Do Your Plants Need More Silicon?

One of the hottest new additives for plants is silicon, which is also incorrectly called silica. It is claimed to reduce pests, improve growth and fend off diseases such as powdery mildew. Although there is some truth to the claims, there is also a lot of misinformation in the gardening community.

What is Silicon, Silica and Silicone?

Let’s first sort out the terminology which gardeners and fertilizer manufacturers tend to use incorrectly.

Silicon is an element in the periodic table with the short form of Si. It has similar chemical characteristics to carbon and is absorbed by plants. Gardeners tend to use the term silicon in the same way as we use iron, potassium, nitrogen etc. These are chemical elements and nutrients that plants use.

Silica is silicon dioxide a compound with the chemical formula SiO₂. It’s a critical component of quartz and sand where it forms complex 3D structures. Many gardeners use the term silica incorrectly. When they say they add “silica” to the garden they usually mean they are adding silicon.

Silicone is a synthetic polymer of silicon that has a rubbery texture. You may be familiar with silicone breast implants and silicone caulking material. Silicone has nothing to do with plants or gardening.

Do Plants Need Silicon (Si)?

Silicon is a nonessential nutrient which means plants do not need it.

However, plants do absorb silicon from soil and recent science shows that it does provide some benefits to plants. It is now called a “beneficial nutrient”.

Soil naturally contains lots of silicon, somewhere around 30% by weight. If soil contains such a high level, why would gardeners need to add more? The reason is that most forms of silicon are not soluble in water and therefore plants can’t absorb it.

Potting media also contains silicon-based ingredients like sand, perlite and vermiculite, but these do not provide plant available silicon. Plants growing in potting media are exposed to almost no silicon unless we add it.

Plant roots, both in a pot and in the ground, are surrounded by compounds containing silicon but almost none of it is useable by them. The only form of silicon they can use is silicic acid (also called mono-silicic acid and orthosilicic acid) with the formula H₄SiO₄, or silicate salts of this acid (see below).

“Most dicots (broad-leaf plants) take up small quantities of silicon and accumulate less than 0.5% in their tissue. Some monocots (grasses) such as rice and other wetland grasses accumulate up to 5-10% silicon in their tissue, which is higher than normal ranges for nitrogen or potassium.”

The concentration of Si varies by organ type with the highest level found in mature leaves.

How Do You Add Silicon to Plants?

Most fertilizers do not contain silicon. Tap water may contain low levels.

Silicon can be added as silicic acid, calcium silicate (Ca₂SiO₄) or potassium silicate (K₂SiO₃). As a buyer you have to be very careful buying these products. Do not go by name on the bag. I found several products that use the term silicon and actually contain silica (SiO₂), which is not plant available. Check the ingredients to make sure it contains silicic acid or a silicate.

Silicon is not very soluble in water and easily precipitates when combined with other fertilizer. If you are mixing a liquid form of Si with fertilizer follow the instructions carefully. If you see a precipitate form you have probably lost the silicon. One way to solve this problem is to alternate between regular fertilizer and silicon, making sure not to use them at the same time.

Alternatively, you can add the dry silicon directly to the soil and mix it into the top layer.

Foliar vs Soil Application

Silicon can be delivered in several ways: foliar spray, incorporation into soil and liquid fertilizer.

Foliar sprays of 100 ppm Si have been used in poinsettia production to prevent bract edge burn. This works for leaves and bracts, but the silicon is not moved by the plant into the roots.

Silicon can also be incorporated into potting media and some companies are now adding it. It is not known how long the material is available to the plant. It may need to be replenished periodically.

A concentration of 50 ppm Si can also be used as a regular weekly fertilizer.

Does Rock Dust Add Silicon?

Rock dust is a very generic term that is used for many types of rock. Will it add silicon for plants? That depends on the source and the chemical analysis of the product.

Wollastonite is a special type of rock dust that is mostly calcium silicate (Ca₂SiO₄) and is a good source of Si for plants. Many research studies use it.

Azomite is a very popular rock dust. It’s chemical analysis shows that it has 66% silicon dioxide ( SiO₂). It does contain lots of silicon but not in a form that plants can use. So it is a poor choice for adding Si to plants, and in fact it is a poor choice for adding any nutrients to most soils.

Diatomaceous Earth is called amorphous silica, but it does contain plant available Si.

Weed Tea

Several plants are known to accumulate higher levels of Si, including horsetail, ferns and some grasses. Can these be used to make a weed tea that will provide Si to plants?

Much of the silicon in plants is accumulated as phytoliths, which are hard forms that damage insect mouthparts. When these were tested from various sources, they were found be soluble at a similar level to amorphous silica (i.e. not very soluble). “The minimal half life time of larch and horsetail phytoliths in the interstitial soil solution ranges from 10–12 years at pH = 2–3 to < 1 year at pH above 6″.

What this means is that weed tea, compost tea or manure tea will contain very little plant available silicon.

Effect on Horticultural Plants

Much of the research is focused on agricultural crops but some studies have looked at horticultural plants. One example is a study done by Cornell University that looked at 21 cultivars growing in a peat-based substrate. They reached the following conclusions.

• Some of the cultivars showed an increased level of Si after treatment, but about half showed almost no change.
• Plant growth characteristics were improved in some cultivars, but were reduced in others. Most cultivars were unaffected by Si.
• Cultivars within a species can show quite different responses to Si.
• Silicon helped about 1/3 of the plants that were stressed with salt (sodium chloride).

