It’s time to speak about another unsung hero of plant nutrition: and what an element it is! Silicon (Si) is the second most abundant element on Earth; so many things contain Si: sands (in the form of quartz), clays, plenty of rocks (granite, obsidian, etc.) and nowadays most computer circuits. And obviously it’s also in plant and animal bodies.
☘ However, until very recently it was completely neglected as a plant nutrient. The main reason for this is that Si is only essential to very few plants: Horsetails (most of whose relatives are extinct) and some wetland grasses (including Rice). Fortunately, in the last three decades we have discovered that it has an outstanding number of
beneficial effects on all plants.
🌿 First, let’s look at what plants do with Silicon. Plants take it up as Silicic acid, which is quite similar to Boric acid (remember Boron - the steering wheel?); they are both very weak acids, not electrically charged, and they are mainly located in the cell walls. This is something that Calcium, Boron and Silicon - a great trio! - have got in common; and it’s for this reason that they are paramount when it comes to plant resilience.
💪 Thus, not surprisingly, optimal Silicon nutrition confers plants the ability to withstand all sorts of stresses. In recent years, numerous papers have shown the efficacy of Silicon in alleviating salt, drought, UV irradiation, cold and heat stress, as well as lodging in cereals.
Silicon gives the cell wall not only strength and rigidity, but also elasticity (very important in cotton production), and this is crucial, because large amounts of Silicon are present in the circulatory system of plants, consisting of two types of piping: the xylem (for water) and the phloem (for nutrients). Therefore, Silicon has a big impact on mineral uptake and transport, as well as keeping stems and leaves solidly in place.
🐛 Perhaps even more importantly, Silicon makes plants really impervious to insect pests and fungal diseases. This happens in three ways: firstly, it gets deposited on the surface of leaves and stems, forming a very tough double layer, which is difficult to penetrate by fungi and insects. Secondly, Silicon is used by the plants to as an additional immune agent (if you have read the Sulphur post, this should ring a bell!) Indeed, it has been shown that when a plant is under attack by a pathogen, it directs loads of Silicon to the attack site to strengthen the surrounding cells and stop or slow the spread of the disease. However, because Silicon is relatively immobile once incorporated into the cell wall, it must be constantly supplied in order for this response to work.
Some diseases that can be reduced by applying Silicon are: Powdery mildew in cucurbits (squash family), Phytopthora in avocados, Fusarium wilt in potatoes, brown rust in sugar cane.
🐞 Things get even more interesting when it comes to insect pests. Si accumulation in plants makes tissues less digestible for insects, but it also interacts with the three major phytohormones (jasmonic acid, salicylic acid and ethylene), and this results in a cascade effect that makes the plant produce HIPVs (herbivory-induced plant volatiles), which attract the natural enemies of the attacker. This mechanism is really effective in grasses (including grains).
🥒 And if all of this weren’t enough, Silicon has proven to give substantial yield increases (up to 15%) in rice, wheat, sugarcane, potatoes and other crops.
🔩 Good amounts of Silicon in the soil have also been shown to mitigate Copper, Cadmium and other heavy metal toxicities, this acting as a detoxifier for plant roots grown in contaminated soils.
And that brings us to the second piece of the puzzle: what’s Silicon like in the soil? Where is it found and how do we get it there?
🌍 You might think that because Silicon is so abundant in most soils, plants should always have enough. However, most of the silicon in soils is found as part of the structures of stable, rather insoluble minerals and only a miniscule fraction is in the form of silicic acid, which plants can take up.
🦠 Conventionally farmed soils are particularly bad at converting this insoluble silicon into the plant available form, probably due to a lack of microbial activity. Only very recently it has been found that silicate-solubilising bacteria (SSB) include a large range of microorganisms: Pseudomonas, Bacillus, Rhizobia, ****Enterobacter, etc. - which are also able to unlock very insoluble phosphates. Good news is that in biologically active soils, such as no-dig ones, these guys are most certainly present. Diatoms (some of the most beautiful microorganisms) have cell walls composed of transparent, opaline silica with intricate and striking patterns.
Disease suppressive soil should contain 100 ppm of monosilicic acid (as measured in a soil analysis) but most soils (especially tilled ones) have only about half of that amount.
Silicon (Si) deficiency affects the development of strong leaves, stems, and roots. As we have seen, it also makes plants susceptible to fungal and bacterial diseases, plus insect and mite pests. This is usually seen in Rice, which is a heavy Silicon feeder.
