When it comes to plant nutrients, there are two main categories: essential nutrients and beneficial nutrients.
There are upwards of 17 essential nutrients, including three macronutrients—nitrogen, potassium and phosphorus—and micronutrients such as calcium, magnesium, sulfur, boron, chlorine, copper, iron, manganese, molybdenum and zinc. Essential nutrients formulated in the proper balance and amount are what make a good hydroponic fertilizer.
The list of nutrients beneficial but not essential to plants is a smaller category consisting of four elements: aluminum, sodium, cobalt and silicon. Before we go on, it has to be said that not all of these are all that beneficial to all plants.
In fact, aluminum, sodium and cobalt can cause severe toxicities in most plants. Silicon, however, has an interesting chemistry that prevents it from causing harm to plants and is extremely beneficial to many types of plants.
Silicon makes up 28% of the Earth’s crust and is a major component of soil. It is generally available to soil-grown plants at low levels, but in some areas, it has been depleted from the soil, which can lead to a number of issues with crop plants.
As hydroponic growing became more popular, the importance of silicon became even more apparent. Even when using hydroponic media such as stonewool or hydroton, which are essentially silicon-dioxide/silica/quartz/sand, silicon does not become available to plants because these media are stable and do not break down into available silicon.
This attribute makes these mediums ideal, as it means they do not degrade or interact with water chemistry, but they are a poor source of silicon for plants.
Plants can either be silicon accumulators or silicon rejecters, which refers to the amount of silicon that accumulates in the tissues of the plant. For example, rice, which is a silicon hyper-accumulator, can have as much as 10% of its tissue dry weight be silicon. Tomato, a silicon rejecter, has around 0.1% silicon in its tissue.
This method of classifying plants may seem over-simplified because although silicon may not be present in some plants at high quantities, this does not mean it is unimportant. Tomatoes, for example, can suffer from silicon deficiency, which can impact flowering, yield and plant quality even though the tissues typically contain a smaller amount of silicon compared to plants like rice.
The bulk of research on silicon in horticulture has been done on rice and sugar cane plants, which are silicon hyper-accumulators. In these crops, silicon has been shown to help with issues such as improved resistance to drought stress, heavy metals and salt stress; increased structural stability; and improved disease resistance.
In recent years, the focus of silicon research has moved from silicon hyper-accumulators to other plants. Current research at the University of California, Davis is focused on horticultural crops such as dwarf citrus, chrysanthemum and roses.
Preliminary results point to a decrease in pest populations on both citrus and chrysanthemums. Researchers have found a decrease in leaf miner populations on these crops when potassium silicate was included in irrigation water. Past research by other scientists has also shown a decrease in powdery mildew, specifically on roses and cucumbers.
There are several theories about how silicon affects plants. First, it is believed silicon simply makes plants tougher. Silicon is taken up by the plant through the roots and accumulates in the tissues.
When the concentration of silicon in plant tissue gets to a certain point, the silicon molecules begin to bind with one another, making crystalline structures in the plant. The silicon structures form around the plant’s epidermal cells, creating structures unique to each plant species (trichomes, sclereites, thicker leaves, etc.).
These structures do not readily break down after the plant dies. Interestingly, many ancient plants were silicon accumulators, so paleobotanists have used these structures to identify from soil samples what ancient plants were growing in an area.
In more recent times, this extra strength in plant tissues makes it harder for pests to eat through the plant. It has even been shown that silicon can wear down the mandibles of chewing pests as they feed, slowing the growth and development of the pests. These tougher plants are also more structurally stable, so in addition to being more pest-resistant, they are less likely to wilt or fall over.
Second, it is believed that silicon helps enhance the plant’s internal plant defenses. This includes a host of compounds that reject fungal infections, deter pests from feeding and may actually signal pests’ natural enemies (predators and parasitoids) to come to the plant to eat the pests.
The third theory is a little more complicated and states that as the concentration of silicon in plant tissue increases, a pest has to eat more tissue to get the same amount of nutrients. Thus, if a plant is 10% silicon, a pest has to eat 10% more plant material to get the same nutrients as if it were eating a plant with no silicon.
This significantly increases the amount of time the insect needs to develop and allows for other control tactics such as biological control to provide pest suppression. This slowdown in individual growth can also slow down population growth and may allow for the crop to outgrow the pest.
It may also force the pest to undergo more mouth-part wear as discussed earlier. This theory is only applicable in a few plants that accumulate very high levels of silicon, such as cattail or equisetum.
The last theory relates to how silicon may chemically react with ions in the water, making them unavailable to plants. This has been shown in soils with high aluminum concentrations. When silicon is added to the soil, the aluminum and silicon bind together and fall out of the solution, making the aluminum inactive.
This is also thought to work positively to alleviate some salt toxicity issues, which brings me to my last point. Because silicon interacts with some ions at high concentrations, it can cause precipitates in nutrient solutions if used too frequently.
Be sure to test any silicon product on a small scale first before adding it to your system, and follow the manufacturer’s directions on the label. Silicon will also interact with itself at high concentrations in a neutral pH.
This is why most silicon products are formulated at a high pH. Silicon is easily buffered in solution, and at low concentrations it will have little effect on your pH levels, but this doesn’t mean you should abandon monitoring your pH levels going forward.
All of the research suggests silicon is a beneficial element when it comes to plants, especially in hydroponic systems. Plants evolved growing in soil containing silicon, so it only makes sense they will continue to need silicon when they are grown in water culture.
Co-authored by Danny Klittich.