"The world's very best fertilizer -- 100 percent, tested for 10,000 years -- is compost," says Eliot Coleman, author of "Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long." "And it's made for free, in your backyard from waste products."
If you do nothing to it -- no equipment, no turning -- what goes into your compost pile will be ready to use in about six months, he says. Gardening doesn't have to be expensive, Coleman stresses. "You can do this without spending any money at all."
If you buy, look for compost "that is loose enough but has the nutrient value that you can mix with your soil," says Gilbertie. Enriching ingredients can include everything from oyster shells or fish to bark, peat moss or sphagnum moss.
Another good fertilizer is manure. It "will also add to the composition and will give you what you need so that these plants really take off," Gilbertie says. And "it goes a long way."
Look for certification from the Organic Materials Review Institute, or OMRI, he says.