If you have poor soil conditions, such as soil that contains a lot of sand or clay, do not simply throw a couple inches of top soil on the surface and plant your plants. This will not be effective and will not help your plants grow and thrive. Rather, you must work the new soil into the existing soil to make any positive difference. However, adding organic matter like compost, well-rotted manure or leaf mold is more beneficial to a garden than adding top soil, according to Jerry Goodspeed, horticulturist with the Utah State University Extension. If you still want to add top soil, because it is cheaper or more easily accessible, then add it in such a way that will help aerate and improve your current soil conditions.
Examine the top soil for any weeds or grass. Look for roots, seeds, green growth or anything that may sprout once it gets some water. It is best not to use soil that is infected with weeds or grass. Often, purchasing top soil labeled as "weed free" is better than obtaining your own.
Determine approximately how much top soil--in cubic feet--you need to buy or obtain. Multiply the area of the garden and the depth of the top soil you wish to add in feet (e.g., 2 inches equals 0.167 feet). For example, to add 2 inches of top soil to a rectangular garden that is 10 feet by 10 feet (100 square feet), you will need about 16.7 cubic feet of top soil (100 X 0.167). This was figured out by multiplying 10 (length), 10 (width) and 0.167 (depth) together. Larger quantities are sold by the cubic yardage, so divide cubic feet by 27, if necessary.
Spread 1 to 3 inches of top soil over the area. Use a hoe, garden fork or garden rake to evenly spread it out. Rent a rototiller and work it into the top 6 inches of the existing soil. You can also use a hoe or shovel, but this will require a lot more labor to work the new top soil into the old soil.