To start an avocado seed in soil, you need these supplies:
One or more fresh avocado pits
One 6 inch diameter garden pot for each seed
A water catchment tray for each pot
Take a fresh avocado, cut it in half, and remove the seed. Then wash the seed in water to clean off any avocado fruit, and soak the seed in water overnight.
The avocado seed has a pointy end where the shoot emerges, and a flat end which is where the roots grow. When you plant the seed in soil, be sure the flatter end in facing down into the soil.
Fill the garden pot to one inch (2.5 cm) from the top with a good quality potting soil mix. Make a hole in the center of the soil mix and place the seed in the soil so that the top inch of the seed remains above the soil line. Fill in around the seed with soil mix, and place the pot on a tray. Then water the seed and soil until water comes out of the bottom of the pot.
Place the pot in a location where it stays between 60ºF (15ºC) and 80ºF (26ºC) and where it receives strong, indirect sunlight during the day and where there are no strong drafts of hot or cold air.
Caring for Your Avocado Seedling as it Grows
Water the seed each day, or whenever the top of the soil dries out, using tepid water. In six to eight weeks a shoot should emerge from the top of the seed. However, if the seed does not produce a shoot in this time, start over with a new seed.
When the shoot gets to approximately 12 inches (30 cm) tall, cut it back by half. This causes the plant to grow side shoots and to become bushier. As the plant grows bigger, carefully transplant it into a larger pot.
After the seedling is well established, give it a weekly application of a fertilizer such as fish emulsion diluted in water, according to the instructions on the fertilizer container. Each week, mist the leaves with water using a spray bottle.
You can continue growing the avocado seedling as an attractive houseplant, or you can transplant it outdoors in the spring, if you live in a USDA climate zone 9-11.