Use a drainage test to diagnose soil problems
To test a soil's drainage rate, dig a hole and fill it with water. Then monitor the time it takes for the water to seep away to determine if your soil is fast- or slow-draining.
Photo/Illustration: Todd Meier
Soil type often determines water needs. Some soils drain quickly and some drain slowly. For example, a sandy soil’s pore spaces are often too large to keep water not immediately claimed by roots from simply slipping away to an area below the root zone, so plants in sandy soils may need to be watered more often.
Slope and orientation can also affect a soil’s water needs. In the far corner of my own backyard, my highest, sandiest bed dries out three days earlier than any other bed. In one client’s hilltop garden, a southwest-facing slope dries out in just two sunny, windy days in midsummer, despite being composed of rich, loose clay blanketed with mulch.
While a lack of water can pose problems for some plants, irrigating more often than necessary can be harmful, too. Tightly packed clay soils hold water so well that any excess displaces what little air there is in the root zone, and adding more water at that time will only raise the level of root-rotting puddles. In these beds, I pay closer attention to water levels than I do in other beds.
A quick test can determine whether your soil has a drainage problem. Dig a 12-inch-wide by 18-inch-deep hole in your garden and fill it to the top with water. Let the hole drain and fill it again. If the water drains away within a few hours, your soil is excessively well drained, apt to dry out quickly unless watered frequently and lightly. If the water drains out in 12 to 24 hours, your soil is well drained, and you can grow almost anything without a problem. If the hole still has water in it after 24 hours, your soil is poorly drained. Install drain tiles, grow wetland plants, or measure how much water is left in the hole and build a raised bed of that height to achieve the 18-inch depth of well-drained soil that most cultivated plants need.