That’s not very reassuring, so let’s see what we can do to improve the situation.
🌋 Normally Silicon is found in good levels in rock dusts such as Wollastonite, as well as in guano. However, this is not the plant available form, and it may take many years for the mineral to become available.
🌾 Another source of Silicon is that deposited by plants in their tissues as biogenic (biologically formed) silica, which solidifies and forms silt-sized particles called phytoliths (”plant rocks”). Mulching with crop residues or compost made from plant materials, will reintroduce phytoliths in the soil. These are really long-lasting and, if microbial activity is right, will slowly re-dissolve, providing an important source of relatively plant-available Si. In some cases, though, even this source of Si, may remain unavailable for thousands, even millions of years.
❗ Fortunately, Boron can come to the rescue. Not only is Boron a calcium synergist, but it has recently been recognized that it also boosts Silicon uptake. For this reason, it is a good idea to combine the great trio (Boron, Calcium and Silicon) to ripe maximum results. A common strategy used by some biodynamic practitioners involves the application of Boron to the soil in late winter to trigger the release of Silicon, which in turn improves the slow uptake of calcium, needed for cell division during the spring flush.
💧 If you need to act quickly, and can’t wait for Silicon to become available from the soil, you can get significant benefits by spraying silicon foliarly as a preventative measure or at the first sign of a disease.
So, where can we get some extra Silicon to boost our plants growth, yield, resilience?
Fortunately for us, there are two better options.
🔬 The first one is diatomaceous earth. This is made of the exoskeletons of diatoms. They contain up to 85% silica dioxide, and because of the microscopic size and sharp shape of the particles, it becomes more quickly available. Diatomaceous earth has been used as a natural insecticide for decades, as the jagged, little razor diatom shells can cut through insects exoskeleton causing them to dehydrate and die. It can also be used in animal and humans control intestinal parasites. It’s important that the diatomaceous earth that you use is micronised, that is milled to a particle size of 5 microns. This can then be applied as a spray or soil drench. Even just 5 litres per hectare have shown to give good results!
🌿 Even better, why not use nature’s powerful Silicon accumulators? Horsetails, which thrive in damp conditions, have developed a unique way to take up Silicon and use it to combat fungal attacks; they typically contain 5–10% of Si in their dry matter. Rudolph Steiner mentioned the fungicidal and beneficial properties of Horsetail already in 1924, and biodynamic practitioners regularly use Horsetail decoctions. How do you make a powerful horsetail spray? First of all you’ve got to identify the correct species: Equisetum arvense. The best time to harvest it is June-July, when Silicon level are highest. Then you can choose whether to make an anaerobic tea (slurry) or a proper biodynamic decoction (boiled). In the first case you macerate (let rot) 1 kg of fresh horsetail in 9 litres of water, so that you get an anaerobically fermented tea which can be strained, diluted 1:10 in water and then sprayed foliarly. A quicker option for real emergencies is to cover 1 kg of horsetail leaves in rainwater, then simmer for 40 minutes, cool down, dilute (1:10) and apply. The latter option provides disease resistance, but does not really provide as much nutrition - because most nutrients get degraded during the boiling process.
👫 Let me finish by saying that, although scientists still consider Silicon only a beneficial, non-essential nutrient, for humans and other animals it is known to be essential! It has been shown to inhibit Aluminium toxicity, which has been linked to Alzheimer’s disease, it is also crucial for the structural integrity of nails, hair, and skin, bone mineralization and health, immune system health, and reduction of the risk for atherosclerosis.
🥗 Great sources of silicon for human consumption are Brown Rice and Oats and whole grains in general, Marjoram, Legumes and in particular Red Lentils, green tea and obviously Horsetail tea! Don’t go crazy though, because too much Silicon intake along with limited water may cause kidney stones both in ruminant animals and humans. As usual, don’t trust me when it comes to human health - chances are your body knows things better than you do. It’s just a matter of learning its language and listening in.
Role of silicon in enhancing the resistance of plants to biotic and abiotic stresses (2011) [link]
Mechanisms of silicon-mediated alleviation of heavy metal toxicity in plants: A review (2015) [link]
Silicon and Mechanisms of Plant Resistance to Insect Pests (2018) [link]
Marschner's Mineral Nutrition of Higher Plants (2011) [link]
Mineral Nutrition and Plant Disease (Huber et al, 2007) [link]
Graeme Sait's Nutrition Farming podcast [link]
The Nature and Structure of Soils (Weil, Brady, 2016) [link]