Regular garden soil is inappropriate for planters because it drains poorly and may contain weeds or pathogens. Buy potting soil at nurseries or garden centres to fill pots and planters. The amount you need depends on the size and shape of the planter, but most small pots need about 20 qts., while large pots need 40 or more qts. of potting soil.
To fill one round pot, approximately 12 inches high and 12 inches in diameter, you'll need almost 20 qts. of potting soil. For large pots, measuring 20 inches high and 20 inches in diameter, buy 90 qts. of potting soil, which is equal to about two large bags. For a rectangular planter, measuring 2 feet long by 10 inches wide, buy 20 qts. of potting soil.
For specific calculations, first figure the area of the planter by multiplying the length of the planter by its width in feet. This will give you the square feet of the planter. Then determine the volume by dividing the depth of the planter by 12. For example, if the planter is 6 inches deep, the calculation would be 6 divided by 12 = 0.5. Multiply this result by the square foot area of the planter to get the volume. The resulting number indicates the cubic feet of potting soil needed to fill your planter.
Potting soil is sometimes sold by cubic foot, cubic yard or even pounds, rather than quarts. Remember these conversions to simplify buying potting soil: One cubic foot equals approximately 26 dry qts., and one cubic yard equals 27 cubic feet. When converting quarts to pounds, remember that the total amount may vary slightly depending on how dense the soil is, but 4.54kg. of potting soil generally converts to about 11 qts. Bagged soil is often soil in cubic feet. Bulk potting soil is almost always sold in cubic yards.
Buying new potting soil every year for your planters quickly adds up, especially if you have large pots that require large amounts of potting mix. Reuse the potting mix if the previous plants were healthy and free of disease. Remove some of the potting mix and add compost, vermiculite or peat moss to restore the soil's draining qualities and fertility. Replace the potting mix after two or three years. Filling a large pot with high-quality potting mix is expensive. Reduce the amount of potting soil needed by filling the bottom of the pot with crushed soda cans, milk cartons or packing peanuts. Lay a piece of landscape fabric over the materials and fill the pot with potting mix.
When buying potting soil, consider not only how much you'll need but the ingredients in it. High-quality potting soil costs a bit more but pays off in the long run because it conserves moisture, drains well and encourages vigorous plant growth. Select a potting medium containing vermiculite or perlite and peat moss. Buy bags of potting soil based on your needs. Small bags may contain 20 qts. or less. Most large bags contain between 32 and 40 qts. of soil. For large projects, consider buying potting mix in bulk from a landscape supply company or make your own.