Silicon Needs of Soils and Crops

Silicon is a mineral present on earth in abundance. The element makes up about 28% of mineral soil by weight.

Until recently, this ubiquitous element was not given much attention as a possible limiting factor in soil fertility and crop production. Agronomists now recognize valuable functions of silicon nutrition in crops and soils, and even animal life.

Research conducted on New Jersey soils and in many places around the world have shown that applying supplemental silicon in a chemically available form can protect plant health and benefit crop production.

Chemical Names and Terminology

Silicon (Si) is elemental silicon also known as the chemical element. Silica, silicon dioxide, or SiO₂, are compounds with silicon and oxygen. Silicate refers to silicon compounds such as CaSiO₃, MgSiO₃, or K₂SiO₃. Silicic acid or mono silicic acid (Si(OH)₄, or H₄SiO4) refers to the soluble, plant-available form of silicon in soils. Silicone refers to R₂SiO, where R is an organic group such as methyl, ethyl, or phenyl.

Function of Silicon in Plants

Silicon is beneficial to many crops when added to some soils as a fertilizer. It is not classified as an essential element for most plants but horsetail (Equisetum) and some types of algae cannot survive without a supply of silicon from the environment.

Many plant species, especially grasses, can take up silicon in amounts comparable to macronutrients. This high concentration of silicon in the plant contributes to plant mechanical strength. Besides a structural role, silicon may protect plants from insect attack, disease, and environmental stress by improving the plant's defense response. For some crops, silicon fertilization of soils increases crop yield even under favorable growing conditions and in the absence of disease.

Specific benefits observed due to silicon nutrition are extensive:

• Direct stimulation of growth and yield
• Counteracts negative effects of excess N nutrition
• Suppression of some plant diseases caused by bacteria and fungi. Diseases suppressed include powdery mildews on cucumber, pumpkin, wheat, and barley and gray leaf spot on perennial ryegrass
• Suppression of stem borers, leaf spider mites, and various hoppers
• Alleviates various environmental stresses. Silicon protects against lodging, drought, temperature extremes, freezing, UV irradiation, and chemical stresses including salt, heavy metals, and nutrient imbalances

In animals, silicon strengthens bones and connective tissue. Vegetables, grains, and fermented grain products are sources of silicon in human nutrition.

Silicon Deficiency Symptoms

Symptoms of silicon deficiency are generally not visually apparent in an obvious way in the field.

Indirectly, silicon deficiency may be exhibited as an increase in susceptibility to certain plant diseases. Crops such as pumpkin, cucumber, wheat, and Kentucky bluegrass are susceptible to a disease called powdery mildew. Providing enhanced levels of silicon nutrition for these crops may suppress or delay the onset of the disease. When crops exhibit a high level of susceptibility to powdery mildew, this may be considered as a sign of silicon deficiency (Figures 1 and 2).

As a result of increasing silicon concentrations in plant tissues the mechanical strength may be increased, helping to protect the plant from infection. Studies have shown that the amount of insect attack on plant tissues may also be inversely related to silicon uptake. So, the response of plants against plant pathogens and insects at the biochemical, physical, and molecular levels are remarkably similar when silicon is taken up by roots and translocated to shoots, suggesting an active role played by this element in plant defense.

Grain crops lacking adequate silicon are more susceptible to lodging.

Soil Analysis

The total elemental analysis of typical mineral soil is about 28% silicon and 47% oxygen. The majority of this silicon is bonded with oxygen and other elements in the crystalline fabric of mineral soil. With weathering and time, some of this vast supply of silicon is released in soluble forms available for uptake by plants.

Although soil testing for silicon availability is not a routine part of soil fertility testing, some laboratories offer an acetic acid extractable soil silicon analysis. At present, the database is very limited in correlating silicon soil test levels with plant silicon uptake. More research is needed to find better soil test methods to predict silicon availability.

Interpretation of any soil test requires years of field research. When eighteen New Jersey soils were collected from across the state and tested using the acetic acid soil extraction method, they exhibited ranges of soil test silicon from 4 to 35 mg/L, with the average soil test silicon level being 14 mg/L.

In our research fields, some crops susceptible to powdery mildew benefited from silicon fertilization when the soil tested greater than 40 mg/L silicon (acetic acid extract). These field trials suggest that many New Jersey soils have less than optimum levels of available silicon for protecting crops normally susceptible to powdery mildew.

Soil pH measurement and tests for lime requirement are useful guides for determining how much of certain types of silicon fertilizer products may be applied to a given field soil. Information on using this soil test will be discussed in the section under application rates.

Factors Affecting Silicon Availability

Soil texture refers to percent sand, silt, and clay particles in a soil. Silicon is a component of these mineral particles of varying size. Although sand is largely composed of silicon dioxide, this material provides very little soluble or plant-available silicon. And it is not unusual for crops grown on sandy soils to benefit from applications of soluble silicon.

