SILICON (Si) Silicon (Si) is not yet classed as an essential nutrient but it exists in all plants  grown in soil and is recognized as a functional nutrient. The benefits of silicon include increasing pest and pathogen resistance, drought  and heavy metal tolerance, and improved quality and yield of agricultural crops. Si is taken up at levels equal or greater than essential nutrients such as Nitrogen  and Potassium in certain plants such as rice and sugarcane, for which it is  considered ergonomically essential for sustainable crop production (Savant et al,  1999). Silicon (Si) is one of the most abundant elements in the earth’s crust. Soils  generally contain from 5 to 40% Si (Kovda, 1973) consisting of mainly poorly  soluble quartz and crystalline silicates, which are inert. Whilet silicon is plentiful,  most sources of silicon are insoluble and not in a plant available form. Plants can only absorb Si in the form of soluble monosilicic acid, a non-charged  molecule. Monosilicic acid, or plant available silicon (PAS), is a product of Si-rich  mineral dissolution (Lindsay, 1979). Different Si sources have different dissolution  rates; the solubility of quartz is low compared to the easily soluble amorphous  silica, diatomaceous earth (Savant et al, 1999). PAS is absorbed by plants, benefiting the plant in terms of growth and resistance  to disease and environmental stresses. PAS also has a significant effect on soil  texture, water holding capacity, adsorption capacity, and soil erosion stability. Silicon and Plant Physiology Soils that have low silicon concentrations are commonly amended with silicon  compounds to increase the quality and quantity of agricultural crops. PAS may be absorbed by roots from the growing medium, but PAS can also be  absorbed as a foliar application (Muir et al, 2001). Approximately 90% of the  absorbed PAS is probably carried via the xylem in the transpiration stream so that  its concentration is greater in the foliar tissues than the root tissues (Jones and  Handreck, 1967). Root absorption by some plants such as oats may be directly  related to water uptake, while other plants such as beans may expend energy for Si to cross the root cell membranes (Jones and Handreck, 1967). Mycorrhizal fungi  may enhance the root uptake of PAS in acid soils (Clark and Zeto, 1996). The  difference in Si accumulation between species has been attributed to differences  in Si uptake ability of the roots (Ma and Yamaji, 2006). Si accumulates in greater concentrations with increasing age of the tissues,  dependent on the species, cultivar and the external availability of PAS. PAS forms deposits of amorphous silica, known as phytoliths (sometimes in  association with cellulose and proteins), in the plant tissues (Neumann et al,  1997). Phytoliths from plant litter may contribute 1-2% to the weight of the soil  and they normally degrade slowly to return soluble Si to the soil. However, the  degradation is too slow to release sufficient PAS to enable benefits in the  following crop of wheat, for example (Rodgers-Gray and Shaw, 2000). Si is accumulated in plants to total concentrations in dry matter similar to those of  essential Macro-nutrients such as Potassium (K), Calcium (Ca), Magnesium (Mg),  Sulfur (S) and Phosphorous (P) (Epstein, 1994). Soil testing is required to measure the amount of Si present in the soil, in order to  optimize the nutrient requirements for the crops’ needs.  Testing of the plant tissues for Si is informative in determining how effectively the  plant is taking up Si from the soil or foliar amendments. It is often recommended that the soil, plants and Si fertilizers br tested for Si  content in order to make Si management in crops efficient and affordable for  farmers (Savant et al, 1999). Plant testing: The amount of Si accumulated by the plant is measurable and generally  recognized as a reflection of the amount of PAS (Andersen et al, 1991). Plant  testing for Si status mainly consists of leaf tissue sampling and its chemical  analysis. X-ray fluorescence is a direct method used on oven dried plant matter.  Alternatively the Si in the plant tissue can be solubilized and indirectly measured  in the extracted solution. Soil Testing: The forms of Si in the soil can be defined as Total, Extractable and Soluble but it is  the Soluble Si (PAS) that is agronomically important and this may have no relation  to the Total Si in the soil (Berthelsen et al, 2003). In addition, the concentration of  Soluble Si in soils is dynamic and affected by the soil type, moisture and  exchangeable/dissolution reactions. The measurement of PAS, relies on extracting monosilicic acid from the soil using  a water-based extractant. Several wet chemical methods are available to measure  monosilicic acid to provide an indication of its availability to plants in the growing  substrate. It is important to carefully choose the extraction method as the  extraction process itself may solubilize more Si compounds in the soil, than  usually available to plants in the natural environment (Muir et al, 2001). 