Silicon Can Reduce Environmental Stresses

Claims for higher levels of silicon in soil:

• Silicon is a beneficial plant nutrient widely recognized for its ability to help plants build stronger roots, stems and foliage.
• Produces superior plant health.
• Helps ensure your plants are getting the nutrition they need.
• It enhances photosynthesis by increasing chlorophyll content and improving the efficiency of light capture.

Each of the above is partially true but only in specific situations. They are not true for every kind of plant and in every situation.

Environmental stress such as temperature extremes, drought, salinity and wind are known as abiotic factors and they make it harder for a plant to grow properly. Some plants that are exposed to higher silicon levels grow better in stressed conditions.

However, the same studies also show that this improved growth only happens in stress conditions. Without stress, the plants don’t grow any better with extra silicon. What this means is that when you care for plants correctly and provide them with normal levels of nutrients and water, adding silicon will not make plants grow better.

Many of the studies have focused on monocots (grasses) such as rice, wheat and sugarcane, probably because these plants absorb silicon more readily. Many garden plants are dicots which have not been studied as well.

Water Stress in Potted Plants

Research conducted at Cornell University found that ornamental crops also benefit from added silicon. Poinsettias raised in a greenhouse and supplemented with 50 ppm silicon not suffered less when they weren’t watered for 18 days, but they also recovered more quickly once watering started again.

Silicon Reduces Diseases

Silicon improves the mechanical strength of plants making them better at fighting diseases. In some cases it is as effective as conventional fungicides and can be used to reduce stem rot, sheath brown rot, cucumber Fusarium wilt, crown spot disease, grain discoloration, gray leaf spot, dollar spot and brown patch and powdery mildew. 

Several studies have shown a reduction of powdery mildew with added silicon. This study by University of Maryland compared a resistant pumpkin cultivar to a non-resistant one, both with and without calcium silicate. Adding silicon did not increase the yield, but it did reduce the amount of powdery mildew. The resistant cultivar did have higher yields than the non-resistant one. Si did not reduce the incidence of downy mildew.

Silicon deficiencies in soil are not visible by looking at plants but Rutgers University suggests that high levels of powdery mildew might be an indicator of such a condition.

It is important to understand that silicon is not a cure and in most studies it did not eliminate the disease. It can reduce the amount of disease and/or delay the onset of the disease. For example, supplemental Si helped control powdery mildew on zinnia, phlox, and sunflower but control was not complete. Initial symptoms were delayed by about a week and the expansion of white spots (colony size) was delayed up to two weeks. That can be of value to greenhouse production, but is it of value to a gardener?

Silicon Reduces Pests

What happens to the silicon absorbed by plants? It is deposited in the walls of epidermal cells, just beneath the cuticle where it helps strengthen cell walls. It also forms hard structures called phytoliths. Think of them as hard sand-like structures of various shapes and sizes. They give the plant rigidity as well as make the leaves and stems less palatable to pests. The phytoliths are strong enough to wear off the mouthparts of chewing insects.

Plants with higher Si levels have fewer pests including both insects and animals. This is due to both damage to mouth parts as well as making the food less digestible. The effectiveness of Si as a deterrent depends on the actual pest. For example aphids are hardly affected by Si.

As plants absorb more Si, the tiny spines on leaves increase in number and size resulting in a much more abrasive surface. This makes them less desirable to herbivory.

Another interesting fact is that a higher level of insect damage on leaves results in the plant absorbing more silicon to defend itself. It is the insect saliva that triggers this response, not the act of chewing.

Should Gardeners Use Silicon?

There have been quite a few studies that show silicon does benefit plants, at least in special cases. It is important to understand that it has limited value when the plant is grown well and has no stresses. And some plants don’t benefit from extra Si even in the presence of stress.

In general, Si is probably not worth using if you grow your plants well. If you garden in difficult climates it is probably worth adding to soil. Plants growing in windy climates will be stronger and better able to withstand the wind. Plants growing in dry climates are less affected by draught. Areas with significant pest problems may also benefit.

Only a few of the studies I looked at reported negative effects, however, some horticultural plants did grow smaller and flower less with added Si.

Several tests have been done on seedlings and they show the same results as for larger plants. There is a benefit if the seedlings are under stress. Since most home grown seedlings endure some stress, are planted in soilless media and are also exposed to stressful conditions as they are moved outdoors, it is probably beneficial to use Si on seedlings.

The type of soil is also important. If the soil is a mucky peat-based soil that contains few minerals it will have lower natural levels of silicon. In that case adding some should have a stronger effect than adding it to mineral soils. For example, rice farmers in Japan found the addition of silicon to be very beneficial when added to their rice paddies that contain high levels of organic matter. Rice also absorbs lots of Si.

There is very little Si in potting media and most house plants are stressed by irregular water levels, higher salt levels and low light conditions. It may be the perfect place to add silicon.

Here are some key take-aways.

• If you grow outdoors, have mineral soil and few stressors, it is probably not worth adding Si.
• If your garden in stressful conditions, either abiotic (climate) or biotic (pests and diseases), give Si a try.
• If you start seedlings indoors, try some Si.
• If you grow houseplants, give Si a try.

Remember that plants grow just fine without Si, so don’t stress about using it.