Why silica additives are needed in hydroponics?

The silica (SiO2) content in the leaves (etc.) of 'soil grown' plants ranges from 1-10% of their dry weight.   This silica is potentially supplied from both the feed water and the soil:
 
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Feed water:  Natural  (i.e. uncontaminated) waters commonly contain around 5 mg/L soluble silica.  Hence soil grown plants potentially enjoy a feed of soluble silica each time the plant is watered.

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Soil:  Sand is composed largely of silica, therefore, the roots of soil grown plants are immersed in a potential "silica reservoir".  Although this form of silica is very insoluble it does dissolve slowly - especially with alkaline waters. 

  
However, plants grown in hydroponic systems without soluble silica supplements, typically contain much less silica in their cells. This occurs because, unlike soil grown plants, silica is virtually absent at the root-zone:
  
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Recycling systems:  Once the plant consumes the silica present in the make-up water,  no more silica is available.   Of course if either rainwater or RO (reverse osmosis) water is used, no soluble silica is present. 

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Inert mediums:  Unlike 'soil', inert mediums are unable to yield silica. 

Research shows that the absence of silica in hydroponics can cause plant health to be less than optimum.


*Note, silica cannot be included in concentrated nutrient formulations because stable silica solutions are by nature highly alkaline. It must therefore be added separately.

When to use Silica?

Silica should be used from seed to harvest: To benefit new growth, silica must be present at all times in the nutrient solution. Electron microscopy and x-ray analysis both confirm that once silica is deposited, it can no longer be redistributed within the plant.

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