10 – Why Do Plants Need Silicon? Si exists in all plants grown in soil (Takahashi, 1990) and its content in plant tissue  ranges from 0.1 to 10% (Epstein, 1999). Si is considered as a nutrient of  agronomic essentiality in that its absence causes imbalances of other nutrients  resulting in poor growth, if not death of the plant (Epstein, 1995; Savant et al,  1997). Numerous laboratory, greenhouse and field experiments have shown the benefits  of silicon fertilizers for agricultural crops and the importance of silicon fertilizers as a component in sustainable agriculture (Matichenkov and Calvert, 1999). There are two different effects on plants due to silicon fertilization: 1. An indirect influence through soil fertility 2. A direct influence on the plants The benefits of Si on plants include (Ma, 2006 and Savant et al, 1999): • Increased growth and fruit yields in some species • Tolerance to abiotic stress: frost, drought and salinity, toxicity by Al, Mn,  and heavy metals • Tolerance to biotic stress: insects and infections • Resistance to lodging The beneficial effect of Si is more evident under stress conditions because Si can protect the plant from multiple biotic and abiotic stresses (Ma and Yamaji, 2006 – Figure 1 below). Abiotic stress is the negative impact of environmental factors upon living  organisms and biotic stress concerns pest pressure Si is accumulated primarily in  the epidermal tissues of both roots and leaves in the form of a silica-gel  (phytoliths). This thickened epidermal silicon-cellulose layer supports the  mechanical stability of plants, thereby resisting lodging and also a greater  retention of seed, especially in grasses (Savant et al, 1999). The increased  mechanical strength also increases the light receiving posture of the plant. The leaves were reported to be darker green, stiffer and slow to senesce,  increasing their potential for photosynthesis and hence growth (Epstein 1994) The deposition of Si in the culms, leaves and hulls also decreases transpiration  from the cuticle and this increases resistance to lodging, low and high  temperature, radiation, UV and drought stress (Ma and Yamaji, 2006). More recent studies suggest that Si also plays an active role in the biochemical  processes of a plant and may play a role in the intracellular synthesis of organic  compounds (Matichenkov et al., 2008). Given that PAS in the sap may constitute  between 0.5-8% of the total Si in the plant, it is likely to play an active role in the  biochemical processes of the plant (Fawe et al, 1998). Si also controls the chemical and biological properties of soil with the following  benefits: • Reduced leaching of phosphorous (P) and potassium (K) (Sadgrove, 2006) • Reduced Aluminum (Al), Iron (Fe), Manganese (Mn) and heavy metal  mobility (Matichenkov, 2002), • Improved microbial activity (Matichenkov, 2002), • Increased stability of soil organic matter, • Improved soil texture (Sadgrove, 2006), • Improved water holding capacity5 (Sadgrove, 2006), • Increased stability against soil erosion (Sadgrove, 2006), and • Increased cationic exchange capacity (CEC) (Camberato, 2001) Therefore even if a plant is a low Si-accumulator, it will benefit from the improved  soil properties resulting from the application of Si. Silicon fertilizers usually possess a very large surface area, therefore their  application increases the water holding capacity of sandy soils and raises the soil’s  adsorption capacity (Savant et al, 1997). The application of silicon to crops is a viable component of an integrated  management program for insect pests and diseases because it leaves no pesticide  residue in food or the environment, is relatively cheap and could easily be  integrated with other pest management practices (Laing et al, 2006). It has been reported that silicon suppresses insect pests such as stem borers,  brown plant hopper, green leafhopper, white backed plant hopper, and noninsect pests such as spider mites (Savant et al, 1997, Ma and Takahashi, 2002). Sugarcane – improved silicon nutrition has been shown to: • Reduce leaf freckling (Elawad et al, 1982), • Increase tolerance to shoot borer (Chilo infuscatelus) (Rao, 1967), • Increase resistance to stem borer (Diatracea saccharalis F.) (Anderson and  Sosa, 2001), and Increase resistance to stalk borer (E.saccharina) (Elawad et  al, 1982, Keeping and Meyer, 2003) Wheat – improved silicon nutrition has been shown to: • Increase tolerance of Hessian fly larvae (Phytophaga destructor Say) (Miller  et al., 1960) • Increased crop resistance and reduced pest infestation, both in the field  (Kordan et al, 2005) and in storage (Korunic, 1997) Rice – improved silicon nutrition has been shown to: • Increase resistance to the African striped borer Chilo zacconius Bleszynski  (Ukwungwu and Odebiyi, 1985) • Prevent attack by the larvae of the yellow rice borer, Scirpophaga incertulas  (Panda et al, 1975) • Reduce susceptibility to the stem borer Chiloe suppressalis Walker  (Sasmoto, 1961) • Inhibit sucking against the brown plant hopper Nilaparvata lugens  (Yoshihara et al, 1979) • Reduce the number of nymphs becoming adults, reduce adult longevity and  female fecundity of plant hoppers (Salim and Saxena, 1992) Maize – improved silicon nutrition has been shown to: • Improve resistance to stalk borer, Chilo zonellus Swinhoe, damage (Sharma  and Chatterji, 1972) • Reduce larval survival of the borer Sesamia calamistis Hampson  (Lepidoptera: Noctuidae) • Increase resistance to Ostrinia nubilalis Hubner (Lepidoptera:Pyralidae Other crops – the contribution of silicon content, to pest resistance has also been  recorded in other crops, such as: • Vegetables (Chelliah, 1972) • Citrus (Matichenkov et al, 2000) • Turf (Korndorfer et al, 2004) Laing et al (2006) report silicon controls red  spider mite on dicotyledonous crops such as green beans, brinjal, tomato  and cucumber. Furthermore, silicon deposits in plant organs were reported in most crops,  including the monad dicotyledonous families (Jones and Handreck, 1967;  Nishimura et al., 1989). This implies that Si plays a role in pest resistance