My goals were:
cheap: I'm not looking to spend much money on this project, so cheap is good, and free is better. After all, that's why we're doing it ourselves, right? I got pretty much all my materials from recycle bins at work or other household items. The few things you would have to buy cost less than $10, and provide enough material for many of these bottles.
It's easy: No expensive tools or equipment, no expertise, just scissors, and an "exacto knife" or other sharp pointed blade.
low maintenance. Once you set it up, it requires no watering, no fertilizing,
no electricity: There are no air or water pumps. The roots get oxygen by forming aerial roots above the surface of the water as the water level drops.
The materials are easy to find: anybody, anywhere can find the materials for this project. You can use almost any kind of plastic bottle with relatively smooth straight sides, from 24 oz up to 2 litre bottles and beyond. Water bottles, Soda bottles, fruit smoothie bottles, square bottles, round bottles, whatever. The growth media is made from easy to find products, for $5 you can buy enough to fill hundreds of these bottles. I've also been experimenting with using water from a fish tank as fertlizer, with good results so far.
It works: So far it's been working great. the plants are healthy and are growing quickly. The roots look healthy and are not rotting or showing signs of lack of oxygen. The leaves are nice and green and show no signs of nutrient deficiencies. A hydroponic system should work as well or better than a soil based system, otherwise, what's the point.
It's compact. the smaller bottles take up about 3 inches of a windowsill, and the larger (2 liter) bottles take up only 5 inches, so you can fit several in a window. You can also hang bottles to use the vertical space of the window. I'm still working on an easy way to hang them. When I get it right I'll post that part. too.
Step 1: Materials Needed
A plastic bottle: Any size larger than 23 oz. Large 2L bottles work very well, but smaller bottle work fine for small plants like lettuce. you're only growing these for about a month, so it doesn't need to be huge. Also, the sides must be fairly straight and smooth, especially near the "shoulder". Bottles with lots of grooves or ridges don't work well. I've used round bottles and square bottles, they both work fine. Bottles with slightly thicker and stiffer plastic work quite well.
A Shower Scrunchie: one of those plastic mesh shower sponge thingies. they consist of about 10 feet of mesh, which is actually a hollow tube of mesh folded and tied into a compact shape. One of these is enough mesh to make dozens of grow bottles.
Scissors: nothing fancy.
a knife: a very sharp utility knife or exacto knife with a sharp tip.
aluminum foil: To protect the roots from direct sunlight. I grew one plant for a few weeks without the foil, and it didn't seem to really harm the roots, but they did grow away from the sun, rather than growing straight down like the others. You are also likely to develop a problem with algae if the roots and liquid media is exposed to direct sunlight, and this will quickly consume the nutrients and foul the water. You could probably use something that looks nicer, if that's important to you.
(update: after a few more weeks, the plants without foil around the water reservoir did develop serious algae problems, and were noticeably smaller than the ones that were covered)
tape: regular clear office tape.
Seeds: I'm using lettuce, because it's fast growing, and has a shallow, fibrous root system. Bibb lettuce apparently works very well, although I'm using a red leaf lettuce. Try other things, and let me know how they work. larger plants would obviously require larger bottles. A packet of lettuce seeds contains hundreds of seeds, so you don't need to buy a lot, and share the extras.
Growth media: You could buy ready made hydroponic growth media, but it's expensive. You can easily make growth media from store bought fertilizer and epsom salts, with home made egg shell extract for calcium and micro-nutrients. A small box of fertilizer costs about $3, and box of epsom salts is about 99 cents. You only use a tiny bit of each, so even the smallest box of fertilizer can make enough media for hundreds of grow bottles. Egg shells are basically free, assuming you eat eggs once in a while. A single egg shell dissolved in lemon juice will be enough micronutrients for several bottles.