Research by APTA Regional de Piracicaba highlights the relevance of silicon in sugarcane fertilization

The São Paulo Agribusiness Technology Agency (APTA), an agency of the Department of Agriculture and Supply of the State of São Paulo, has been testing the effects of silicon (Si) supplementation on biotic and abiotic factors that affect sugarcane. Promising results have led to publications in important scientific journals.

"Silicon is an element that is present in all soils, being its main form. However, most tropical soils have a low amount of soluble ones, that is, available to the plant", comments the researcher from APTA Piracicaba Regional Mônica Sartori de Camargo. She uses beach sand as an example, which has a high total concentration of Si (in the form of silica, SiO2), but where the element is not available for absorption by plant roots. "To be available, soil particles must be finer, as occurs in clayey textures," she says.

In her research, Mônica works precisely with sandy soils, which have become widely used in the interior of São Paulo for sugarcane cultivation, her focus of study. "Sugar cane is considered a Si accumulator", says the expert. Despite this, she mentions, supplementation with this element is normally not considered essential for the culture, despite being seen as beneficial. The researcher's aim is precisely to show how positive it can be for sugarcane farmers to invest in this type of fertilization.

In her experiments, Mônica uses ground steel slag as a source of this element. "Slag is considered an environmental liability, coming from the manufacture of iron and steel, and is rich in Si, Calcium (Ca) and Magnesium (Mg)", explains the APTA Regional researcher. As he says, the material has various origins and must be approved for agricultural use, being free of heavy metals and other contaminants. "This silicate can be used as a source of Si and as a soil acidity corrector, as it contains limestone."

More production and less drill

Initially, Mônica's efforts focused on testing Si absorption in different sugarcane varieties. "We tested varieties from the Agronomic Institute (IAC-APTA) and, over 3 cycles, we wanted to study whether there was any relationship between absorbing more silicon from the soil and the plant's ability to have less sugarcane borer (Diatraea saccharalis) ", points out Monica. According to her, once absorbed by the roots, silicon rises through the plant through the xylem and is deposited in the leaves in the form of a polymer (which can be compared to silicone, for example). "It forms a double layer of silica and cuticle in the epidermis and insects find it more difficult to enter this tissue", explains the specialist. She found that some varieties had higher Si levels when compared to others, such as IAC91-1099, but did not find a direct relationship between Si content in the soil and D. saccharalis attack. Although the soil had an adequate amount of Si for the plant, it might not be enough to reduce the damage from the borer.

Secondly, the researcher decided to apply Si to the soil in order to try to reduce the impacts caused by the insect. The experiments were carried out on two different types of soil, with application of Si in the planting furrows, and all treatments received the same amount of Ca and Mg as the control group, so that it was possible to specifically analyze the effect of Si. "We noticed that there was a reduction in the amount of borer attack in both cycles, especially for the SP89-1115 variety, which was susceptible to borer attack. The results showed that it was indeed advantageous to use Si fertilization. It was something very important to observe in a field experiment", emphasizes Mônica, suggesting that Si can be a good ally in biological pest control strategies. Some results of the work were published in the scientific journal Field Crops Research (access, in English), of great relevance in the area.

Rust tolerance

Among the factors of biological origin that can interfere with the growth of a plant (the so-called biotic stress), insect attacks and diseases are among the most important. With a positive outcome in the case of the borer, the APTA Regional researcher thought it would be a good idea to evaluate whether Si could also contribute to sugarcane's tolerance to brown rust, a disease caused by the fungus Puccinia melanocephala. "When varieties come out of genetic improvement programs, brown rust is one of the diseases evaluated in experimental fields, but the big problem is that in commercial planting there are environmental pressures and the plants are exposed to various strains of the fungus", informs Mônica. According to her, in recent years rust has greatly affected plantations and, although it does not kill the plant, it greatly reduces photosynthesis and can harm growth in some situations.

"Thus, sugarcane plants were evaluated in 100 L pots with various types of soil. We saw that as the concentration of Si in the soil increased, the natural incidence of brown rust decreased", says the APTA Regional researcher. This encouraged her to evaluate, under controlled conditions, 10-week-old plants in soils with and without Si supplementation. "The severity of the disease was evaluated after inoculating an equal amount of disease spores in all plants after 14 days", describes the specialist. As he says, the presence of Si caused the plant, in contact with the fungus, to start producing specific metabolic compounds, which reduced the severity of the disease.

As a complement to the work, the researcher sent samples of the plant leaves to Unesp. With the collaboration of university doctoral student Isabel Coutinho, it was possible to identify in the laboratory which chemical compounds the plants were producing and which had the ability to combat the fungus that causes rust. "We were able to prove that these compounds are natural fungicides, with an effect comparable to that of synthetic products sold today", highlights the APTA Regional specialist. The research also resulted in publication in a specialized journal (check it out here).

Minimizing abiotic factors

If, on the one hand, silicon showed promise in reducing the impacts caused by biotic factors, it remained to be evaluated how it could contribute to those of physical and chemical origin - the abiotic ones. According to the researcher, in this regard, water stress has been a pressing problem lately, due to the decrease in rainfall. "We have studied tolerant and sensitive varieties and carried out experiments to evaluate whether or not the use of Si had an effect on the plant's physiology and production", she explains.

According to Mônica, the team has achieved good results. "In pots, after evaluating the plants subjected to water deficit at 3 and 6 months, we returned to supplying the usual water and the plant fertilized with Si proved to be more productive even after experiencing stress", he celebrates. The hypothesis is that Si is acting as an ally, stimulating photosynthesis in times of reduced water availability. "The plant becomes better prepared for stress, which can even result in greater sugar production", highlights the researcher. Some of the results obtained can be read in this article.

Despite the benefits indicated by research, the use of Si in the form of silicates (slag) on ​​a larger scale still ends up encountering some bottlenecks. "As it is considered more advantageous than limestone, some commercial plantations already use Si supplementation in some regions", says the specialist. "However, use still comes up against the issue of shipping in other regions, which can increase costs", she ponders. In her opinion, this shows the need to evaluate other forms of application and also other sources of Si. Another point that the producer must pay attention to is the quality of the slag. "Some producers end up using low-solubility slags, which don't work very well, as the Si takes a long time for the plant to absorb through the roots", concludes Mônica.