in most,  if not all, cultivated crops. Several mechanisms have been proposed to explain the tolerance and resistance  of plants to insect pests. According to Bernays and Barbehenn (1987) most of the  plant silicon occurs in the epidermis, which might dislodge young larvae before  they can establish in the stem. Various studies have demonstrated that silicon  increases the hardness of plant tissue, which negatively impacts insect larval  boring and feeding ability. Djamin and Pathak (1967) showed that increased silicon content in rice plants resulted in mandibular teeth loss of stem borer  larvae. Recently, a parallel mechanism as that seen in the resistance of plants to diseases  via an activation of the plant’s own defense mechanisms by soluble silicon has  been observed for insect pests. Sieburth et al (1990b) reported such a mode of  action against insects such as the noctuid Trichoplusia ni, the coccinellid Epilachna  varivestis, the aphid Acyrthosiphon pisum, and the cockroach Periplaneta  americana. Similarly, Keeping and Meyer (2005) reported the resistance of sugarcane to E.saccharina. Silica in the form of diatomaceous earth sprayed or dusted onto plants has also  been reported to kill insect pests such as; Cryptolestes ferrugineus, Rhyzopertha  dominica, Sitophilus oryzae and Sitophilus granarius (Saez and Mora, 2007),  amongst others. This mechanism differs from the soil amended Si application and  works by desiccating the insects when they physically contact the silica dust  (Korunic, 1997). Diatomaceous earths are the most common inert dusts  registered for protection of grain in storage (CSIRO, 2001). Aluminum, Iron and Manganese Many metals found naturally in the soil can be potentially toxic to plants. These  metals often become a problem when there is a change in the soil environment,  such as the acidification of soil. This low pH environment solubilises iron (Fe),  Managanese (Mn) and Aluminium (Al). Excess Al is toxic to plants causing: • Stunted roots • Reduced availability of phosphorus (P), • Reduced availability of sulfur (S), • Reduced availability of other nutrient cations through competitive  interaction Multiple laboratory and field experiments have shown that silicon fertilization is  more effective than liming for reducing aluminum toxicity (Matichenkov and  Calvert, 1999). Five different mechanisms of Al toxicity reduction involve Si-rich compounds: 1. PAS can increase the pH of acid soils 2. PAS can be adsorbed onto aluminum hydroxides impairing their mobility 3. PAS can form ions with Al, thereby precipitating it out of solution 4. The mobile Al ion can adsorb onto a surface of strong adsorption capacity, such  as a silica surface 5. Mobile Si compounds can increase a plant’s tolerance to Al. These mechanisms can work simultaneously although there is usually a prevalent  one. Manganese deficiency occurs in plants grown in alkaline soils and toxicity occurs  on very acid and poorly drained soils. A deficiency of Si causes an increased  uptake of Manganese in rice, barley, rye and ryegrass causing toxicities (Lewin  and Reimann, 1969). Si fertilization relieves this toxicity (El-Jaoual and Cox, 1998). The proposed mechanism may be increased oxidation of Mn at the root surface if  there is sufficient oxygen present, and redistribution of Mn to prevent necrosis  (Lewin and Reimann, 1969; El-Jaoual and Cox, 1998). Iron is classified as a trace element, or micro-nutrient, because it is only needed in  small amounts. Too much iron can be toxic to plants, producing stunted growth of  roots and tops, dark green foliage, or dark brown to purple leaves on some plants.  Iron toxicity is a particular problem in rice paddies that show the symptom of  brown leaves, called “bronzing”. Silicon deficiency also causes an increased  uptake of Iron (Fe) in rice while adding Fe oxides to soil will reduce the plant availability of Si, (Jones and Handreck, 1995; Lewin and Reimann, 1969). The  presence of sufficient PAS at the root surface may increase the oxidative power to  precipitate toxic levels of Fe as for Mn (Jones and Handreck, 1967; Perry and  Keeling-Tucker, 1998). Recently, Ma and Yamaji (2006) suggested that the deposition of Si in the roots  reduces Apoplast bypass flow and provides binding sites for metals, resulting in a  decreased uptake of toxic metals and salts from the roots to the shoots. Mining, manufacturing, and the use of synthetic products can result in heavy  metal contamination of urban and agricultural soils. Heavy metals also occur  naturally, but rarely at toxic levels. Heavy metals enter the food chain through  the soil and become hazardous contaminants of food, entering the human body  as a cumulative poison (Benavides et al., 2005). The rehabilitation of soil contaminated by heavy metals relies on methods such as  managing the mobility of heavy metals in the soil so that they don’t leach into  waterways or get taken up by plants. This can be achieved by changing the pH  (heavy metals are less mobile at high pH) or by adding soil amendments to  increase the soil’s adsorption capacity thereby reducing the plant’s ability to  access the heavy metal. However, these measures may not be sufficient or are  costly and inefficient. A recent report (Matichenkov and Bocharnikova, 2010) demonstrated that the  leaching of heavy metals (Cu, Pb, Cr, Ni, and Co) was reduced significantly, by over  50%, with the addition of a Si fertilizer (diatomaceous earth & liquid activated  Silicon complex solution). This reduction in leaching of heavy metals may be  explained by the interaction