In general, older and more highly weathered soils are more depleted of silicon than geologically young soils. Many of New Jersey's soils are classified as Ultisols. These soils, which have been subjected to leaching in a humid environment for a very long time, tend to have less remaining weatherable minerals. Consequently, Ultisols tend to be relatively depleted of silicon.

Silicon is not a major component of soil organic matter. Soils composed almost entirely of humus and organic matter are called muck soils or Histosols. Because the substrates of such soils are almost devoid of minerals, they are inherently low in silicon content.

The use of peat-based soilless mixes in greenhouse production means that very little silicon is being supplied from the growth medium. Greenhouse production systems have also been shown to benefit from silicon fertilization. Some commercial greenhouse mixes are now pre-amended with silicon fertilizer.

Silicon availability does not change markedly across the soil pH spectrum used to grow crops. Many of the commonly used silicon fertilizer materials also serve as liming agents and their application results in neutralization of soil acidity.

Soils amended with plant-available silicon fertilizers will typically enhance uptake of silicon by crops for a period of several years. Thus, growers may plan a rotation cycle where a series of responsive crops can take advantage of the potential residual benefit of previously applied silicon.

In summary, field research suggests that many New Jersey soils have less than optimum levels of available silicon for crop production, especially plants most susceptible to powdery mildew disease.

Plant Tissue Analysis and Crop Uptake

Silicon concentrations in plants, in some cases, may reach levels comparable to or above those for macronutrients nitrogen, phosphorus, or potassium.

Concentrations of silicon in plant tissue can vary widely depending on plant species and silicon availability from the soil. Grasses and monocots in general tend to accumulate silicon. Concentrations as high as 10% silicon are possible in some plant species such as Equisetum. Concentrations near 1% are common among grasses.

Dicotyledonous plant species in general have fewer tendencies to accumulate silicon and some species may grow adequately with levels at about 0.1% Si in plant tissue.

Optimum silicon concentration levels have not been established for many crops grown in New Jersey. However, research conducted on local soils and crops suggest the concentration ranges that may occur for some crops. For example, supplying supplemental silicon to a Quakertown soil used to grow pumpkin, corn, and wheat resulted in large increases in concentrations of silicon in the plant tissue. Silicon concentrations in pumpkin leaf tissue increased from 700 ppm to 3,500 ppm; in corn stem tissue from 1,300 ppm to 3,300 ppm; wheat flag leaves from 1,530 ppm to 11,750 ppm; and Kentucky bluegrass leaves from 4,200 ppm to 7,200 ppm.

For optimum disease suppression and grain yield of wheat, a silicon concentration of 1% (10,000 ppm) or more in the flag leaf is recommended.

The harvest of wheat straw can uptake and remove from soil about 40 lb per acre of silicon.

Crops Responsiveness to Silicon

Crops may benefit from disease suppression, reduced injury from insect pests, stronger stems, and tolerance to stress, or direct stimulation of yield from supplemental silicon applications.

Around the world, rice and sugarcane are the crops that are well-known to exhibit beneficial responses to silicon fertilization. Of crops commonly grown in New Jersey, pumpkin, corn, wheat, oats, Kentucky bluegrass, and dogwood, may benefit from silicon fertilization.

Crop groups that are considered good candidates for silicon fertilization include cucurbits, grasses, and small grains. Any crop susceptible to powdery mildew and/or grey leaf spot would appear to be good candidates for field responses to silicon fertilization.

Silicon Fertilizers

Silicon is now officially designated as a plant beneficial substance by the Association of American Plant Food Control Officials (AAPFCO). Plant-available silicon may now be listed on fertilizer labels.

To be an effective source for crops, a silicon fertilizer should provide a high percentage of silicon in soluble form. Other characteristics to consider are cost of material, physical properties, and ease of application. Some silicon fertilizers supply other nutrients, neutralize soil acidity, and serve as liming materials.

Because silicon in nature is always combined with other chemical elements, the agronomic value of the other elements that accompany the product should also be considered. Some of these elements may be valuable plant macro and micronutrients.

Commercial silicon products are marketed as either solids or liquids. In the case of solids, plant-available silicon increases as particle size decreases.

Calcium silicate products are the most commonly applied silicon fertilizers for field application. Steel mill slags are a rich source of calcium silicate. Because calcium silicates neutralize soil acidity and supply calcium, they are commonly applied to soil as an alternative liming agent in much the same way as agricultural limestone or calcium carbonate. Slags vary in purity, silicon availability, and liming ability (rated as calcium carbonate equivalent or CCE). A fine particle size, purity, and a high percent concentration of soluble silicon are desirable properties of a calcium silicate or slag byproduct.

