Even though silicon is the second most abundant element in the world’s soils, most of Africa’s soils are actually deficient in it, say plant pathologists researching the role silicon plays in plant growth and health. “Plants indigenous to Africa have adapted to low silicon concentrations, but imported commercial crops don’t perform as well as they should on most of the continent, including in SA, because of this deficiency,” says Prof Mark Laing, head of the University of KwaZulu-Natal (UKZN’s) Plant Pathology Department.
Prof Laing and his colleague, Dr Pat Caldwell, explain that because there are no obvious symptoms of silicon deficiency in plants, for a long time it wasn’t recognised as an essential element for plant nutrition and health. Recent research has found that silicon is essential to plants’ biochemistry, promoting resistance to diseases, insects and nematodes. “Silicon seems to act as a catalyst for triggering the plant’s immune system against pest and disease threats,” Prof Laing explains.
“For example, when pests attack, plants will initiate self-defence using chemical compounds, releasing tannins and phenols into the leaves or roots to make them unpalatable. Silicon speeds up the self-defence process, thereby reducing damage to the plants.” A similar process occurs in the case of disease. However, instead of tannins and phenols, the plant releases phytoalexins to attack and kill fungi, bacteria and viruses. Once again, silicon acts as the catalyst in this self-defence process, officially called Systemically Acquired Resistance (SAR).
Prof Laing and Dr Calwell say silicon can also reduce soil acidity. Soils with acidity problems usually also experience aluminium and manganese toxicity. Traditionally, lime has been used to counter low soil pH and bind excess aluminium and manganese. “The problem is that when acidifying fertilisers, like ammonium, are applied to these lime-treated soils, they release the aluminium and manganese back into the soil,” explains Prof Laing. “However, if silicate is added to soil, it binds the aluminium and manganese permanently. First add silicon to bind toxic elements, then to feed the crop.”
Research at UKZN
Trials at the UKZN demonstrated that silicon helps crops deal with the stresses associated with drought, heat and cold. Tissue-cultured banana plants were kept for six weeks in a cold room at 4ºC. Control plants suddenly exposed to warmer temperatures died, but plants that had been drenched with a 2 000 parts per million silicon solution twice before being put in the cold room did not.
This experiment has positive implications for the SA banana industry, which currently loses much of its crop every few years to unusual frosts. Prof Laing says, “Silicon can be used on any crop, and can even extend their ability to tolerate earlier or later growth periods. Trials are also underway to see what it can do to help crops cope with heat stress.
It’s also been found to help reduce lodging in high-yield crops such as maize and sugarcane. Applying silicon actually strengthens the plants’ cell walls.” The UKZN Plant Pathology department and PHP are also conducting trials on a wide variety of tree, field and greenhouse crops to determine the cost-effectiveness of various silicon formulations and their application frequencies.
Sources of silicon
A number of South American countries have realised the importance of providing silicon to commercially important crops. Silicon is provided via the widely, and relatively cheaply, available forms of cement waste, or slag which is the byproduct of smelting ore.
There have been experiments in SA using cement waste and slag as silicon sources for commercial crops, but the costs and logistics of transporting these materials are prohibitive here. Cement waste and slag also carry a meagre 2% silicon content, so large volumes need to be applied for crops to benefit. “Currently, the only commercially available silicon product for agricultural application in SA is Calmasil, which is used predominantly by the sugarcane industry,” said Prof Laing. “Silicon application has been a very effective control measure against eldana, a pest insect.
The sugar industry is now researching silicon’s effectiveness against sugarcane rust to improve the resistance of newer cane varieties.” Calmasil is only available as a slow release silicon application. However, the UKZN Plant Pathology department, together with the company Plant Health Products (PHP), based in KZN, are developing a liquid potassium silicate product that will provide a 20% concentration of both these important nutrients, and can also be used in the horticulture industry.
The liquid form gives more immediate results. Prof Laing says that the university and PHP are also developing a granular formulation to be added to field crops at planting, which provides a continuous supply of potassium (K) and silicon for up to nine months. Prof Laing cautions, “Silicon must always be available to plants, so the choice between the fast-release liquid or slow-release granular formulations will depend on the crop and its particular requirements.” Prof Laing and Dr Caldwell conclude that silicon is not meant to be a silver bullet for plant production and health problems, but will definitely enhance plant potential.
If agriculture is going to move away from chemical controls against pests and disease because of costs, environmental threats, human health issues, and increased pest and disease resistance problems, farmers will need effective alternative control methods.
The pathologists believe a combination of the best available biocontrol agents and the best effects of silicon holds the most promise. Used together, biocontrol and silicon multiply each other’s positive effects. Contact Prof Mark Laing on (033) 260 5526, fax (033) 260 6465, or e-mail [email protected]. Contact Dr Pat Caldwell on (033) 260 5822, fax (033) 260 5919 or e-mail [email protected]. |fw