between the heavy metals and Si-rich substances. Several mechanisms are proposed in this recent report as responsible for  inactivating the heavy metals: a weak physical or strong chemical adsorption  between the heavy metals and the diatomaceous earth and a reaction between  PAS (monosilicic acid) and the heavy metals. How Silicon Enhances Plant Disease Resistance There is substantial evidence that when several high Si accumulator plants are  fertilized with Si, they benefit from reduced severity of disease, with associated  yield increases over infected plants that weren’t fertilized with Si (Belanger et al,  1995). Several fungal diseases of rice, sugarcane, cereals, roses and lettuces, amongst others, are reduced in severity, as listed in Table 1. Mostly the findings are for foliar pathogens and there are some instances where  root diseases caused by Fusarium and Pythium are reduced. The protective effect  has been reported for hydroponics systems, soil applications (e.g. rice) and foliar applications (e.g. grape). The effect may occur within 24 hours of spray treatment  for foliar pathogens, and can take up to three weeks against root pathogens by  root uptake (Bowen et al, 1992). There is increasing evidence that PAS is required continually, in sufficient  concentration in sap to provide protection (Samuels et al, 1991b). Two mechanisms for Si-enhanced resistance to diseases have been proposed (Ma  and Yamaji, 2006). One is that Si acts as a physical barrier, where Si is deposited beneath the cuticle  such that the Si layer mechanically impedes penetration by fungi, thereby  disrupting the infection process. Another mechanism proposed recently is that soluble Si acts as a modulator of  host resistance to pathogens (Ma and Yamaji, 2006). Foliar fungal pathogens live  parasitically in living plant tissues, removing cell nutrients until the sporulation  process on the surface of the plant excludes light and the tissues senesce (Muir et  al, 2001). Plants respond to foliar pathogens by releasing chitinases and other  proteins and phenolic compounds to kill its own cells, isolate the pathogen and  prevent the infection of adjacent cells. Several studies in monocots (rice and wheat) have shown that plants supplied  with Si can produce phenolic and phytoalexins in response to fungal infection  such as those causing rice blast and powdery mildew (Fawe et al, 1998; Belanger  et al, 2003; Remus-Borel et al, 2005; Rodrigues et al, 2004). Similarly, it has been  shown (Samuels et al, 1991; Cherif et al, 1994) that nutrient solutions amended  with Si activate defense mechanisms in dicots (cucumber) by enhancing the  activity of chitinases, peroxidises and polyphenoloxydases. These biochemical responses are only induced by soluble Si and the biochemical  pathways of the plant that lead to disease resistance remain unknown, although  several potential mechanisms have been proposed (Ma and Yamaji, 2006). Effect Of Silicon On The Uptake Of Other Nutrients The presence of Si in nutrient solutions affects the absorption and translocation  of several macro and micro-nutrients (Epstein, 1994). Increased Si fertilization increases Zinc (Zn) uptake if deficient, especially if P is excessive (Marschner et al,  1990). Si fertilization retards the toxic uptake of Phosphorous (P) by roots, such as  in cucumbers (Marschner, 1990), while promoting its translocation to grain in rice  and wheat (Lewin and Reimann, 1969). Cultivated plants can use only about 30%  of applied Phosphate fertilizer, if leaching is low. The mixture of active Si with P  fertilizer can increase the efficiency of P fertilization by 40-60% (Matichenkov et  al, 1997b). Si interaction with Potassium (K) varies, depending on the anion in the fertilizer (Jones and Handreck, 1967). Transpiration decreases if the chloride salt is used,  yet increases if the sulfate salt is used so there is greater uptake and deposition of  Si for the latter K fertilizer. Importantly, Si-rich amendments are recommended for the reduction in leaching  of nitrogen, phosphorous and potassium based fertilizers (see Section: “The role  of silicon in improving the efficiency of NPK fertilizers”). The damage to plants caused by salt (NaCl) depends considerably upon the salt  tolerance of plants. One of the mechanisms considered as being responsible for salt toxicity to plants is ionic phyto-toxicity which is caused by excess amounts of  salt ions (Na and Cl) in the plants (Liang and Ding, 2002). Si-fertilization has been  shown to alleviate Sodium (Na) uptake in rice, wheat and barley (Savant et al,  1997a). Liang et al (1996) proposes a mechanism for salt tolerance as a reduction  in membrane permeability of the leaf cells of the salt-stressed plant, reducing the uptake of Na. Liang and Ding (2002) also proposed that Si causes sodium and  chloride ions to be more evenly distributed over the whole root section, which  improves the salt tolerance of the plants. Therefore ensuring that a plant has  sufficient PAS will reduce the effects of salinity. In the soil solution, or liquid phase, Si is present as monosilicic acid (Si (OH) 4,  referred to as PAS) and polysilicic acid (the polymer of PAS) as well as complexes  with organic and inorganic compounds such as Al oxides and hydroxides  (Berthlesen et al, 2003). While it is the PAS that is taken up by the plant and has a  direct influence on crop growth, the polysilicic acid and inorganic and organic  complexes are important sources/sinks that replenish the PAS following crop use. They also have an important and significant effect on the soil properties, such as  improving soil aggregation and increasing soil water holding capacity as wells as  increasing the exchange and buffering capacity of the soils (Berthelsen et al,  2003). The solubility of Si in the soil is affected by a number of dynamic processes  occurring in the soil (see Figure below) including the particle size of the Si  fertilizer, the soil pH7, organic complexes, presence of Al, Fe and phosphate ions,  temperature, exchangeable/dissolution reactions and soil moisture8 (Berthelsen  et al, 2003). 