Soil Amendments: Silicon and plant health

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Strength & Resilience: Silicon and Plant Health

Although silicon is abundant in the soil, only a very small percentage is actually plant-available. Continued cropping, particularly of silicon-accumulating crops, can reduce this amount. Progressive growers know that promoting plant and soil health is the first and most important step in pest management. The basis of plant health is soil health, and nutrient balance is a key aspect. Silicon is an important part of that balance.

The Role of Silicon

When most of us think of silicon, we probably think of silicone kitchenware – the flexible, yet strong and temperature-resistant material popular for baking utensils (silicone contains silicon, but is a manmade compound). Or we think of the silica in sand, the primary component of glass and semi-conductors. Both of these images can be helpful when considering the role of silicon in plants.

Silicon is incorporated by plants into the cell walls, thickening and strengthening them on the molecular level. It is logical then to think that these thicker, stronger cell walls would benefit the plant, and research supports this assumption. Studies show that plants rich in silicon resist lodging (where the stalk of a plant, usually a grass or cereal, is unable to support the maturing head and lays flat) and are better able to withstand drought, frost, and other abiotic stresses.  Cereals, grains, grasses, cucurbits, and grapes can particularly benefit from silica applications; however, increased silica in pastures can also decrease livestock palatability and grazing.

Stronger cell walls also act as a barrier against invaders. Insect pests find it harder to poke or chew through the cell walls to reach the life-giving fluids within, and even fungal hyphae (the branching filaments responsible for fungal growth) have more difficulty penetrating the cell wall to infect the contents and spread disease. This passive resistance to disease infection has been observed under electron microscopes.

There’s more: the silicon ion also plays a role in the immune systems of plants. Unable to flee “predators”, plants have developed complex biochemical defenses that activate under attack. In fact, some of these phenolic compounds are actually exuded through plant roots and become chemical “messengers” in the soil, alerting neighbouring plants to the danger and activating their own defences. Another role of these phenolic compounds is to trigger the plant into increased production of lignin and other chemicals at the site of attack, laying down barriers against the attackers.  Silicon plays a role in the manufacture, or synthesis, of these compounds, as well as helping them move around the plant more effectively and efficiently.

The Research

A growing body of research demonstrates silicon’s effect in passive and active resistance against abiotic and biotic stresses. The following are a few highlights (links to these and other studies can be found in our research section):

  • In a study comparing treatment of soil with calcium carbonate and calcium silicate on disease development in soybeans, calcium silicate reduced incidence of frogeye spot and downy mildew; other research showed increased resistance to stem canker;
  • Silicon supplementation decreased bract edge burn in poinsettia; decreased powdery mildew in zinnia, sunflower, and phlox; enhanced flower size of gerbera; increased resistance to metal toxicity in zinnia; decreased population growth of aphids on zinnia; improved salt-tolerance in New Guinea impatiens; and improved shelf-life of poinsettia;
  • Calcium silicate suppressed development and decreased final severity of leaf spot in Bermudagrass;
  • Wollastonite soil amendment significantly decreased gray leaf spot incidence and severity in perennial ryegrass;

Greenhouse work and multi-year field trials by Dr. Joseph Heckman at Rutgers University, New Jersey, demonstrated numerous benefits over the course of a crop rotation including:

  • Significant increase in total dry matter yield in Kentucky bluegrass with calcium silicate vs. calcium carbonate amendments, with no greater powdery mildew pressure under the denser canopy;
  • Powdery mildew suppression (delayed onset and reduced incidence) in pumpkins, resulting in an average 18% yield increase over 2 years (equivalent to the yield increase gained with fungicide applications);
  • Residual calcium silicate in the soil following the pumpkin crop reduced the length of corn borer tunnels in corn stems (no impact on yield observed);
  • Calcium silicate amendments reduced powdery mildew lesions on wheat 29% and 44%, and resulted in a 10% yield increase in one of the two years studied.

These results indicate that increasing silicon levels in soils and plants could be of benefit to agricultural producers, particularly in organic and Integrated Pest Management oriented systems.

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Sourcing Silicon

Older, more highly-weathered soils are more likely to be deficient in silicon, as will soils that have been continually cropped, particularly to grasses or other plants that tend to accumulate large amounts of silicon. Soils low in organic matter also tend to be lower in plant-available silicon. Organic matter itself is not a source of silicon, but the biological activity stimulated by higher organic matter levels can make existing reserves of silicon more available. Improving plant health with silicon supplementation can happen by either feeding the plant or feeding the soil.

Crop residues and mineral sources can be used to increase silicon levels in the soil. Because grasses tend to take up a lot of silicon, straw can be a good source, assuming, of course, that it was grown on silicon-rich soil to begin with, and that there is sufficient biological activity to cycle the nutrients back into the system. It may take several years for the silicon content of crop residues to become available to growing plants again, and some of the silicon found in plant residues like straw is in the form of “phytoliths” which are very resistant to breakdown.

Quartz-containing minerals, like sand and granite, are high in silicon, but these silicon dioxide crystals are not readily-available to plants. Research indicates calcium silicate is the most common source of silicon as a soil amendment. This silicon is in a non-crystalline, amorphous form, meaning that in the presence of water, it is available to plants as silicic acid. In addition, the calcium component acts much the same way as the calcium from limestone: to balance soil pH and supply calcium for plant growth.

To date, most of the calcium silicate used in agriculture has been sourced as slag from stainless steel manufacturing and is not permitted for use in organic agriculture. Wollastonite from Canadian Wollastonite is the natural form of calcium silicate. It is OMRI-listed for use in both the United States and Canada and approved for use by EcoCert Canada.

For a more detailed description of the various roles of silicon in improving the health and yield of crops around the globe, please read this blog post from Nutrition Matters.