What is a Lysimeter Anyway?

excerpt from the Soil Report Newsletter of Soilmoisture Equipment Corp.

Definition: "A device for collecting water from the pore spaces of soils and for determining the soluble constituents removed in the drainage". Early devices, called pan lysimeters, collected soil water as it percolated down via gravity through saturated soils. The major limiting factor of a pan lysimeter was that fluids could only be gathered under saturated gravity flow.

In 1961, P.E. Skaling (Soilmoisture's founder) and Dr. George H. Wagner, of the University of Missouri, fashioned the first suction lysimeter. The suction lysimeter was a cylindrical device consisting of a porous ceramic cup (to withdraw soil pore water); a body tube to act as a reservoir; and a simple stopper assembly with a single hole for pulling a vacuum and retrieving the sample. These early suction lysimeters allowed pore water to be "pulled" from unsaturated soils near the soil surface. Subsequent changes to the suction lysimeter added a "pressure" port to the stopper assembly and the "pressure/vacuum" was born. The new pressure/vacuum lysimeters extend the use of lysimeters to greater depths or even remote locations. Today, measurements are carried out in the "parts per billion" range. In order to achieve that kind of precision, it is necessary to use samplers like Soilmoisture's Ultra Samplers made of materials that will not interfere with the super sensitive chemical analyses. What are the important factors to consider in selecting a lysimeter? There are three primary factors to consider: porous interface materials; air entry values; and bonds and configurations.

Porous interface materials are a lysimeter's most important feature since it must have a hydrophilic (water loving) surface with numerous pore channels to transport soil water fluids without alteration or leaking. Volumetric porosities of the interface material should be greater than 10% and pores small enough to sustain air pressures "or bubbling pressures" greater than 10 psi. Generally there are four materials considered for interface materials: porous plastic films; porous plastic shapes; sintered metals; and ceramics. The first three materials have significant drawbacks to functionality. Delicate plastic films tear easily and cannot be well supported. Porous plastic shapes have large nonuniform pores and require special surface treatments to become hydrophilic. Sintered metals have high exchange capacities, will frequently oxidize, and again have nonuniform pore sizes. Ceramics have historically found application and long term use, with Alumina and Porcelains preferred for their inert and tough characteristics.

Air entry values of the porous interface material directly affects the range of the lysimeter's operations (maximum of full vacuum at sea level - 14.7 psi differential). Early pan lysimeters could only function when overlying soil became saturated and fluids "rained" into the catchment pans. Today=s suction lysimeters and pressure/vacuum lysimeters, rely on a surface of uniform wetted pores that act as contact points pulling fluids from soils, through interface pore channels into the reservoir of the sampler (lysimeter). If the pores of the sampler's interface material are either not small enough in size or not uniform, the water links break between the soil and the sampler. Then air, not water enters the sampler under vacuum. To test a sampler for range of operation wet or soak the porous surface for 15 minutes in water, seal the device, slowly apply increasing pressure, while holding the device under water. There should be no signs of leaks (stream of air bubbles) below 14.7 psi coming from the porous interface material. If your device shows significant leaking at say 5 psi, it means you will probably only be able to sample conditions near saturation and not the drier conditions associated with elevated differentials of 10-14 psi.

Bonds and Configurations can determine the long term success of a sampling program. Two-part epoxy resins that form a semi-flexible bond between materials of dissimilar coefficients of expansion have been found to be most satisfactory in sampler manufacture. High-grade epoxy bonds are impregnable to fluid intrusions, do not act as reservoirs for ion exchange, and have no sustained volatiles or organic hydrocarbons given off years after their manufacture. Wherever possible, materials should be mechanically joined or made from the same materials, thereby preventing any possible contamination or leakage sites.

Today, there are a wide range of sizes and shapes, from the older pan lysimeters to very small cup samplers, as well as the larger standard suction and pressure/vacuum lysimeter. From plate samplers to the more familiar cylindrical samplers, lysimeter technology is evolving to meet the needs of people who want and need to know more about the fluids that sustain our plants and microscopic subterranean organisms.