22 The main processes influencing Si concentration in the soil (extracted from Savant  et al, 1997), where Si (OH) 4 is PAS. Si can be added via water irrigation and fertilization but it is lost through plant absorption and leaching. PAS is only present in solution at less than pH 9 and has a solubility of 65mg/L,  which is constant between pH2-8.5 (Jones and Handreck, 1967). There is a  polymerization of PAS to form a silica-gel if it exceeds a concentration of 65mg/L  or if there is dehydration of the soil, which is reversible on dilution (Savant et al,  1999).  Why Is There a Need for Silicon Fertilization? Silicon deficiency in crops has been recognized since the 1970s. The optimization of silicon nutrition has been shown to have positive effects on plants. In  particular, substantial research on rice and sugarcane has shown that silicon  application can significantly enhance insect pest and disease resistance with  consequent yield increases. Plants differ in their ability to accumulate Si (Ma and Yamaji, 2006) but in order  for any plant to benefit from Si it must be able to acquire this element in high  concentrations. The concentration of PAS in the soil is dynamic and influenced by soil pH,  temperature, composition of the soil and moisture, amongst others. Si fertilizer is  necessary to improve soils deficient in Si and replace Si removed by cropping and  leaching. The composition of soils in terms of the level of Si is an important parameter to  measure in order to determine its Si-deficiency. For example, Queensland  sugarcane soils are considered deficient in Si if the concentration is less than 10- 15mg Si/kg dry soil following extraction with 0.01M CaCl2 (Muir et al, 2001). Berthelsen et al. (2003) analyzed three different Australian soils: Bundaberg  (Hydrosol soil), Mossman (Tenosol) and Innisfail (Ferrosol). These soils varied in  their levels of PAS in the order: Hydrosol>Tenosol>Ferrosol. Areas of high rainfall and temperature undergo significant weathering where  important nutrients (Ca2+, Mg2+, K+ and Na+) are stripped from the soil resulting  in acidification of the soil, which in turn dissolves aluminosilicate clay minerals  with the concomitant leaching of Si. Matichenkov & Calvert (2002) report that 210-224 million tons of plant-available  Si is removed from arable soils globally on an annual basis, assuming 70-800kg ha1 of plant available silica is removed with the harvesting of crops. Harvesting  cultivated plants usually results in Si being removed from the soil. In most cases  much more Si is removed than other macronutrients (Savant et al., 1997; Datnoff,  2005): – Potatoes remove 50 to 70 kg Si ha-1, – Cereals remove 100 to 300 kg Si ha-1, – Rice removes 230 – 470kg Si ha – Sugarcane removes 500 to 700 kg Si ha-1 (Anderson, 1991). In continuous cropping with high Si-accumulator species such as sugarcane and  rice, the removal of PAS can be greater than the supply via natural processes  releasing it into the soil unless fertilized with Si (Savant et al, 1997b) While other plant-available elements are restored by standard fertilization, Si is  not. Silicon exists in a variety of forms but most are poorly soluble, and therefore  do not contribute significantly to the PAS. Quartz (SiO2) is commonly found in  sandy soils; however, this inert form of Si has a poor adsorption capacity, low  water holding capacity and very low solubility. While Si is an abundant element it is not found free in nature but combined as  silicates or oxides. Many sources have been assessed for use as an agricultural  amendment. Before any source can be considered for agricultural applications, it  must meet a number of criteria, such as: solubility, availability, have suitable  physical properties and be free of contaminants (Gascho, 2001). One of the most  important, and most difficult to achieve, is solubility. Liquid silicates such as sodium silicate and potassium silicates are effective for  foliar applications and used in greenhouses but are generally uneconomical to use  for the rates needed for soil application (Berthelsen et al, 2003). Calcium metasilicate (CaSiO3, often referred to as simply calcium silicate) from  slag has been used by the Hawaii sugar industry for years (Medina-Gonzales,  1988). Calcium silicate occurs naturally as wollastonite, however, the availability  and solubility of wollastonite depends on the degree of metamorphosis involved  during its geological formation (Muir et al, 2001), and can be low compared to  some synthetic or slag silicates. Silicate slag has been used extensively in the USA; however, the furnace  temperatures influence the formation of insoluble silicate glasses (Prakash 1999).  Slags can be variable in composition and although they have high concentrations  of total Si, often only a small proportion is easily solubilized (Gascho, 2001). An  important consideration with silicate sources derived from industrial byproducts is the possible high level of heavy metals associated with their  origin or processing (Berthelsen et al, 2003). These are not only toxic to  plants but leach into waterways causing environmental damage.  