How to Amend Potting Soil

Pour a 40-pound bag of standard grade potting soil into a wheelbarrow, large bucket or a potting table. In most cases, this will be the cheapest potting soil on the shelf or look for a variety that is not indicated for a specific plant type.

Inspect the potting soil to determine the basic composition. If you notice a lot of mulch, for example, this tells you that you won't need to add as much drainage material, if any at all. It may also help to put some of the soil in a pot with holes in the bottom, pour water over the soil and observe how well or poorly the soil retains the moisture.

Add equal parts of organic compost or aged manure to the potting soil to add nutrients to the soil which serve as plant food; use only aged manure because fresh manure increases the ammonia levels in the soil. If you don't have a large enough area to mix the entire bag of potting soil and an equal amount of compost, use part of the bag and measure equal parts of amendments.

Mix in a material with a high water retention rate, such as peat moss or vermiculite if you need to improve the soil to drain more slowly. Add about as much peat moss or vermiculite as the amount of potting soil.

Add a material with low water retention and high permeability, such as perlite or wood chips, if your plants require more drainage. Use the same mulch that you use in your flower beds as the wood chip amendment. Add about the same amount of material as the amount of soil you used.

Add lime and dry fertilizer to the soil mixture as needed. Contact your local extension agent for information about testing the soil so you know exactly how much lime and fertilizer the soil needs.

Mix all the ingredients until they are thoroughly combined. Vermiculite makes it easy to tell when the soil is well mixed when the white particles are evenly distributed throughout the soil.