Myths and truths about the use of agricultural gypsum in farming

The deficit of calcium and sulfur and the presence of toxic aluminum are some of the main characteristics of Brazilian soils. To replace these nutrients and combat aluminum, one of the most efficient mineral compounds available on the market today is calcium sulfate, also called agricultural gypsum or phosphogypsum.

The product is obtained during the production of phosphoric acid, in which phosphate rock reacts with sulfuric acid, resulting in phosphoric acid and generating calcium sulfate (gypsum) as a co-product.  

According to experts, the application of agricultural gypsum to the soil helps replace calcium and sulfur and also improves the subsurface environment. For saline and sodic soils, gypsum is also used as a conditioner and corrector. Acting as an ally for agricultural limestone, even though it does not correct pH, gypsum is an important source of soluble calcium and sulfur, as recalled by the technical director of the Institute of Agricultural Sciences (INCIA), Elmar Floss.

"Gypsum does not change the pH, but it is a source of calcium and sulfur in the soil. Furthermore, sulfate, which has a negative charge, carries calcium and magnesium in depth. And calcium is the main activator of root growth. root grows when touched by calcium and, at the same time, there will be a reduction in aluminum activity", emphasizes Floss.

Agricultural gypsum has been studied since 1970, with trials on plots that received simple superphosphate (which contains around 50% calcium sulfate in its composition). The researchers observed that the greater the depth of the root system, the greater the water absorption, the lower the aluminum content and the higher the calcium content. The conclusion of these studies proved that these benefits came from the plaster. 

Therefore, the use of agricultural gypsum is not fashionable. It has been the subject of research since the 1970s and, increasingly, is becoming an agronomically and economically viable alternative for the producer, due to its cost-benefit.

Compared to other products for replacing calcium and sulfur, the main advantage of gypsum is its dissolution, which allows it to reach the deeper layers of the soil. The application of agricultural gypsum reaches a depth of up to 60 centimeters in the soil, as the product is 150 times more soluble than limestone. This stimulates the growth of the plants' root system and the absorption of nutrients is greater, as explained by professor and researcher at Esalq/USP, Godofredo Cesar Vitti: "agricultural gypsum or phosphogypsum is a miracle! The producer does not need to incorporate it, it goes down the profile. Like any fertilizer, agricultural gypsum is divided into calcium and sulfur. Part of it goes underground, carried away by rainwater. I use that part of the gypsum that remains on top as fertilizer, mainly as a source of sulfur, and the part that came down I use to improve the subsoil", he highlights. 

Discover now some myths and truths about the product, answered by the technical director of SulGesso SA, Eduardo Silva e Silva:

- Calcium sulfate is only used to neutralize aluminum in depth. Myth or truth? 

Myth. The Ministry of Agriculture, Livestock and Supply (MAPA) authorizes the registration of calcium sulfate as a fertilizer. Therefore, in addition to conditioning the soil, neutralizing aluminum, it acts as a supplier of the nutrients calcium and sulfur.

- Agricultural gypsum does not provide calcium and sulfur. Myth or truth?     

Myth. Calcium sulfate provides calcium and sulfur in forms readily available to the plant. Its chemical composition is CaSO₄.2H₂O

- Is it true that gypsum only provides calcium and sulfur when there is aluminum in the soil?     

Myth. Calcium sulfate (gypsum), whether in granulated, crumbled or powdered form, will provide calcium and sulfur whenever there is hydrogen and/or aluminum, that is, whenever there is exchangeable acidity (Al + H) and active acidity (H). As most Brazilian soils are acidic, in the superficial and deeper layers, calcium sulfate will provide calcium and sulfur in these two layers.

For the producer who adheres to the Direct Planting System (SPD) and applies limestone to the surface: is it true that the presence of limestone will prevent the gypsum from releasing calcium and sulfur to the plant, in the surface layers?

Myth - It would be true if the limestone was incorporated, well homogenized and applied in precisely the correct dose. It turns out that limestone applied to the surface, as it is practically insoluble, takes years to sink from 0cm to 3cm into the soil. Therefore, the gypsum, as it is up to 150% more soluble, will pass through this layer of limestone in the form of CaSO4 and as soon as it encounters hydrogen and aluminum, from 3cm onwards, it will begin to "break" and, therefore, release calcium. and sulfur to the plant.

There are videos on the internet where it is stated that plaster is only used to neutralize aluminum. That is true?

Myth. Agricultural gypsum has multiple applications. Serves to neutralize sodium; serves to flocculate clays, "loosening the soil"; serves to increase calcium in solution; serves to induce rooting; serves to protect the phosphorus; serves to maintain nitrogen in the form of NH4+ for longer; serves to induce greater plant resistance to dry spells; it serves to nourish the plant with calcium and sulfur, in readily soluble forms and, finally, it also serves to build the soil profile.