Likewise, cement and cement building board waste can contain heavy  metals (Muir et al, 2001). Other overseas sources of Si include magnesium silicate, basalt dust, dolomite  and rock phosphate, but these only contain traces of PAS (Savant et al, 1999). Since 1970, Hawaiian Sugar Planter Assoc. researchers have tested several silicate  materials and their findings include (Savant et al, 1999): The degree of Si solubility  from siliceous materials is dependent on particle size and chemical composition  (HSPA, 1979), NPK (Nitrogen, Phosphorous, and Potassium) fertilizers are synthetic, inorganic  fertilizers which are used to improve crop yields. Their widespread use is blamed  for the degradation of natural resources, especially soil (Bunemann et al, 2006). There are three considerations regarding the impact of fertilizers: 1. How they are made, 2. How they affect long term soil nutrition, and 3. How they affect waterways through runoff Ammonia is the major feedstock used to produce the nitrogen based fertilizer but  the production of ammonia is a very energy intensive process, relying  predominantly on natural gas. Potassium and Phosphorous come from limited  natural sources. Manure is also a source of N, P and K. Si fertilizers are an abundant natural resource found in the earth’s crust and an  organic fertilizer, not requiring a synthetic route of manufacture. This means no emissions are generated and minimal energy is required in its production. Various practices in agriculture are productive in the short term but increase soil  acidity (Mason et al, 1994). A significant concern regarding nitrogen based  fertilizers is that they cause soil Acidification. Acidic soils cause many problems  such as (Mason et al, 1994): • Limited growth and production of crops, • Toxic levels of aluminum and manganese which are more soluble at low pH, • Phosphorous deficiency caused by aluminum toxicity, • Deficiencies of cationic nutrients (calcium, magnesium and potassium), and • Reduced activity of micro-organisms (such as the critical nitrification of  ammonium-N to nitrate) It has been shown, for example in a long term trial in Western Australia on wheat,  (Mason et al 1994) that the continued use of a nitrogen and phosphorous based  fertilizer progressively lowered the soil pH giving rise to the problems listed  above. Limestone was shown to only have short term effectiveness and required  repeated applications. Also, many synthetic fertilizers may not replace trace elements which are  depleted by crop uptake. Studies on sandy loam soils in Australia (Hati et al, 2006)  showed that a balanced application of mineral (essential plant nutrients) fertilizer  with organic manure yielded higher crop productivity compared to no fertilizer,  although the impacts on soil acidification and leaching were not Australian soils  often lack essential nutrients such as nitrogen, phosphorous and potassium, especially after years of intensive agriculture (Sadgrove, 2006).  NPK (Nitrogen, Phosphorous, and Potassium) fertilizers are synthetic, inorganic  fertilizers which are used to improve crop yields. Their widespread use is blamed  for the degradation of natural resources, especially soil (Bunemann et al, 2006). Silicon based fertilizers do not have any long term negative impacts on soils since  they are a natural product. Not only do they improve soil nutrition through a higher cationic exchange capacity and ability to retain moisture, it provides plant available silica, an important nutrient in crop productivity. Calcium silicate slag, a potential source of Si, often contains  contaminants such as heavy metals which affect soil health and  productivity. The final consideration in terms of fertilizers is their effect on the greater  environment, such as waterways. During high periods of rainfall, varying amounts  of nitrate are washed from the farmland into nearby waterways, such as oceans,  lakes and groundwater. The addition of excess nitrates or phosphates into the  water system through fertilizers is known as eutrophication. It results in the death  of fish and other organisms (Presley et al, 2006). High levels of nitrates stimulate the growth of plankton, which may initially result  in increased fish numbers with the excess food availability; however, eventually  oxygen may be depleted to a fatal level, a situation in which masses of fish die  (Presley et al, 2006). There is evidence of chronic eutrophication in the Great  Barrier Reef lagoon and Queensland marine ecosystems, which is attributed to  runoff of fertilizer from cropping and pastoral lands (Gabric and Bell, 1993; Lobb, 2002). Silicon based fertilizers, pose no such risk to waterways. The Role of Silicon In Improving The Efficiency of NPK Fertilizers NPK (Nitrogen, Phosphorous, Potassium) based fertilizers are often considered a  necessary part of intensive crop cultivation. These macronutrients are consumed  in large quantities by plants and NPK fertilization is a way of improving crop  production. Organic manures are also used as a source of these macronutrients,  alone or in combination with synthetic NPK fertilizers. Blood and bone meal is  another source of these macronutrients, though predominantly of organic  nitrogen (nitrogen combined with carbon). The application of fertilizers for plant nutrition and animal farming, are major  sources of natural water pollution. Many soils are susceptible to leaching due to  the high rainfall and widespread use of irrigation and drainage can lead to  