Almost every gardening book and magazine article you read says you need well-drained soil. What is it? Well-drained soil is a soil where water infiltrates at a medium rate -- somewhere between running off and draining as though someone pulled a plug. The best soil is about half air space and half solid mineral, with 2-5 percent organic matter. Under ideal growing conditions, about half of the air space will be filled with water. If the soil is too wet, plant roots fail to get the oxygen they need. If the soil has too much air space and drains too quickly, plant roots dry out.
Here's a simple test to find out how well your soil drains. Dig a hole that is 12-18 inches across and 12-18 inches deep. Fill the hole with water. If water drains from the hole in 10 minutes or less, you have fast drainage. If the water takes an hour or more to drain, you have poorly drained soil. Improve soil drainage by building raised beds or by adding organic matter to existing soil in the form of well-rotted manure, compost, or peat moss.