Measuring Soil's Drainage Capacity


Good soil drainage can be the difference between a garden's success and failure. Well-draining soil retains enough moisture for plants' needs but lets excess moisture dispel before plant roots drown. Measuring the drainage capacity of your garden's soil will help you choose plants appropriate for your growing conditions and help you determine the materials needed to make the soil even better.

Digging Drainage Holes

  • The basic soil drainage, or perk, test requires digging a hole in a planting area, filling it with water and charting how long it takes the water to move into the soil. In order to obtain a consistent picture of soil's drainage rate, dig the hole 1 foot deep and 1 foot in diameter in a place where you want to measure drainage. Keeping the hole's sides straight and its bottom flat gives the drainage measurement an additional element of consistency. You are likely discover that the rate at which a particular volume of water drains varies from one part of your yard to another part. So digging several holes is helpful to get a clear picture of how the whole area drains, especially when planning a new flowerbed or other large planting area. If you plan to plant a tree or large shrub, then testing drainage in a hole that is 1 feet wide and 1 1/2 to 2 feet deep, instead of 1 foot in diameter and 1 foot deep, will give you more specific information about your site.

Measuring Results

  • A clock and a carpenter's tape measure or a stick on which you marked inches are all the measurement tools needed to determine inches of water drainage per hour. Experienced gardeners make this a hands-free project by laying a stick across the drainage hole's rim and tying the measuring stick so that it stands vertically in the hole. If possible, check and record measurements once each hour, especially if you assess the drainage rate of rapidly draining soil.

Determining Drainage Rates

  • Drainage rate is determined by soil porosity. The spaces between soil particles, or pores, hold both water and the oxygen plant roots need to process soil nutrients. Clay and silt soils have fine particles, and their low porosity is reflected in their slow water drainage. Sandy soil, on the other hand, is highly, sometimes excessively, porous, letting water drain almost instantly. Any hole that still has water in it after 12 hours is in very poorly draining soil. A hole that drains immediately is of equal concern in an area where plants are desired, and a drainage rate of 6 or more inches per hour may be hospitable to solely drought-tolerant plants. A drainage rate of 1 to 6 inches of water per hour is best, according to a Cornell University website article. Soil that drains at a rate less than 2 inches per hour has poor drainage, a University of California-Davis online article states.

Improving Drainage

  • Reduce the drainage rate of fast-draining soil by adding organic matter such as rotted leaf compost. Rotted sawdust, homemade compost, shredded evergreen bark and grain meals all increase soil's water-holding capacity. Incorporating sand into slow-draining soil increases porosity, letting soil release excess water more rapidly, but adding organic matter as well lightens soil texture effectively while improving nutrients. When improving soil drainage for a single plant, add an equal volume of organic matter and soil in an area at least as deep as your drainage test hole and two to three times the hole's diameter. Adding a 3- to 6-inch-thick layer of organic material to the full area of a new planting bed can improve drainage substantially. Plan to dig drainage test holes annually and renew organic materials every year to maintain soil good drainage.

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