leaching from well-drained soils to water of 20% to 80% of added nutrients and  agrochemicals (Matichenkov and Bocharnikova, 2010). However minimizing fertilizer application may have a negative impact on yield. Two independent studies into the application of NPK fertilizer with diatomaceous  earth found: (i) Reduced nutrient and pollutant leaching, a. Reductions in leaching of P by 60%, of K by 60%, and of N by 54% at 100kg/ha NPK + DE in Grey Forest Soil (Matichenkov and Bocharnikova, 2010) b. Reductions in leaching of (Sadgrove, 2006): i. N by 60% for sandy soil and 10% for potting mix, ii. P by 30% for sandy soil and by 95% for clay, iii. K by 60% for sandy soil and 15% for potting mix (ii) Improved crop productivity a. Increased barley biomass (Matichenkov and Bocharnikova, 2010), b. Increased quality and quantity of rice, grass, citrus, tomato and corn  (Matichenkov 2001, 2002) c. Rhoades grass grown in the DE amended soils was significantly taller and darker green than the grass in unamend soils (Kerr, 2007) Si-rich amendments are therefore recommended for the reduction in leaching of  N, P and K. Southern Cross University Study indicated that Synthetic fertilizers and manure  supply nitrogen in the form of ammonium (NH4)+ (blood and bone Meal) supply  organic nitrogen which is converted by soil bacteria into ammonium). Ammonium can realize one of several fates, once in the soil: 1. ‘Volatilized’ into ammonia, 2. ‘Nitrified’ into nitrite (NO2) -) then nitrate (NO3) -) by bacteria, 3. Fixated by high cationic exchange materials, 4. Taken up by plants, or 5. ‘Immobilized’ by bacteria that turns it back into organic nitrogen. Nitrate and ammonium are the plant available forms of nitrogen; however, once  ammonium is converted into nitrate, it is very soluble and easily leached from the  soil. Diatomaceous earth when added with a NPK fertilizer was found to interrupt the  first three nitrogen fates listed above and improve the efficiency of nitrogen  uptake by plants through the following mechanisms: 1. A reduction of volatilization, therefore reducing loss of nitrogen in the form of  ammonia from the soil (Sadgrove, 2006) 2. Diatomaceous earth amended soils show a reduced loss of nitrate, particularly  the sandy soils and potting mix (Sadgrove, 2006). This is significant given that  nitrification (conversion of ammonium to nitrites then to nitrates by bacteria) is  generally promoted in well aerated soils, so these normally nitrification  susceptible soils were improved through the addition of diatomaceous earth 3. Retention of the ammonium ions by an order of magnitude (Kerr, 2007). The results show that DE improves the efficiency of nitrogen fertilizer by reducing  losses via leaching and volatilization and increasing its availability by immobilizing  ammonium for plant uptake. Rhoades grass grown in the DE amended soils was significantly taller and darker  green than the grass in un-amended soils. Furthermore, potassium and phosphate  levels improved for all soil types amended with DE (Sadgrove, 2006). This is  attributed to DE’s ability to retain moisture as well as its high cationic exchange  capacity which enables DE to retain plant available nutrients such as ammonium,  potassium and phosphate. Soil acidification arises in areas of high rainfall and temperature undergoing  significant weathering where important nutrients (Ca2+ Mg2+, K+ and Na+) are  stripped from the soil. In turn, the acidic conditions in the soil, dissolve aluminosilicate clay minerals causing leaching of Si. The acidification of soil and leaching of Si have several impacts: • A greater availability of potentially plant toxic ions such as Al, Fe and Mn,  which are more soluble at low pH, • Limited growth and production of crops, • Phosphorous deficiency caused by aluminum toxicity, • Deficiencies of cationic nutrients (calcium, magnesium and potassium), • Reduced activity of micro-organisms (such as the critical nitrification of  ammonium-N to nitrate) • Reduced cationic exchange capacity • Low levels of PAS In order to combat the loss of nutrients, fertilizers are added, such as ammonium  (Synthetic fertilizers and manure supply nitrogen in the form of ammonium).  However, ammonium releases hydrogen cations which further increase the  acidity of the soil. It is common practice to lime soils to increase the pH and thus  the soil’s CEC. The amount of lime that is required to change the soil’s pH  depends on the soil type, the more clayey the soil the more lime it requires.  However, often soils have a high pH buffering capacity (meaning that the soil  resists a change in pH when chemicals are added to it) it is often uneconomical to  increase the pH above 6. It is often more efficient to raise the CEC by adding  amendments such as diatomaceous earth or calcium silicate which also supply  PAS to these Si-deficient soils. In fact, too much liming can have a marked (negative) effect on the availability of  Si. Multiple laboratory and field experiments have shown that Si fertilization is  more effective than liming for reducing aluminum toxicity. Silicon application rates are mainly influenced by the chemical makeup of the Si  source, Si levels in the soil, and in the plant (Savant et al, 1999). It is important to determine a soil’s responsiveness to Si; soils that are not  deficient in Si are likely to have a low response to Si amendments (Datnoff and  Snyder, 1991). Many soils are deficient in Si due to high leaching conditions which  results in a loss of plant available silica. Si deficiency occurs more in highly  weathered, low base saturation and low pH soils such as Oxisols and Ultisols  (Datnoff, 2005).  