A Protocol for Collecting and Constructing Soil Core Lysimeters

The overall goal of this procedure is to demonstrate the collection, assembly, and irrigation of an intact soil core lysimeter to study nitrogen leaching through undisturbed agricultural soils. This method is designed to answer questions concerning nutrients and other chemicals that can leach below the root zone to shallow ground water. The main advantage of this technique is that it is quick and easy.

We have found that we need four or five replicates to produce significant results. Although in this video we were looking at nitrate leaching, the method can be used to study many other compounds, such as phosphorus, pharmaceuticals, hormones, or metals such as arsenic. To begin, remove surface vegitation, rocks and other debris from the collection area.

Position two lysimeter bodies on level ground where lysimeters are to be taken. Ensure that lysimeters are level, so that soil within the column is of a uniform depth. Drive a specially designed, trailer-mounted drop hammer into place over the lysimeter bodies.

When the drop hammer is in place, deploy hydraulically powered outriggers to level the steel plate with the ground and the top of the lysimeter bodies. The outriggers also provide stability for the drop hammer. Partially hoist the steel plate weighing 1, 180 kilograms up a 3-meter tower with the aid of a mechanical winch.

Release the steel plate to hammer to columns into the soil. Repeat this step several times, until the column rim is 2 cm above the soil surface. Check for soil compaction inside the lysimeter by measuring the depth of the soil inside and outside of the column.

If the soil inside the column is more than 1 cm lower than the soil outside the column, soils are compacted, and are not suitable for research. Place a plywood disc inside the rim of the column to prevent contamination. Dig a trench beside the soil core and slightly deeper than the column bottom with a backhoe.

Widen the hole with a shovel or pick, and expose as much of the outside of the cylinder as possible. Push a heavy metal digging bar down along the entire length of the side of the column, so that it is between the soil and the outside column wall. Pry the digging bar back and forth until the soil interface at the bottom of the column is broken, and slide the column into the center of the trench.

Frame the lifting scissors around the top of the lysimeter in preparation for soil core removal. With a person holding each bar, pull up until the scissors close tightly around the column, and lift the lysimeter out of the hole. Place the lysimeter on a flat working surface, such as a flat board.

After flipping the soil core over so that the bottom side is up, gently level the soil even with the rim of the PVC with a straight edge. Remove stones protruding above the plane of the rim with a pen knife or a screwdriver. Also loosen the soil and expose the natural surface textures therein.

Fill any voids with chemically inert play sand and gently pack it. Grade the sand even with the column bottom with a straight edge, and remove any excess sand. Next, extrude a continuous round bead of clear silicone caulk around the rim of the lysimeter.

The caulk should be thick enough to seal the perforated disk bottom to the lysimeter’s, and prevent leaking. Lay the perforated disk onto the rim, with the filter fabric facing the sand, and press down firmly to allow good contact of the plate and lysimeter. Then drill eight evenly spaced pilot holes around the edge of the plate, and fasten the perforated disk with 1-inch stainless steel screws and a drill driver.

Slip the flexible pipe coupling onto the lysimeter base, so that about 2 cm of the coupling are projecting above the lysimeter rim. Fit the modified PVC cap into the flexible pipe coupling, and push the cap down until it makes contact with the lysimeter body. Then place the fastening bands in the grooves of the coupling and lightly secure without constricting the coupling.

Tighten the metal bands with a handheld hex driver until the lysimeter cap is held firmly in place. The lysimeter is ready to be flipped, and transported to a climate controlled facility. To install moisture sensors, first scribe a 5 cm long horizontal line on the lysimeter wall at 5 and 25 cm depths.

Measure from the soil surface, and not the rim of the lysimeter. Drill a 1 cm diameter hole through the wall of the lysimeter at each end of the marked lines. Cut the remaining 3 cm of plastic between the drilled holes away with a rotary cutting tool.

Next, chisel a 1-cm thick by 5-cm long slit into the soil to accommodate the casing of a moisture sensor. Push the moisture sensor into the hole, into the cleaned out slot, until the sensor prongs are firmly buried in the soil and only the wire is sticking out of the lysimeter. Clean the soil from the walls of the slot with a brush or rag.

Apply a thick bead of silicone caulk into the slot to prevent water from leaking out. After the caulk has dried, apply a second cycle of silicone to ensure that all gaps in the hole surrounding the sensor are sealed. Seal gaps between the soil and the lysimeter wall with caulk to reduce the risk of preferential flow down the inside walls of the lysimeter.

First, pierce and load a tube of clear silicone caulk into a standard caulk gun. Place the tip of the caulk tube between the void in the soil to be filled and the inside face of the lysimeter body. Then push the tip of the caulk gun below the soil about 2 cm.

Squeeze the caulk out of the tube until it fills the void and oozes above the soil surface. After checking that the soil core is level as described in the text protocol, wrap Teflon tape around the threaded nylon tube fitting, and turn the fitting clockwise into the cap. Tighten the fitting with an adjustable wrench until none of the threads are visible.

Push a 0.5-inch hose onto the barbed end of the nylon fitting, and cut the hose so that it passes approximately 4 cm into the mouth of the collection jug. Set the container under the lysimeter, and place the hose inside the collection jug. Cover the soil surface with cheesecloth, or another chemically inert mesh such as a window screen, to protect and preserve soil aggregates and surface residue.

Measure 1, 450 ml of simulated rain water into a graduated cylinder, and pour it into a watering can equipped with a shower head. Gently and evenly sprinkle the water over the window screen at a rate that does not disturb the soil surface. After waiting a period of time for the water to infiltrate and percolate through the soil column into the cap and collection container, tip the lysimeter in toward the outlet hole until all water is drained from the lysimeter reservoir cap into the collection vessel.

Measure the mass of leachate collected with a scale, and convert mass in grams to milliliters. Pour the leachate sample into a 350 ml sterile plastic sample bottle. Immediately filter 50 ml with a suction funnel equipped with 0.45 micron filter paper in preparation for nitrate analysis using color imagery via flow injection analysis.

Representative results of the irrigation and leaching experiments are shown here. Average weekly leachate depths ranged from 1.12 to 1.95 cm for the four soils. Leachate recovery expresses percentage of irrigation water applied, followed a general trend related to soil texture, with recoveries from the sandy Evesboro and Sassafras soils being slightly more efficient than from the finer textured Bojac and Quindocqua soils.

This figure shows that changes in soil water content at 5-cm and 25-cm depths, following irrigation, demonstrate differences in water transmission between coarser and finer textured soils. Moisture profiles indicate rapid movement of irrigation water through the coarser textured Evesboro sand and Sassafras sandy loam soils. This figure shows change in nitrate concentration before and after poultry litter application.

Nitrate concentrations in leachate increased after litter application, but followed different temporal patterns between soils. Differences in leachate nitrate mass loss reflect not only trends in nitrate concentrations in leachate, but also differences in leachate depths. To assess the role of poultry litter application and leachate volume on nitrate flux, soil nitrate fluxes from before litter application were subtracted from subsequent weekly fluxes.

While attempting this procedure, it is important to be safe. Always wear hearing protection while operating drills or the drop hammer. For safety, never work underneath the drop hammer.

Be aware of other people in the area when you’re operating the drop hammer, and maintain a safe distance from the drop hammer when operating. After watching this video you should have a good idea how to collect an intact soil core, construct a lysimeter, install soil moisture sensors, carry out irrigation, and collect leachate.