Soil Water Retention Curves (Testing in “Slow Motion”)

The properties of water flow through a soil can be determined by testing the saturated hydraulic conductivity, developing a water retention curve and then testing the unsaturated hydraulic conductivity. AGVISE Laboratories has been developing the methods for determining the water retention curve and the unsaturated hydraulic conductivity of soil samples under GLP guidelines for the past several months. This has been a long process and we are now just about finished with method development.

One of the difficult aspects of developing methods like these is the amount of time required to complete the method so that the test results can be evaluated. For most of the GLP soil characterization methods of analysis, the time required to complete a test may range from a few hours to a couple of days. One example is the soil texture analysis method that takes about three hours to complete. In the case of the method for determining a soil water retention curve, it can take many months to complete the test.

Determining the soil water retention curve for a soil sample starts with a saturated soil column. Often we use the same soil wateerretpic column which was used for the saturated hydraulic conductivity test. This test shows how much water will flow through a saturated column of soil in a given amount of time. The next step is to determine how much water the column of soil will continue to hold when a known amount of vacuum is placed on the soil column (see picture). In this method, a vacuum is applied continuously to the bottom of the column of soil until the water loss has stopped. The water loss is determined by weighing the entire soil column apparatus each day until it stops losing weight. Depending on the texture of the soil, this may take up to a month or more. Once the water loss has stopped, the amount of water remaining in the soil column is calculated. This entire process is then repeated at several different levels of vacuum until a moisture retention curve is developed for that soil sample. The time required to develop a water retention curve for a sample can easily be a few months or more. Once a moisture retention curve is developed, the unsaturated hydraulic conductivity test is then conducted. This method determines how fast water will flow through a soil that has different initial water contents. This information is important for predicting how nutrients or pesticides may move through a soil profile. While these testing methods allow us to learn a lot about how nutrients and pesticides may move through a soil profile, we now know that there is no way to rush this type of analysis. Every soil will require a different amount of time in the laboratory for these procedures to be completed.