Organic soils (Histosols) are also deficient in PAS because of the greater content  of organic matter and low content of minerals. Entisol soils have a high content of  quartz sand (SiO2) but are low in PAS.  Si can be applied as a soil amendment or foliar spray, depending on the form of  the Si fertilizer. Si fertilizers that are applied as a soil amendment: -Broadcast spread and incorporated into the top 10-20cm using an off-set disc  plough or rotary hue for sugarcane (Berthelsen et al, 2003) -Generally, all Si is applied to soil before planting (Savant et al, 1999) Hydroponics growers have generally used 19-50mg Si/L in hydroponic solutions  for increased plant growth and reduced disease (Menzies and Belanger, 1996).  There is little research on the fertilization of potting mixes with Si to increase yield  or plant health (Chen et al, 2001). Some compost has biological resistance against  various pathogens due to biocontrol agents, but this isn’t always present (Hoitink  et al, 1997). Given that the estimated loss of ornamental and nursery plants due  to Phytophthora root rots is worth millions of dollars per year, the need to control  pathogens in potting mixes is important (Muir et al, 2001). This loss is greater if  Pythium damping off, Fusarium root and crown rots and various fungal foliar  diseases are included. If sufficient Si is available to container grown plants, it may  assist in the suppression of such diseases and increase plant productivity (Muir et  al, 2001). A study (Muir et al, 2001) on Si fertilized mixes, found: • Increased dry matter and fruit yields of cucumbers • Increased dry matter and flowers of native daisy • Reduction in powdery mildew and root infection in cucumbers • Less severe black mold on native daisies • Increased favorable biological responses (production of Chitinase and beta1,3- • Glucanase) in pea • Fewer lesions on pea leaves inoculated with a fungal pathogen • It is important to correctly amend potting mixes to ensure the Si availability  is not compromised by the lack or excess of other nutrients which affect  results. Compared to a Si deficient control. Si has been proven to be influential in maintaining the health of many plant  species for decades (Muir et al, 2001). Rice and horsetail will not grow without Si  and cucumbers, soybeans, strawberries and tomatoes have been shown to suffer  adverse effects on growth if grown without Si (Epstein, 1994). Plants differ in their ability to accumulate Si (Ma and Yamaji, 2006). There are  some general trends in silicon accumulation in plants: monocots tend to be high  accumulators and dicots poor accumulators. There are, however, exceptions to  these trends with silicon accumulation varying among ecotypes of the same  species (Epstein, 1999). Plants can be categorized in terms of Si-accumulation (Jones and Handreck, 1967): • Wetland grasses (rice and horsetail): 10-15% dry matter (High Si  accumulator) • Dry- land grasses (sugarcane, cereal and turf): 1-3% dry matter (medium Si  accumulator) • Dicots (especially legumes): less than 1% dry matter (low SI accumulator) Members of the grass family, accumulate Si and several reports demonstrate the importance of Si nutrition for rice and sugarcane. Large growth and yield  responses appear to occur more rapidly with Si fertilization in high Si-accumulator  plants than others, but low Si-accumulator species also show increased growth  and health in the presence of added Si (Epstein, 1999). Table 1: Si is attributed with improvements in many plants (further details  included for highlighted plants in following sections Tomatoes: Fail to fruit or deformed fruit if Si deficient Epstein 1994; Miyake and Takahashi, 1978 Strawberries Increased flower/fruit yields, reduced powdery mildew by Si in  nutrient solution or mixed rice hulls and peat (Voogt 1991a, 1991b; Wang and Galetta, 1998in 1994) Rice: Increased growth, reduced insect and disease damage (Epstein 1999; Savant  et al 1997a, 1999; Datnoff et al, 1992) Lobolly pine (Pinus taeda): Increased growth (Epstein 1999) Annual brome (Bromus secalinus): Increased growth (Epstein 1999) Poinsettia (Euphorbia pulcherrima): Increased growth (Epstein 1999) Cotton: Increased growth, reduced impact of Al toxicity (Epstein 1999) Sugarcane: Increased growth, reduced insect and disease damage (Epstein 1999;  Savant et al 1997a, 1999) Apples: Apples treated by foliar spray are not as severely russeted in cold  conditions, yielding almost double fruit yield (Meador, 1977) Maize: Reduced impact of Al toxicity; reduced witch weed parasitisation; reduced  disease (Epstein, 1994; Hodson and Evans, 1995; Walker and Morey, 1999) Sorghum: Reduced witch weed parasitisation and insect pest damage, less leaf  anthracnose (olletotrichum graminicolum) (Walker and Morey, 1999; Savant et al,  1999; Narwal, 1973) Millet: Reduced witch weed parasitisation ( Walker and Morey, 1999) Pearl Mille: ,Enhanced resistance to Sclerospora graminicola, reduction by 78% of  powdery mildew (Deepak et al, 2008) Beans: Reduced rust (Uromyces phaseoli var. typical) in hydroponic systems  (Heath 1981) Dandelion: Reduced powdery mildew in hydroponic systems ( Belanger et al  1995) Grape vine: reduced powdery mildew (Uncinula necator) with Si addition in foliar  spray (Bowen et al, 1992) Lettuce: Reduced Pythium root rot by Si addition to nutrient solution (Belanger et  al, 1995) Muskmelon: Reduced powdery mildew (Sphaerotheca fuliginea) with Si addition  to foliar spray (Menzies et al, 1992; Belanger et al, 1995)