Why add silicon to nutrient solution?

Although silicon has not been recognized as an essential element for higher plants, its beneficial effects have been shown in many plants.
Silicon is abundant in all field grown plants, but it is not present in most hydroponic solutions. Silicon has long been recognized as particularly important to rice growth, but a recent study indicated that it may only be important during pollination in rice.
The beneficial effects of silicon (Si) are twofold:

1) it protects against insect and disease attack, and 2) it protects against toxicity of metals.

For these reasons, I recommend adding silicon (about 0.1 mM) to nutrient solutions for all plants unless the added cost outweighs its advantages.