Crossover® (Harsco, Camp Hill, PA) is a commercially available silicon product for agronomic crops. It is made from stainless steel slag that has been subjected to processing to remove metals resulting in a calcium and magnesium silicate product that typically contains 30% Ca, 7% Mg, and 12% Si. With a calcium carbonate equivalent value of 93%, Crossover® can be used as a liming material at about the same application rate as would be recommended for calcium carbonate or dolomite liming materials. A calcium and magnesium silicate fertilizer with added micronutrients called Excellerator® is also available from the same manufacture designed mainly for the turf industry.

Wollastonite is naturally occurring mined calcium silicate. Mined minerals are usually permitted for use in organic farming. Organic farmers should check with their certifier to be sure that a particular source of silicon fertilizer is permitted for use in organic farming. The Organic Materials Review Institute (OMRI) has listed a commercially available wollastonite product approved for use in organic agriculture. Research conducted at Rutgers NJAES has demonstrated that finely ground wollastonite is an excellent source of plant-available silicon.

Potassium silicate and sodium silicate are more commonly used for horticultural or greenhouse crop applications. They are soluble products that can be added to nutrient solutions or used as foliar sprays. Plants benefit more from soil rather foliar applications of silicon. This is because the supply of silicon to plant roots must be continuously present or it will be less effective at disease suppression.

Fertilizer sources that are by-products of industry may contain high levels of heavy metals. Such materials if applied as fertilizer will also add heavy metals to soil. Materials containing heavy metal concentrations greater than that allowed by regulators, or believed to be unsafe relative to other soil amendments, should not be used in agriculture. Samples of questionable products can be collected and tested by the New Jersey Department of Agriculture, P.O. Box 330, Trenton, NJ 08625. Phone: 609-984-2222. In Washington State, all commercial fertilizer products must be tested. The analytical results are available on the internet. Because many of the fertilizers listed are national brands, New Jersey growers can use this information for selecting products with low heavy metal content.

Application of crop residues, manures, and compost also add silicon to soil. Straw from wheat and other small grain crops may contain significant amounts of silicon. Wheat straw silicon concentrations may range from 0.15 to 1.2% Si depending on the silicon fertility level of the soil on which it was produced. Demand for silicon by crops on some soils may exceed the ability of plant residues and compost to supply available silicon. Increased soil biological activity associated with organic matter may improve solubility of silicon from soil; however, it may take many years for silicon from crop residues to become available for plant uptake.

Some of the silicon in plant residues occurs in the form of "plant stones" or phytoliths. These silicon structures are very resistant to decomposition and many persist in soils for very long periods.

Silicon Fertilizer Practice and Rates of Application

In general, silicon fertilizers should be applied to the soil, soilless mixes, or added to nutrient solutions. Spraying silicon fertilizers on plant foliage is generally not effective.

The need for silicon fertilizer is not easily predicted by currently available soil tests for extractable silicon. But soil testing for soil pH and need for liming can be very useful in determining the proper application rates for calcium silicate sources.

A practical approach to managing soil fertility for enhanced silicon nutrition of crops is to use calcium silicate products, such as wollastonite, as liming materials. Appropriate application rates can be determined by the need for soil pH adjustment or lime requirement of the soil. The greater the lime requirement of the soil, the higher the application rate possible for calcium silicate.

Another soil test factor to consider is the percent saturation of the soil colloids with calcium, magnesium, and potassium. Silicate products containing these cations can be used to supplement the balance of soil fertility on the cation exchange complex (CEC).

Over-application of silicon to soil from calcium silicate is generally not a concern because target soil pH levels would limit how much can be applied. Thus, application rates for calcium silicate may range from 1 to 6 tons per acre depending on the initial soil pH level and the target soil pH range for the crop to be grown.

When heavy application rates of liming are needed, use calcium silicate or wollastonite and direct it to the crop fields most likely to benefit from the silicon application. For example, fields to be planted to pumpkin, wheat, or other crops are known to benefit from silicon fertilization. High-value horticultural crops may benefit from soluble silicon fertilizers, such as potassium silicate or sodium silicate, applied through drip irrigation systems or through calcium silicate additions to soilless mixes.

References

1. Provance-Bowley, M, JR Heckman, and EF Durner. 2010. Calcium silicate suppresses powdery mildew and increases yield of field grown wheat. Soil Science Society America Journal 74:1652-1661.
2. Heckman, JR., S Johnston, and W Cowgill. 2003. Pumpkin yield and disease response to amending soil with silicon. HortScience. 38(4):552-554.
3. Hamel, SC and J.R. Heckman. 1999. Impact of mineral silicon products on powdery mildew in greenhouse grown turf. Rutgers Turfgrass Proceedings 31:215-220.
4. Torlon, JT, JR Heckman, JE Simon, and CA Wyenandt. 2016. Silicon soil amendments for suppressing powdery mildew on pumpkin. Sustainability. 2016, 8, 293; doi:10.3390/su8040293.
5. Tubaña. BS and JR Heckman. 2015. Silicon in Soils and Plants. 45 pages. In: FA Rodrigues, and LE Datnoff (eds), Silicon and Plant Diseases. Springer.

October 2017