Infiltration Rate

Infiltration rate is a measurement of how quickly water can enter soil. 

Infiltration rates are reported as the depth water (in millimeters) can reach within one hour. For example, an infiltration rate of 10 mm per hour means that a 10 mm layer of water on top of the soil will take one hour to soak in. Understanding the infiltration rate of your garden or landscape can mean the difference between irrigation and flooding.

Soil types & infiltration

When soil is extremely dry, it won’t absorb water at all because it becomes hydrophobic. Before that point is reached, water is absorbed quickly in what is called the initial infiltration rate. It happens quickly because the macropores in the soil contain only air, giving the water plenty of places to go. As the pores begin to fill, absorption slows down to a steady rate called the basic infiltration rate. According to the Food and Agriculture Organization of the United Nations (I never knew there was such a group until today!), different types of soil have different basic infiltration rates:

• sand - more than 30 mm/hr
• sandy loam - 20 to 30 mm/hr
• loam - 10 to 20 mm/hr
• clay loam - 5 to 10 mm/hr
• clay - 1 to 5 mm/hr

Clearly, taking an infiltration test can help you have a better understanding of what type of soil you have. It also helps you to select plants that are best suited to your soil.

Benefits of better infiltration

Understanding and amending the infiltration rate of your soil can provide many benefits:

• improved soil structure
• soil moisture tension (ability of plants to pull water from the soil)
• a better habitat for beneficial microorganisms
• reduced water consumption
• more effective drainage
• reduce soil compaction
• it can make the soil easier to work

How to conduct an infiltration field test
Generally speaking, most of these tests are done with specialized equipment. You can, however, follow these steps to perform a modified version that will give you useful information:

Another testing method you can try only requires a shovel and a watch:

1. Dig a hole, 6 inches across and 6 inches deep.
2. Fill it with measured water and allow it to stand for one hour.
3. Measure the remaining water and then refill the hole.
4. Allow it to stand for a second hour and measure the water depth again.
5. Add the two measurements for a reasonable infiltration rate.

If your soil is like mine, compacted heavy clay when we moved in, adding organic material is the best way to improve the infiltration rate. If you plan on installing a rain garden, checking the infiltration rate is critical. Standing water can drown even the healthiest plant, given enough time.

Help your garden and your landscape with improved permeability and infiltration rates. Your plants will be healthier, more beautiful, and more productive.

1. Select a level location.
2. Expose bare soil, disturbing it as little as possible.
3. Use a large metal coffee can or restaurant size vegetable can with both ends cut out.
4. Place a board on top of the can (to prevent bending the can) and start hammering in circles until it is buried 2 to 3 inches into the soil.
5. Measure the above ground portion of the can and subtract one inch.
6. Have a watch or timer ready and fill the can with water to one inch from the top.
7. Record the amount of time it take for all the water, including puddles, to be absorbed.

Infiltration test (USDA) Public Domain