Hydroponics as we know it today was developed during the 1930s at the University of California, where Dr. William F. Gericke achieved higher yielding vegetables by planting them in a soil-less solution of nutrients. Today, hydroponics can be used to describe plants grown in a substrate such as rockwool or perlite and fed liquid nutrients, or bare-root systems where the roots are suspended directly in a liquid solution. No matter which solution you use, getting plants started hydroponically is the same process.
Wet a paper towel to the point that it is saturated and place it under a grow light.
Place the vegetable seeds on the paper towel. Until the seeds germinate, spacing will not matter.
Monitor the paper towel carefully to ensure that it never dries out. Continue to do so until the seeds germinate. If the paper towel dries completely, seeds won't germinate.
Place the germinated seeds into cubes of rockwool and mist with a pre-mixed solution of nutrients and water. The pre-mixed solution will replace all the nutrients that are not present in the soil. You can purchase a pre-mixed solution from a hydroponics specialty retailer.
Transplant the entire rockwool cube into a larger rockwool cube when the plant has outgrown the smaller cube by digging a hole into the larger rockwool cube that is slightly larger than the smaller cube and placing the smaller cube inside that hole.
When the plant has reached maturity, transplant the final rockwool cube into the hydroponic substrate by digging a hole into the substrate that is slightly larger than the rockwool cube and placing the cube into the substrate, or gently wash away the rockwool and place the bare root plant into a bare root hydroponic system.