Sub-irrigation is just a complicated term for self-watering. This type of watering has several advantages for plants, as well as plant owners. The method described here encourages roots to grow downward toward water rather than wrapping around the plant. It also allows the plants to absorb as much water as they need without over or underwatering and without pouring water on top of the soil, which can wash away soil at the top of the root structure. The advantages for plant owners, besides happier plants, are that you don't have to worry about forgetting to water your plant
Remove the cap from your soda bottle and discard. Run your tap water until it gets hot, but not scalding hot. Fill your bottle with the hot water and let it sit for 30 seconds to 1 minute. This will melt the glue on the label and allow you to remove it easily. Remove the label and discard. Pour out the hot water. Shake out as much of the excess water as you can. Note that if the tap water is too hot, it will warp or shrink the bottle.
Place the bottle on a flat surface. Next to the bottle, stack books or old magazines until they are approximately half as tall as the bottle. Now take your marker, remove the cap, and tape it to the top of the stack, with the tip hanging off about one inch. You will rotate the bottle, with the bottom of the bottle flat against the hard surface and the side of the bottle pressed to the marker to draw a black line all around the bottle. This is the line you will use to cut your bottle in half.
Use either scissors, a craft knife, or a solderer with a blade to cut the bottle in half along the line. This will yield you two halves: the top is the planter and the bottom is the watering chamber.
This part requires a lot of measuring and cutting. Invert the top of the bottle and put it inside of the bottom of the bottle. The top where the lid screws on should touch the bottom of the bottle. If it does not, trim the sides of the bottom half and try again. Repeat until the the top of the bottle, when turned upside down and placed in the bottom half, touches the bottom. You can measure by removing books and magazines from your stack and using the marker to draw another line.
Now, you need to add holes to the top of the bottle to allow oxygen to get to the roots. Use your solderer or craft knife to poke two rows of holes, about an inch apart, at the round part of the neck.
Now you are ready to plant in the container! Cut a fabric or cotton square large enough to cover the hole and come up the neck of the bottle. Use the marker to push the square through the neck of the bottle, making the fabric stick out a tine bit. This fabric will keep the soil from falling out of the bottle opening, and serve as a wick to touch the water.
Fill the top of the container with moist (not wet) potting mix, and your favorite plant. Fill the bottom half full with water. Place the top of the bottle inside the bottom of the bottle and watch. You will see the water in the bottom chamber go down as your plants wick it upward. You want to make sure your plants are not getting overwatered by keeping the chamber full at all times. It is best to pick a day each week to add water to them, then pour the excess water our the following day. For example, I fill my chambers on Monday, and pour the excess out on Tuesday. This keeps the plants from being overwatered.