Gary Breitenbeck and Joseph Kraska
Silicon, a common element in our daily lives, generally exists in nature as silicon dioxide – or silica. Silicon is a major constituent of glass, ceramics and computer chips. Next to oxygen, silicon is the earth’s most abundant element and is found in sand, silt and clays – the mineral constituents of soils. The solubility of silicon in water varies greatly among its various forms. A coffee mug, for example, is mostly silica, but we don’t think of it as soluble. Clays in soil are slightly more soluble, and ground water often contains more than 100 parts per million of silicon.
Most plants take up dissolved silica. The silica content of many plants is less than 1 percent of their dry weight, but some species take up large quantities. Such plants are called silica accumulators, where silica can represent 10 percent or more of their dry weight. Silica accumulators include rice, sugarcane, wheat and many other grasses as well as broadleaf plants such as cucumbers. Unlike nitrogen and phosphorus, silica is not essential for a plant’s metabolic function and growth. Even silica accumulators can complete their life cycle when denied silica in their rooting medium. These plants, however, typically grow more vigorously when supplied with silica.
While scientists don’t completely understand the role of silica in plants, it is well-known that adequate silica can cause stronger, more upright stems and leaves that capture sunlight more efficiently and, thereby, increase yield. The principal roles of silica appear to be largely protective. Ample evidence indicates adequate silica reduces susceptibility of silica accumulators to insects and disease (biotic stresses) and to drought, metal toxicities and salt (abiotic stresses).
In many parts of the world, silica fertilizers are applied to ensure adequate uptake to protect crops. For example, many of the ancient rice paddies of Asia receive routine applications of silica to compensate for removal rates. Organic soils and extremely weathered soils of the tropics containing low amounts of silicate clays also respond to silica fertilization. Similarly, crops grown on sandy soils may also respond favorably to silica fertilization.
While silica is a principal constituent of quartz sands, this form of silica is highly insoluble and unavailable for plant uptake. Conventional wisdom has maintained that silica fertilization is not necessary for soils containing appreciable amounts of silica-rich clays, such as the alluvial soils that comprise much of Louisiana’s agricultural land.
LSU AgCenter researchers were first drawn to the possibility that silica deficiency may occur in Louisiana while pursuing the mysterious "localized decline" syndrome that affects isolated rice fields in southwest Louisiana. Toxic accumulations of iron and aluminum are symptoms of this disorder and are the probable cause of the crop’s decline. The cause of this increased uptake of toxic metals, however, remains unclear.
Because several scientific reports have linked iron toxicity with a deficiency in silica, the AgCenter researchers conducted a survey to determine silica levels in affected crops. A value of 5 percent silica is an accepted standard for silica sufficiency in rice. When rice plants begin to spread by tillering – sending out shoots – those that display symptoms of localized decline contain an average of 3.2 percent silica – not an extremely low value, but possibly deficient.
For a source of silica to conduct greenhouse trials, the researchers obtained rice hull ash from a co-generation utility in the Lake Charles area that burns rice hulls to generate electricity. They also found out that a local cooperative had conducted an informal study, applying three rates of rice hull ash to a rice field south of Iowa, La., three years earlier. The exact application rates were unknown, and crop responses had not been measured. AgCenter researchers collected samples from those fields prior to harvest in the third season after ash was applied. The differences were astounding. Rice hull ash had not only increased silica uptake by as much as 111 percent but resulted in taller, stiffer plants with larger stems. Moreover, seed head (panicle) weights of maturing rice were 77 percent greater in areas receiving the highest rate of hull ash than in areas receiving no hull ash. These findings and those of the initial AgCenter survey indicated further investigation was merited.
In 2007, AgCenter researchers and extension agents sampled rice fields representative of each rice-growing area in Louisiana. Young plants were collected at mid-tiller, and straw and seed heads were collected immediately before harvest. Soil and floodwater samples also were collected. Water was extracted from the soil pores in flooded soils as an additional index of silica availability in each of the 126 fields sampled. The silica content was measured in all tissue and water samples.
The principal findings of this survey are shown in Table 1. The most striking finding is that young rice contained less that 5 percent silica in more than 90 percent of the field samples. While a 5 percent level has not been confirmed deficient in Louisiana, a wealth of evidence from other rice-growing regions indicates that young rice containing less than 3 percent silica is severely deficient with increased susceptibility to disease, insects and other stresses. By maturity, the silica accumulated in straw and seed heads in many of the fields had increased, often above the 5 percent level – sometimes in rice severely deficient early in the season.
Silica deficiency is most severe early in the season. The cause of this deficiency is less clear but may be related to the discharge of irrigation and rainwater laden with the most soluble forms of soil silica. On one farm, the reuse of warm, silica-rich irrigation water from a nearby crawfish pond substantially reduced the symptoms of localized decline in a field severely affected by this disorder for several seasons. While it is tempting to attribute this to greater silica availability, the specific cause for this response is uncertain because no clear relationships were evident between silica concentrations in flood or soil pore water and silica accumulation in rice plants at most locations. The influence of pH was the only significant relationship between silica uptake and the various soil properties measured. As a rule, silica uptake tended to increase as pH increased.
Sufficient evidence is not yet available to recommend silica fertilization for Louisiana, but the relationship between silica and pH suggests that perhaps slag should be substituted for limestone when "liming" fields used for silica-accumulating crops. Slag is a byproduct of metal smelting and is the most common form of silica fertilizer worldwide. Slag is produced when ground limestone is added to heated ores. Calcium combines with the silicates in these ores, causing the pure metals to sink to the bottom of the cauldron and calcium silicate slag to rise to the surface. The slag is then skimmed and finely ground for use as a silica fertilizer. When applied to soils, slag increases pH and available calcium in addition to supplying a fairly soluble form of silica. Depending on the ore, slag can supply a number of micronutrients as well. The price of slag is somewhat greater than that of agricultural lime because of higher shipping and processing costs. Even so, slag merits consideration as a lime alternative, especially in rice-soybean rotations where rice may benefit from greater silica availability and the soybean crop from elevated pH and added calcium.
LSU AgCenter researchers have evaluated a number of methods for assessing silica availability using the diverse soil samples collected in the 2007 survey. Unfortunately, none of these tests proved reliable, and research to identify a suitable soil test continues. The most reliable means of assessing silica status is tissue testing of the plant. Initially, this too proved problematic because the methods most commonly used proved highly variable and unreliable. Since then AgCenter researchers have developed a rapid, precise and inexpensive method for measuring silica accumulation in plants. That method was used for the analyses presented here and possibly will be available soon through the LSU AgCenter’s Soil and Plant Analysis Laboratory.
Gary Breitenbeck, Professor, and Joseph Kraska, Graduate Research Associate, School of Plant, Environmental & Soil Sciences, LSU AgCenter, Baton Rouge, La.
(This article was published in the winter 2009 issue of Louisiana Agriculture.)