past century. Ground phosphate rock is reacted with sulfuric acid to form a semi-solid which cools for several hours in a den. The plastic-like material is then conveyed to a storage heap for several weeks of additional curing. The hardened material is then granulated or screened to the appropriate particle size.
The general chemical reaction is: Ca3(PO4)2 [rock phosphate] + 2 H2SO4 [sulfuric acid] ? Ca(H2PO4)2 [monocalcium phosphate] + 2 CaSO4 [gypsum]
Since SSP contains both monocalcium phosphate (MCP, also called calcium dihydrogen phosphate) and gypsum, there are no problems with phosphogypsum by-product disposal as occurs with the manufacture of other common P fertilizers.
SSP is also known as ordinary superphosphate and normal superphosphate. It is sometimes confused with triple superphosphate (TSP), which is created when rock phosphate reacts with phosphoric acid.
Agricultural Uses of SSP
SSP is a great source of three plant nutrients. The P component reacts in soil similarly to other soluble fertilizers. The presence of both P and sulfur (S) in SSP can be an agronomic advantage in soils where both of these nutrients are deficient. In agronomic studies where SSP is demonstrated to be superior to other P fertilizers, it is usually due to the S and/or Ca that it contains. When locally available, SSP widely used for fertilizing pastures where both P and S are needed. As a source of P alone, SSP is often more costly than some of the more concentrated fertilizers, which may explain why it’s popularity has been declining.
Handling SSP does not require any special precautions. Its agronomic effectiveness is similar to other dry or liquid phosphate fertilizers. The loss of P in surface runoff from fertilized fields can contribute to water quality problems.
SSP is primarily used as a crop nutrient source. However, MCP and gypsum, the two primary ingredients in SSP, are widely used in many products. For instance, MCP is commonly added to enrich animal feed. It is also routinely used as a leavening agent to cause baked goods to rise. Gypsum is extensively used in the construction industry, as well as in the food and pharmaceuticals.
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